an engineering geological characterisation of tropical clays - GBV

AN ENGINEERING GEOLOGICAL CHARACTERISATION OF TROPICAL
CLAYS
CASE STUDY: CLAY SOILS OF NAIROBI, KENYA
Dissertation
zur Erlangung des Grades eines
Doktors der Naturwissenschaften
Vorgelegt von
Simon Wanyonyi Otando, M.Sc.
aus Bungoma, Kenya
Genehmigt von der
Mathematisch-Naturwissenschaftlichen Fakultät
der Technischen Universität Clausthal
Tag der mündlichen Prüfung
01.10.2004

Referent: Prof. G. Reik, Ph.D.
Korreferent: Prof. Dr. H.-J. Gursky
Dekan: Prof. Dr. D. Mayer
Die Arbeit wurde an der Abteilung für Ingenieurgeologie des Institutes für Geologie und
Paläontologie der TU Clausthal angefertigt.

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Contents
Page
Acknowledgements
Summary (Abstract)
Chapter 1. Introduction 1
1.1 Scope of study 1
1.2 Location of the study area 2
1.3 Definition of study problem 2
1.4 Summary of case problems 4
1.5 Physiography 8
1.6 Climate 11
1.7 Method of survey 13
Chapter 2. Previous works 15
2.1 Summary 15
Chapter 3. Geology 19
3.1 Introduction 19
3.2 Precambrian rocks of the Mozambique Belt 20
3.3 Volcanic rocks 21
3.3.1 Kapiti phonolite 24
3.3.2 Nairobi phonolite 25
3.3.3 Nairobi trachytes 27
Chapter 4. Field methods 30
4.1 Introduction 30
4.2 Investigation and sampling scheme 30
4.3 Cone penetration depth sounding 34
4.4 Field vane test 38
4.4.1 Introduction 38
4.4.2 Testing procedure 38
4.4.3 Results of study 40
4.4.4 Applications 43
Chapter 5. Types of soil 45
5.1 Superficial deposits 45
5.2 Soils 46
5.2.1 Red friable clays 48
5.2.2 Black clays (black cotton soils) 53

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Chapter 6. Chemical and mineralogical analysis 58
6.1 Introduction 58
6.2 X-ray fluorescence (XRF) studies 58
6.2.1 Scope and method 58
6.2.2 Results 58
6.3 X-ray diffraction (XRD) studies 59
6.3.1 Scope and method 59
6.3.2 Results 60
6.4 Scanning electron microscope (SEM) analysis 67
6.4.1 Scope and method 67
6.4.2 Results 68
6.4.2.1 Nairobi phonolite 68
6.4.2.2 Nairobi trachytes 70
6.4.2.3 Black clays 71
6.4.2.4 Red soils 75
6.5 Importance and influence of clay minerals on engineering
properties of black clays and red soils 77
6.6 Carbon and sulphur content of soils 78
6.6.1 Method of analysis 78
6.6.2 Results 79
6.6.3 Analysis of results 80
Chapter 7. Laboratory soils index and engineering properties determination 83
7.1 Moisture content and index tests 83
7.1.1 Introduction 83
7.1.2 Natural moisture content 83
7.1.3 Atterberg limits and consistency of clay soils 84
7.1.4 Free swell test (after Gibbs & Holtz, 1956) 91
7.1.5 Linear shrinkage 95
7.2 Grain/ particle size analysis 96
7.2.1 Scope 96
7.2.2 Procedure and results 97
7.3 Direct shear tests 104
7.3.1 Scope 104
7.3.2 Procedure 104
7.3.3 Results 106
7.3.4 Evaluation and analysis of results 110
7.4 Oedometer consolidation tests 115
7.4.1 Scope 115
7.4.2 Theory of consolidation 115
7.4.3 Consolidation coefficients and other parameters 119
7.4.4 Procedure 124
7.4.5 Analysis of consolidation test data 125
7.4.6 Results of consolidation tests 126
7.4.7 Swelling pressure and percentage swelling 138
7.4.8 Evaluation and application of results of oedometer consolidation,
swelling pressure and percentage swelling 152

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Chapter 8. Distribution of index and strength properties in black clays 155
8.1 Natural moisture content 155
8.2 Liquid limit 158
8.3 Plasticity index 161
8.4 Linear shrinkage 164
8.5 Free swell 166
8.6 Clay and fine fraction 168
8.7 Shear strength 173
Chapter 9. Implications of index/ engineering properties of black clays and red soils
on construction practice 176
9.1 Atterberg limits and other index parameters 176
9.2 Grain size distribution 177
9.3 Shear strength parameters 177
9.4 Consolidation-settlement characteristics 178
9.5 Percentage swelling and swelling pressure effects 180
Chapter 10. Correlation of index properties 181
10.1 Linear shrinkage and plasticity index 181
10.2 Liquid limit and plasticity index 187
10.3 Liquid limit and linear shrinkage 191
10.4 Free swell and other index properties 194
10.5 Clay fraction and linear shrinkage, free swell 198
Chapter 11. Conclusions 200
Chapter 12. Recommendations 207
References
Appendices
Appendix A. Results of oedometer consolidation tests
Appendix B. Results of swelling tests on black clays
Appendix C. Results of swelling tests on black clays
Appendix D. Distribution/ variation of index and engineering properties of
black clays in Nairobi area
Appendix E. Geotechnical soil map of Nairobi area
Figures
1.1 Location map of Nairobi city and study area 3
3.1 Geology: relative position of Great Rift Valley, extent of early lava flows and
ages of rock formations in Nairobi area 22-23
4.1 Site investigation/ sampling scheme employed for black clays in this study 31
4.2 Field depth sounding results for black clays and red soils 35
4.3 Contoured depth variations in black clays and red soils 36
4.4 Block diagrams of depth variations in black clays and red soils 37

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4.5 Principle of vane shear test 38
4.6 Variation of field vane shear strength with soil depth 41
4.7 Variation of field vane shear strength with natural moisture content of soils 42
4.8 Variation of field vane shear strength with bulk density of soils 43
5.1 Distribution of soils of the study area 47
6.1 – 6.6 X-ray diffraction diagrams of black clays 61-64
6.7 – 6.10 X-ray diffraction diagrams of red soils 65-67
6.11 Variation of carbon content of red soils with depth 80
6.12 Variation of carbon content of black clays with depth 81
6.13 Variation of carbon content of both black clays and red soils with depth 82
7.1 A plasticity chart classification of black clays and red soils 90
7.2 Activity level classification of black clays and red soils 91
7.3a Particle size distribution curves for black clays and red soils 102
7.3b Triangular classification chart for black clays and red soils 103
7.4 Shear stress/ displacement curves of black clays and red soils 107
7.5 Coulomb or shear failure envelopes obtained for black clays and red soils 110
7.6 Correlation between shear paramters (c´, φ´) and relative consistency,
Cr, as well as moisture content, Wn, of black clays 111-112
7.7 Correlation between shear paramters (c´, φ´) and plasticity index (PI)
of black clays 112-113
7.8 Correlation between shear paramters (c´, φ´) and reciprocal of
plasticity index (1/PI) of black clays 113
7.9. Correlation between shear paramters (c´, φ´) and ratio Cr/PI
for black clays 114
7.10 – 7.12 Time factor Tv related to degree of consolidation U% 117-118
7.13 Log-time/settlement curve analysis method showing phases
of consolidation 118
7.14 Analysis of square-root time/ settlement curve 118
7.15a. Log-pressure/ voids ratio (e/ log p) curve 122
7.15b. Determination of coefficient of secondary compression cα 122
7.16 Conventional log-time/settlement curves for black clays during
loading and unloading stages 127
7.17 Unconventional log-time/settlement curves for red soils
during loading and unloading stages 127
7.18 Voids ratio/ log Pressure (e/ log p) curves for black clays and red soils 128
7.19 - 7.20 Permeability/ log pressure (K/log p) curves for black clays
and red soils 130-131
7.21 Coefficient of volume compressibility/ log pressure (mv/log p)
curves for black clays and red soils 131
7.22 Coefficient of consolidation/ log pressure (cv/log p) curves for black
clays and red soils 132
7.23 – 7.24 Correlation between laboratory determined liquid limits and
compression indices for black clays and red soils 134
7.25 – 7.26 Correlation between swell index and Atterberg
limits for black clays and red soils 137
7.27 Correlation between swell index and free swell for black clays
and red soils 137
7.28 Swelling pressure curves obtained for black clays 139

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7.29 – 7.33 Correlation between swelling pressure and index properties
of black clays 140- 141
7.34 Variation of percentage swelling with loading pressure in black clays 143
7.35 Correlation; % swelling and loading pressure of black clay samples 144
7.36 Correlation between % swelling and loading pressure of black clays 146
7.37 Correlation; percentage swelling in relation to initial specimen height, Ho,
and extent of external loading for black clays 149
7.38 Estimation of swelling properties of black clays using Greek method 151
8.1-8.2 Natural moisture content variation in black clays across the study area for
depths less than 0,50m as well as those greater than 0,50m 155-157
8.3-8.4 Liquid limit variation in black clays across the study area for depths of
less than 0,50m as well as those greater than 0,50m 158-161
183
8.5-8.6 Plasticity index variation in black clays across the study area for the depth of
less than 0,5m as well as that greater than 0,5m 162-163
8.7-8.8 Linear shrinkage variation across the study area for the depth of
less than 0,50m as well as that greater than 0,50m 164-165
8.9-8.10 Free swell variation in black clays across the study area at depths
of less than 0,5m as well as those greater than 0,50m 166-168
8.11 Variation of clay proportion in black clays across the study area at depths of less
than 0,5m as well as those greater than 0,50m 170
8.12-8.13 Variation of fines proportion in black clays across the study area at depths
of less than 0,5m as well as those greater than 0,50m 171-172
8.14 Distribution and variation of coarse fraction in black clays 173
8.15 Distribution and variation of shear strength parameters (c´, φ´) in black
clays 174-175
10.1 Correlation between laboratory measured values of plasticity index and linear
shrinkage of black clays
10.2 Correlation between calculated and laboratory measured plasticity indices
of black clays 184
10.3 Correlation between laboratory measured values of plasticity index and linear
shrinkage of red soils 186
10.4 Correlation between calculated and laboratory measured plasticity indices
of red soils 187
10.5 Correlation between laboratory measured values of plasticity index
and liquid limit of black clays and red soils 187
10.6 Correlation between calculated and laboratory determined plasticity indices
of black clays and red soils 191
10.7 Correlation: laboratory measured values of linear shrinkage and liquid
limit of black clays and red soils 191
10.8 A correlation between calculated and laboratory determined linear
shrinkage of black clays and red soils 194
10.9 Correlation: laboratory measured values of free swell and those of liquid
limit, plasticity index and linear shrinkage of black clays and red soils 197
10.10 Correlation between calculated and laboratory determined free
swell values of black clays and red soils 198
10.11 Correlation between laboratory measured values of linear shrinkage and
clay fraction of black clays and red soils 198
10.12 Correlation between laboratory measured values of free swell and clay
fraction of black clays and red soils 199

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Tables
1.1 Mean maximum and minimum temperatures for each month of the
year over a period of 50 years, for Nairobi city and the present study area 11
1.2 Average rainfall, mean relative humidity and amount of bright sunshine
for each month of the year, for Nairobi city and the present study area 12
2.1 Stratigraphic correlation of rocks in the Nairobi region 17
3.1 Chemical analyses of alkali-feldspars from the Kapiti phonolite
and similar rocks 25
3.2 Chemical analyses of strongly alkaline lavas of Nairobi phonolite
and other rocks 27
3.3 Chemical analyses of mildly alkaline lavas of Nairobi trachyte
and other volcanic rocks 28
3.4 Mineral compositions of trachytes and phonolites from Nairobi and
the study area 29
4.1 Classification of soft soils based on vane shear strength 40
4.2 Results of field vane shear tests in soft black clays at Madaraka West 40
4.3 Results of field vane shear tests in soft red soils at Arboretum 40
4.4 Results of field vane shear tests in soft red soils at Kenya High School 41
5.1 Profile description of a red friable clay 49
5.2 Soluble base content in soils of the study area 49
5.3 Profile description of a shallow yellow-brown clay overlying laterite 50
5.4 Results of chemical analyses of red soils obtained in this study 51
5.5 Earlier results of chemical analyses of clays and red soil 52
5.6 Some results of soil classification tests on red soils in present study 52
5.7 Earlier results of soil classification tests on red clays from the Nairobi area 53
5.8 Results of chemical analyses of black clays obtained in this study 54
5.9 Clay mineral composition of black clays 56
5.10 Profile description of black clay 57
6.1 Results of chemical analyses of red soils obtained in this study 58
6.2 Results of chemical analyses of black clays obtained in this study 59
6.3 Clay mineral composition of black clays 61
6.4 Estimated mineralogical composition of red soils 64
6.5 Theoretical compositions of kaolinite, illite and montmorillonite 65
6.6. Carbon content of soils of the study area 80
7.1 Activity of clay soils and clay minerals 85
7.2 Results of index tests performed on black clays and red soils 87-88
7.3 Atterberg limits, activity/ free swell classification and assessment
of possible clay mineralogy of black clays and red soils 92
7.4 Free swell classification of clay soils (Gibbs & Holtz, 1956) 94
7.5 Soil classification by particle size (BS 1377: 1975) 97
7.6 Viscosity and density of water (Kaye and Laby, 1973) 99
7.7 Results of grain size analysis for black clays and red soils 100-101
7.8 Typical values of φ for dry noncohesive soils and clay (after Lambe and
Whitman, 1979) 106
7.9 Shear strength parameters of black clays and red soils 108
7.10 Distribution and/variation of shear strength parameter (φ´) across study area 109
7.11 Time factors for one-dimensional consolidation (Leonards, 1962) 117
7.12 Derived parameters from results of oedometer consolidation tests
for the loading range of 25-800 kPa 128

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136
139
7.13 Compressibility classification of black clays and red soils 129
7.14 Rates of consolidation-settlement of black clays and red soils 130
7.15a Compression indices, Cc, and compressibility classification of clay soils 132
7.15b Results of compression indices correlated with Atterberg limits 133
7.16 Correlation between calculated and laboratory derived compression indices 135
7.17 Classification of black clays and red soils based on coefficient of secondary
compression 136
7.18 Results of swell indices for black clays and red soils correlated with Atterberg
limits and free swell
7.19 Results of swelling pressures for black clays, alongside corresponding
index properties and clay mineralogy
7.20 Percentage swelling of black clays under various load decrements 142
7.21 Relative degree of swelling of black clays under various loads 143
7.22 Classification of relative degree of swelling of black clays based on
percentage swelling 144
7.23 Percentage swelling of black clays under various load decrements (S% is in
relation to initial specimen height, Ho, i.e. 2 nd method in this study) 147
7.24 Classification of relative degree of swelling of black clays using the
newly derived/ 2 nd method 147
7.25 Computed values for estimating swelling characteristics of black clays
using the Greek method 150
8.1 Values of index properties of soils along field profiles in this study 156
8.2 Values of clay and fines fractions of soils along field profiles in this study 169
10.1 Calculated and laboratory measured values of plasticity index of black clays 181
10.2 Calculated and laboratory measured values of plasticity index of red soils 182
10.3 Calculated and laboratory measured plasticity indices of black clays 188
10.4 Calculated and laboratory measured plasticity indices of red soils 190
10.5 Calculated and laboratory measured linear shrinkage values of black clays
and red soils 192
10.6 Calculated and laboratory measured free swell values of black clays and
red soils 195
Plates
1.1 (a) & (b) New stone structures coming up in the study area 4
1.2 (a) & (b) Strong shrinkage cracks in black cotton soils during dry months 5
1.3 (a) & (b) Cracked and peeled-off asphalt on tarmac road giving way
to formation of pot holes in a zone of black clays in the Nairobi city centre 5
1.4 A slightly tilting building in a zone of black clays in the study area 6
1.5 (a) & (b) Light/ low rise buildings located in black clays with poor drainage 6
1.6 (a) & (b) Heavy constructed structures in a zone of black clays in
the study area 7
1.7 (a) & (b) View of the Great Rift Valley from its eastern flank, to the west of
Nairobi area 8
1.8 (a) & (b) Flat plains characterising areas of black cotton soils in this study 9
4.1 (a) & (b) Hand-dug excavation pits and undisturbed sampling in black clays 32
4.2 (a) & (b) Hand-dug excavation pits and undisturbed sampling in red soils 33

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4.3 Field augering of soils 33
4.4 Cone penetration sounding of soil depths 33
4.5 (a) & (b) Vane shear testing in soft black clays 39
6.1 – 6.5 Results of SEM analysis of Nairobi phonolite 68-69
6.6 – 6.9 Results of SEM analysis of Nairobi trachyte 70
6.10 - 6.24 Results of SEM analysis of black clays 71-75
6.25 – 6.31 Results of SEM analysis of red soils 75-77
7.1 Plastic limit determination of clay soils 86
7.2 Liquid limit determination of clay soils 89
7.3 Free swell testing of clay soils 94
7.4 Linear shrinkage testing of clay soils 96
7.5 Sieves used for grain size analysis of soils 97
7.6 Sedimentation procedure in grain size analysis of soils 97
7.7 Direct shear testing of clay soils 105
7.8 Oedometer consolidation testing of clay soils 125
7.9 Arrangement for swelling pressure determination of black clays 138

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Acknowledgements
I would like to thank all those who contributed in different ways towards making my Ph.D.
research work a success.
On the German side, my gratitude goes to the following: the German Katholic Academic
Exchange Programme (KAAD), in Bonn, for funding my studies through a four-year Ph.D.
scholarship; staff of the African Department, KAAD, Bonn (Dr. T. Scheidtweiler and MS.
Simone Saure) for valuable general information, advice and guidance throughout the
scholarship period; the Institute of Geology and Palaeontology, Technical Universty of
Clausthal, for availing facilities and equipment for field and laboratory investigations; the
Institute of Mineralogy (through Prof. K. Mengel), Technical Universty of Clausthal, for
facilitating chemical and mineralogical analyses of soil and rock materials; Prof. G. Reik
(Institute of Geology and Palaeontology, TU-Clausthal) for supervision of field and
laboratory studies as well as reading and correcting the thesis; Prof. H. J. Gursky (Institute of
Geology and Palaeontology, TU-Clausthal) for reading and correcting the thesis; and the
Faculty Examination Board under Prof. D. Mayer (Faculty of Mathematics and Natural
Sciences, TU-Clausthal), for examining the thesis work. I also appreciate the assistance
rendered by other members of staff, research assistants and students of the Institute of
Geology and Palaeontology, TU-Clausthal, in areas of samples preparation and analysis, data
analysis and interpretation, as well as printing of the thesis.
On the Kenyan side, my profound gratitude goes to the following: the Ministry of Education
for granting me a research permit to conduct fieldwork in Nairobi; the Kenya Airports
Authority (KAA) for permitting field investigations around JKIA and Wilson airports; the
Survey of Kenya for availing topographic maps; Department of Geology, University of
Nairobi, for storage of field equipment; Dr. Patrick L. Legge (Department of Geology,
University of Nairobi,) for assisting in the field study phase; Mines and Geology Department,
Nairobi, for permitting shipment of soil and rock samples to Germany for laboratory analysis.
I also thank Dr. Chris Nyamai (Department of Geology, University of Nairobi) as well as the
field investigation and sampling crew for their contributions.
I would also like to thank my wife (Ragchaasuren S. ) and parents, for their tireless support,
encouragement and patience. Contributions and assistance rendered by friends in Clausthal,
Gaston Schettkatt and Nassim Ouassa, are highly appreciated.

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Summary (Abstract)
An engineering geological characterisation of tropical clays. Case study: Clay soils of Nairobi, Kenya
Simon Wanyonyi Otando, M.Sc.
The purpose of this project was to study and evaluate soils engineering geological characteristics with a view to determining their form of distribution and
spatial variation across the study area. It also aimed at assessing the suitability, capacity and/ or limitations of the soils to support low/ light constructed
engineering structures, as well as recommending best suited and economical soil improvement/stabilisation techniques where necessary. The study and
investigation was therefore centered on determining/ developing statistical models to describe and predict the distribution of soils engineering properties.
Results of statistical analyses served to determine the degree of homogeneity or uniformity of the soils. It also aimed at developing/ preparing small-scale
and site specific engineering geological maps to provide spatially continouos ground information, soil data and depth variations across the study area. The
study also sought to present results of soil classifications and correlations between soil index and/ or engineering properties. A detailed chemical/
mineralogical analysis and evaluation of the soils and underlying bedrock was also carried out to investigate and establish the genesis of the soils.
The current study area lies mainly to the south of the fast growing Nairobi city, Kenya. It is covered by topographic sheet 148/4 of Nairobi from Survey of
Kenya, bounded by latitudes 1°15´S and 1°24´S to the north and south; and longitudes 36°45´E and 36°58´E to the west and east ; respectively, and
occupies an area of about 367 km². The area is covered mainly by varying thickness of black cotton soils/ black clays. Red soils are limited to the northwestern
part of the area. Underlying the soils are Tertiary volcanics which inturn overly a Basement system of folded Precambrian metamorphic rocks. The
research and study was centered on black clays with red soils included for comparison purposes.
The present area was chosen because it is mainly located on the outskirts of Nairobi city which are projected for future expansion of the town, as regards
construction of new social-economic and industrial/residential facilities. There is also a shiftimg trend in building tradition from timber and iron-sheet
structures on strip/ pad footings towards masonry construction of low buildings on slab foundations using dimension stone and bricks, hence the need for an
engineering properties characterisation of the soils. Current field observations also show the black clays to be potentially expansive/ reactive and exhibit
ground instabilities, failures and movements; with serious environmental, engineering and social-economic implications.
The study incorporated suitable field procedures, sampling criteria and laboratory chemical/ mineralogical and engineering soils testing methods. The field
study methods included cone penetration sounding, soil augering, in-situ description of soils and underlying rock, tests for possible carbonate content, as
well as undisturbed and disturbed soil sampling. This study phase had the objectives of providing information on the nature and distribution of expansive/
reactive black cotton soils as well as red soils of the study area. It also served to acquire site specific information on environmental characteristics
(groundwater conditions, climate, vegetation) which impact on in situ soil behaviour, investigate and model depth variation of soils; as well as compare in
situ soil properties/behaviour and derived parameters with those predicted and/ or derived from laboratory studies.
Laboratory studies performed on soil samples collected from the field included index tests of natural moisture content, Atterberg/plasticity limits, swelling
capability/free swell and linear shrinkage, as well as grain size analysis. Direct shear strength tests, Oedometer consolidation/ compressibilty tests,
percentage swelling and swelling pressure tests were also carried out by performing the tests on undisturbed samples. Laboratory chemical/ mineralogical
analyses and/ or studies employed the use of X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques.
The field and laboratory investigations/ studies of soils of the project area have provided a range of soil index and engineering properties as well as
chemical and mineralogical composition. The results of study show the distribution and spatial variation of index properties and chemical/ mineralogical
composition of black clays to be generally uniform across the study area, implying a generally homogeneous occurrence and engineering character of the
clays. Results of chemical/mineralogical analyses show the black clays to be composed mainly of smectites (montmorillonite) as well as having been
largely genetically derived from the gradual conversion of lacustrine deposits of volcanic ash, colluvium, alluvium and soft Pleistocene materials previously
deposited in a basin-like lake and swampy environment during a pluvial period. Contributions by leached components from areas of red soils as well as
eroded materials from surrounding areas are to a lesser extent.
Index properties of red soils point to a comparatively more stable and reliable engineering character of the soils. Results of chemical/ mineralogical analyses
show these soils to be mainly kaolinite in composition and polygenetic in origin. The soils must have derived/developed and formed under humid conditions
by weathering/alteration of underlying Nairobi trachytes/ volcanic tuff and volcanic ash. A limited contribution was derived from eroded and watertransported
detrital materials.
The index properties and grain sizes have been used to classify black clays into very high to extremely high plasticity clays and/ or silty clays exhibiting
medium to very high levels of activity as well as high swelling capabilities/ potential expansiveness on wetting from a dry condition. This engineering
behaviour could be accounted for by the high content of the more expansive clay minerals of smectites (90% and over) in the black clays. Likewise, the red
soils have been classified as medium to high plasticity clayey silts and silty clays with sand showing medium activity levels and generally low swelling
capabilities/ potential expansiveness when allowed free access to water. These characteristics have been attributed to the high content (80% and over) of the
rather less expansive clay mineral, kaolinite, in the red soils.
Results of laboratory shear strength investigations showed the black clays to be limited in strength and have relatively low angles of shear resistance
averaging about 18°. The red soils were found to be relatively more stable, with angles of shear resistance of 28° to 29°. On the other hand, results of
laboratory oedometer consolidation tests show the red soils to exhibit medium to very high compressibility and medium to high rates of consolidationsettlement
on normal external loading. This could be attributed to their loose friable nature and relatively higher porosity and permeability which facilitate
faster drainage on external loading. These soils would therefore be expected to undergo rapid consolidation – settlements, especially during the construction
stage, without posing long-term instability problems to constructed structures. However, the black clays would exhibit slightly lower compressibility and
low rates of consolidation-settlement due to their compact/cohesive nature as well as relatively lower porosity and permeability which tend to limit
drainage. In practice therefore, settlement and instability of structures located on these clays would be expected to persist beyond the construction stage.
Results of statistical analysis and correlations show that most soil index properties could be characterised and/ or estimated with a high degree of
approximation from results of Atterberg limits and other index tests performed on disturbed and fractioned samples. New relationships have also been
derived and developed to estimate and predict percentage swelling and swelling pressure characteristics of black clays in situ under various loading
conditions. However, in situ characterisation of engineering behaviour of soils as regards shear strength, consolidation-settlement, swelling pressure and
percentage swelling was shown to be better accomplished by direct measurements on undisturbed core samples rather than through correlation with results
of index tests performed on disturbed/ part samples which harbour destroyed soil fabric and structure.
The vast data base of soil index and engineering properties as well as chemical/ mineralogical composition obtained from field and laboratory
investigations/ studies of soils of the project area, together with prepared engineering geological/ geotechnical maps would be readily useful and applicable
by engineering geologists, geo-engineers and planners of future-intended construction projects in the area. The results of the investigations together with
related engineering geological assessments would assist especially at the stages of planning, designing and construction of projected facilities; and also
subsequent maintenance after completion. The results will also serve to complement on-going and future research on the characterisation of expansive and
reactive black cotton soils, especially as regards their negative environmental, engineering and social-economic implications.

1
Chapter 1
Introduction
1.1 Scope of study
The purpose of this project was to study and evaluate engineering geological characteristics of soils
found in the Nairobi area, Kenya. The information and data so obtained would be useful for
assessing the suitability and capacity of soils found regarding the possibilities and / or limitations of
developing and constructing low buildings and other light engineering structures ( roads, pavements
etc) in the study area and geologically similar neighbourhoods. Attempts have been made by the
author to present the geological data and soil parameters in a spatial continuous form that would be
readily useful and applicable by engineering geologists, geo-engineers and planners of futureintended
construction projects in the area. The results of the investigations together with related
engineering geological assessments would assist especially at the stages of planning, designing and
construction of projected facilities; and also subsequent maintenance after completion.
The current study area lies mainly to the south of the fast growing Nairobi city, Kenya. However,
the southern half of the Nairobi metropolitan, including the City centre, is also covered. The area is
covered by topographic sheet 148/4 of Nairobi, at a scale of 1:50 000; and printed by the Survey of
Kenya (1991). The exact location and extent of the study area is described in the next section. The
area is covered mainly by varying thickness of black cotton soils. Occurrences of red soils are also
found, but limited in the north-western part of the area. Underlying the soils are Tertiary volcanics
which in turn overly a Basement system of folded Precambrian metamorphic rocks.
The present area was chosen because it is mainly located on the outskirts of Nairobi city which are
projected for future expansion of the town, as regards construction of new social-economic and
industrial facilities, residential houses and related infrastructure. It is also largely covered by the
potentially expansive and reactive, relatively unstable and problematic black cotton soils. Previous
works and studies in adjacent areas and sections of the present area (Anyumba, 1991; Manga,
1988; Mowlem, 1985), together with recent field observations by the author reveal problems of
ground instabilities, failures and movements in areas of black cotton soils with serious engineering,
environmental and social-economic implications.
The field study phase of this project was carried out in Nairobi (Kenya) in the period of April-
September, 1999; and December, 2000-January, 2001. The laboratory study phase was undertaken
in the period of November, 1999-March, 2001; based at the Department of Engineering Geology,
Institute of Geology and Palaeontology, Technical University of Clausthal-Germany. The field
methods and laboratory procedures followed in the investigation, study and analysis of the soils and
underlying bedrock are summarised in the relevant sections in later chapters.
Adequate and proper engineering geological studies and investigations should serve to provide
useful and detailed information on aspects of geology (rock types and lithostratigraphy), types of
soil, geomorphology and geological structures, hydrology and hydrogeology, and geotechnical
characteristics and potential behaviour of rocks and/ or soils; followed by an assessment of potential
instabilities and possible natural hazards, and a subsequent consideration of suitable development
projects (Attewell, 1976; Bell and Pettinga, 1985; Blyth, 1974; Hedberg 1976; Sowers, 1994;
Tilford, 1994). In addition to the above investigations, this study also includes a detailed chemical
and mineralogical analysis and evaluation of the soils of the area. Genetic aspects of the soils are
also discussed.

2
In most developing countries including Kenya, there are difficulties of undertaking and / or
executing an exhaustive and comprehensive soils investigation programme time and again, due to
economic constraints related to limited finance as well as lack of adequate expertise and/ or
equipment. In such cases, the use of soil index properties to infer for soil reactivity and stability
characteristics could be partly a solution to the problem. This study attempts to correlate soil index
parameters with those related to shrink-swell potential, swelling pressures, shear strength and
consolidation-settlement (compressibility); in an effort to find closely approximating and/ or fitting,
and therefore predictive relationships between them.
Engineering geological investigations and mapping of soils carried out in this study was in
accordance with known standard practices and/ or procedures (IAEG, 1981a/ b; IAEG, 1976 ).
1.2 Location of the study area
Nairobi city is currently bounded by latitudes 1°15´S and 1°20´S to the north and south ; and by
longitudes 36°45´E and 36°55´E to the west and east; respectively (Fig. 1.1). It is the largest town
in Kenya and is located about 30 km to the east of the north - south running Great Rift Valley
(Saggerson, 1991; and Fig. 3.1). It extends for about 18,7 km east - west and about 9,3 km north -
south, covering an area of about 174 km² (Nairobi City Council, 1999). The western and northwestern
sections of the city are a part of the Kikuyu highlands that rise from an altitude of about
1660 m at the City centre to over 1800 m above sea level on the immediate eastern flanks of the Rift
Valley. These sections are covered by thick deposits of red soils that increase in their depth
westwards and north-westwards from about 4 m at the City centre to over 10 m in the vicinity of the
Rift Valley, outside the present area. The soils are underlain by a series of Tertiary volcanic rocks, a
result of several episodes of volcanic activity that accompanied the formation of the Great Rift
Valley (Fairburn, 1963; Gregory, 1921; Saggerson, 1991; Sikes, 1939).
The southern and eastern sections of the city are a part of the generally flat to gently rolling Athi
and Kapiti plains. The plains are covered by thick surficial deposits of black cotton soils that are
underlain mainly by volcanic rocks; while isolated exposures of Basement system Precambrian
metamorphic rocks also occur (Saggerson, 1991).
The present study area is largely located to the immediate south of Nairobi metropolitan which is a
part of the Athi and Kapiti plains. The remaining part of the area to the north forms the southern
half of Nairobi city. The whole project area is bounded by latitudes 1°15´S and 1°24´S to the north
and south; and longitudes 36°45´E and 36°58´E to the west and east ; respectively. It occupies an
area of about 367,38 km² in a zone of mainly black cotton soils; and to a lesser extent, red soils
which are confined in the north-western section of the area (Scott, 1963; Surv. of Kenya, 1990).
1.3 Definition of study problem
The undertaking of the present research study has been prompted by a number of factors, and
include:-
- changing residential construction practice
- widespread damage to roads/ pavements, residential and low rise buildings as a result of
expansive movements in areas of black clays
- lack of sufficient data to facilitate an engineering characterisation of the soils found
- predicted urban population growth of the Nairobi city
- predicted growth of services and industrial activities in the city
- predicted growth and expansion of the Nairobi city into its surrounding areas

Figure1.1 Map of Nairobi region showing location of present study area (Otando, 2003; Surv. of Kenya, 1991).
3

4
1.4 Summary of case problems
The Nairobi city has a current population of about 3 million people (Nairobi City Council, 1999). It
is also characterised by a number of social - economic activities which involve construction of
residential housing and industrial facilities. Other activities involve construction of roads, rail lines,
bridges, airports and sports facilities; power and telephone lines; water and oil pipelines; as well as
sewerage lines. Tunnelling and dam projects are also found. The population of the city is expected
to grow in the coming years. It is also expected that the above - mentioned activities will expand
and/ or multiply, while totally new ones may also be introduced. It is therefore anticipated that there
will be increasing pressure on currently available space and facilities in the city, prompting the need
by the Nairobi City Council and City planners to consider expanding into adjacent areas.
In the past, residential construction on the outskirts of Nairobi city, which include the present study
area, were predominated by structures constructed out of timber and iron sheets, with strip/ pad
footings. However, the recent ten years have witnessed strict regulations put in place by the Nairobi
City Council aimed at encouraging construction of more durable low-rise buildings and bungalows
(Plates 1.1 (a) & (b)) on slab foundations using dimension stones and / or bricks to cater for the
increasing urban population. This shifting trend towards masonry construction justifies a close
study, examination, engineering properties characterisation and assessment of the potential
capabilities and/ or limitations of the soils found, especially the expansive black cotton soils.
Plates 1.1 (a) & (b) New stone structures coming up in the study area.
The current project area is therefore also a potential area of expansion of the Nairobi metropolitan
due to its proximity to the city. It is especially favoured by a number of major communication lines
already traversing it and include the north - south running Nairobi - Mombasa road and the Kenya -
Uganda railway; while the Jomo Kenyatta International Airport and Wilson Aiport are also found
within. However, the ambitious plans by the Nairobi City Council to locate new and light
constructed facilities in the study area may be complicated by the fact that the area is covered by
black cotton soils and also receives heavy rainfall during the wet season. These soils are highly
expansive and reactive, and would usually exhibit very strong shrink-swell capabilities during
alternating dry and wet months of the year, with accompanying general lowering or rise of the
ground surface, respectively (Mitchell,1993; Nelson and Miller, 1992).

5
Plates 1.2 (a) & (b) Strong shrinkage cracks in black cotton soils (black clays) during dry months.
Examples of structural damage triggered off by ground movements associated with expansive and
reactive soils in the Nairobi area include cracked pavements as well as cracked and peeled-off
asphalt on tarmac roads giving way to formation of pot holes (Plates 1.3 (a) & (b)). The cause is
strong shrinkage cracks (Plates 1.2 (a) & (b)) that usually characterise black cotton soils during the
dry months. In light residential and low-rise buildings, the instabilities manifest themselves in the
form of cracked floors and walls and/ or tilting buildings (Plate 1.4). As part of an effort to offset
the huge costs the government has to incur every year in rehabilitating and maintaining damaged
roads, pavements and other infrastructure, the ministries of Environment and Public Works are
emphasising on soil research aimed at providing data and information relevant to producing suitable
foundation designs capable of withstanding the otherwise destructive effects of the reactive black
cotton soils found. Best suited and economical ground and/ or soil stabilisation techniques are also
to be sought. This study therefore forms a part of the collective efforts geared towards fulfilling
these goals.
Plate 1.3 (a) Peeling asphalt on tarmac road.
Plate 1.3 (b) Pothole formed on tarmac road.
Light and surface engineering structures are usually constructed on/ or within the upper layers of
soils (Galster, 1977; Maharaj, 1995; Plate 1.5). Because of the above stated potential instability
problems that would be posed to such structures in the current study area, a prior systematic and
sound mapping, characterisation and evaluation of the black cotton soils found together with the
underlying bedrock, just as was undertaken in this study, are an essential prerequisite. The results of
study and investigations so performed would be crucial to a sound engineering geological
assessment of the soils. These results would assist geo-engineers and city planners to anticipate in
advance potential limitations and engineering behaviour of the soils at sites intended for selected
projects. The possible alternatives at this stage would be for the geo-engineers and planners to use
the ground information and data so provided to recommend appropriate ground stabilisation/
improvement techniques aimed at improving the capacity of the soils to support increased loads

6
from intended structures; and/ or design and construct suitable foundation structures capable of
withstanding destabilising effects that may be caused by expansive ground movements.
Plate 1.4 Slightly tilting structure (towards the background) in a zone of black clays.
The available geological and soils maps covering the study area, are at their best, medium scale. For
engineering geological purposes, they can be described as regional and therefore less useful and
inadequate. Apart from presenting information mostly on lithology, geological structure and
stratigraphy of rock formations found, the maps show and give little on surficial deposits and soils.
There is also an obvious and total omission of information on geotechnical characteristics of the
soils and rocks. Within the current study, small scale engineering geological maps that are site
specific have been developed to provide and present important ground information and data on soils
in a continouos spatial form interpolated and extrapolated over the project area. The small - scale
detailed engineering geological soil maps will contain information sorted out by geotechnical
criteria for easy interpretation and readily application by geo - engineers and planners. The use of
these maps and application of contained information could also be extended to surrounding areas
with similar lithology.
Plates 1.5 (a) & (b) Foundation structures of light and low rise buildings located in black clays with
impeded drainage (foreground of Plate(b) with stagnant water).

7
Available literature in the form of geological reports (Fairburn, 1963; Gregory, 1921; Saggerson,
1991; Sikes, 1939) is also regional in approach and dwell mainly on rock formations, while the
soils are merely described as recent to present surficial deposits. Two reports are available on soils
(Gethin - Jones, 1952; Scott, 1963) but the aspects contained are discussed and treated from an
agricultural perspective and therefore almost entirely irrelevant and of little value to the needs of
geo-engineers and city planners. Site investigation reports from isolated areas over the city are also
available (Anyumba, 1991; Manga, 1988; Mowlem, 1985). They report mainly on the state and
condition of the underlying bedrock, the investigations having originally been meant for more
versatile engineering projects involving heavy foundation loads (Plates 1.6 (a) & (b)). As a result,
little useful information, if any, on geotechnical characteristics of soils is included.
Plates 1.6 (a) & (b) Heavy structures/ buildings in a zone of black clays in the study area.
The present study and research therefore aimed to investigate, assess and establish the engineering
geological characteristics of soils, and especially as regards the potentially expansive and reactive
black cotton soils in the present area, for geotechnical purposes. Attempts were made to present the
results of the investigations in geotechnical criteria and/ or such a similar form that would be readily
interpreted and applied by geo-engineers and city planners.
Best fitting and/ or closely approximating models to describe, present and predict distribution and
spatial variation of important ground information and engineering soil properties across the study
area have been developed. Use was made of systematically determined soil parameters, both
laterally as well as with depth, across the area.
Indirect characterisation of engineering soil behaviour in situ (shear strength, compressibility,
shrink-swell potential, swelling pressures) has also been made through correlation between results
of index tests performed on disturbed and part samples on one hand, and those of the above
mentioned engineering properties performed on core and/ or undisturbed samples, on the other
hand. The modelled results may be usefully applied by geo-engineers and city planners in readily
anticipating probable geotechnical soil behaviour with respect to projected structures at chosen
sites, based on results of soil index tests. Possible forms of interrelationship between soil index
properties and Atterberg limits were also investigated.
In short therefore, this study embarked on accumulating relevant site specific information across the
study area. The information involved soil properties as well as environmental characteristics such as
climate, groundwater conditions and vegetation. The approach followed in site characterisation
included among others the examination, assessment and interpretation of the performance of
existing constructed structures and buildings; examination, description and classification of soil
profiles; as well as computation and prediction of potential ground movements

8
and soil behaviour, based on results of field and laboratory investigations. The investigations would
be specific in seeking to establish and/ or model moisture variation with depth, as well as providing
details on depth of cracking, soil reactivity in terms of shrink-swell potential, active depth; and
other essential variables necessary in the assessment and/ or computation of potential ground
surface movement.
In conclusion, this study sought to investigate and present a background on the nature of expansive
and/ or reactive soils of the study area, by following suitable field procedures and sampling criteria
as well as laboratory chemical/ mineralogical and engineering soils testing methods. It also sought
to present results of soil classifications and correlations between soil index and/ or engineering
properties. Results of statistical analyses involving hypothesis testing seeking to determine the
degree of homogeneity or uniformity of the soils found, have also been presented. The undertaking
of this project by the author was also an initiative aimed at providing an approach of study that
would serve as a basis and/ or framework for comprehensive and detailed engineering geological
investigation and assessment of expansive and reactive soils in lithologically similar areas around
Nairobi, and Kenya at large.
1.5 Physiography
The geological history of Nairobi, including the current study area, has been dominated by volcanic
activity since Miocene times. These areas are currently underlain by a series of volcanic rocks as a
result of successive lava flows that originated from centres and fissures on the high eastern flank of
the Rift region to the west (Gregory, 1921; Saggerson, 1991; Fig. 3.1; Plates 1.7 (a) & (b)).
Plates 1.7 (a) & (b) View of the Great Rift Valley from its eastern flank, to the west of Nairobi area.
The flow of lava was generally eastwards, onto a warped and partly dissected pre-Miocene erosion
surface below which were older crystalline metamorphic rocks. A few isolated occurrences of rocks
of Precambrian and Quaternary age are also found.
The geomorphological evolution of Nairobi and the study area has therefore been mainly controlled
by the volcanic activity that accompanied the Rift Valley formation. The physiography of these
areas is consequent upon the volcanic rocks found, and the tectonic movements which have affected
them (Saggerson, 1991; Sikes, 1926).
The study area is a part of the lava plains (Plate 1.8 (a) & (b))that are bordered to the north-west and
west by the Kikuyu highlands, an extension of the high ground of the eastern flank of the Rift
Valley; and to the south-west by the Ngong hills. The Kikuyu highlands are characterised by a steep
downward slope in an easterly direction. The drainage is consequent, follows the slope and
comprises of a number of streams giving rise to parallel ridges, some of which get broader as the
streams converge. The streams are still down-cutting giving rise to ridges with convex to uniform

9
slopes. The northern boundary between the two physiographic units of the Kikuyu highlands and
the plains is roughly along the east - west line across the Nairobi city centre (Sikes, 1926). The
plains are made up of two parts, i.e. the Athi plains and the northern section of the Kapiti plains,
both of which extend further southwards and eastwards, respectively, beyond the study area. These
plains are underlain by a succession of lava flows which usually alternate and/ or intermingle with
lake beds, stream deposits, tuffs and volcanic ash (Saggerson, 1991).
Plate 1.8 (a) Flat plains around Embakasi station. Plate 1.8 (b) Flat plains around J.K.I.A. Airport.
The Athi plains rise from an altitude of about 1500 m at Athi River, located about 8 km south-east
of the present area; to about 1800 m above sea level farther west in the faulted region near Ngong.
The plains form gently rolling grasslands with a fairly even surface. They end in an abrupt
escarpment just west of the Athi river. The abrupt escarpment represents the end of a lava flow. In
addition, some smaller and low east-facing scarps also occur within, and represent the margins of
partly eroded lava flows (Gregory, 1921; Saggerson, 1991). The plains are being dissected by
down-cutting rivers (over 30 m below the general land surface in some places) currently flowing
into the main Athi river. Some steep-sided valleys are also found in the form of gullies and canyonlike
gorges cutting into lavas such as along the Nairobi, Mathare and Gitathuru river valleys in the
northern part, as well as the tributaries of the Athi river which include Mokoyeti, Sosian and
Mbagathi river valleys in the southern part of the study area, respectively. On the other hand, areas
with soft material are characterised by relatively wide valleys, and this is noticeable in lower
sections of the Nairobi river valley, as well as along other stream courses in the southern and southwestern
parts of the study area. The plains are mainly underlain by phonolitic volcanic rocks and
tuff (Saggerson, 1991).
The Kapiti plains occur to the east of the Athi river, where the topography is more variable, from
gently undulating to flat with a number of small hills protruding above the general land surface.
One large hill mass, the Lukenya, is found located in a metamorphic region of the plains to the east
of the present area. Underlying the plains are volcanic rocks, tuff and metamorphic rocks. The
plains are located in a drier region having no permanent rivers; and the seasonal stream-beds found
contain water for only short periods of time in a year. The dissection of these plains is therefore
relatively less severe. Gully erosion is, however, common and characteristic.
The Athi and Kapiti plains extend farther south-eastwards and eastwards beyond the present study
area, where they are bounded by the central hill masses of Machakos District (Machakos hills).
These consist of the Ithanga, Mua and Iveti hills as well as the Kanzalu range and Donyo Sabuk.
The hills rise abruptly above the surrounding plains. They are also steep-sided and being actively
dissected, partly as a result of existing poor land management.
There is also a general rise in relief from the south-east towards the north-west, across the study
area. Elevations of 1540 - 1600 m above sea level occur in the south-eastern parts; 1620 - 1680 m

10
above sea level from Jomo Kenyatta International Airport to Wilson Airport; and 1740 - 1800 m
above sea level in the north-western parts of the study area; respectively (Survey of Kenya, 1990).
More than 80% of the study area, including the southern half, has therefore an altitude of between
1540 m and 1740 m above sea level. An average elevation of about 1650 m above mean sea level is
recorded for the study area; with the lowest elevations (1540 - 1580 m above sea level) in the southeastern
(around Beacon Ranch Farm) and north –eastern parts of the study area. The highest
elevations (1740 - 1800 m above sea level) occur at the far western and north-western sections of
the study area.
The main drainage in Nairobi, including the study area, is consequent upon the regional topography
and prevailing slope of the volcanic rocks. The streams are generally easterly-flowing, but a few
cases of drainage and flows to the south-east and south also occur (Morgan, 1967; Survey of Kenya,
1990; Fig. 1.1). The north-western and western parts of the study area bordering the Kikuyu
highlands are characterised by streams that show features of having been deeply incised and to have
re-excavated their former courses. According to Saggerson (1991), this could be attributed to the
continued uplift of the Rift region during the late Tertiary, and probably early Pleistocene; and also
to the continual deposition of lavas and tuffs onto an easterly - inclined surface. This is especially
observable in the Nairobi river valley and its main tributaries of Kerichwa Kubwa and Kerichwa
Dogo, as well as in the Mathare, Gitathuru and Ngong river valleys. Lavas, welded tuffs and ash
flows therefore poured down the valleys causing their inundation and choking with volcanic rock.
Lateral erosion was limited, and the erosional effect of the tuff-flows was mainly concentrated in
the stream-beds where channelling and corasion were at maximum, causing re-excavation of the
stream valleys.
The uplifting and deposition of volcanic materials have therefore given rise to streams that are
characterised by young valleys with steep gradients and narrow and/ or sharp V-shapes in the northwestern
and western parts of the study area. In addition, the rapid down-cutting, together with the
relatively soft character of the younger volcanic rocks have resulted in the streams flowing in
generally parallel courses, with limited instances of river capture (Gregory, 1921; Morgan, 1967).
Many parts of the study area receive rainfall amounts of around 900 mm per year, with the rainy
seasons concentrated in the periods of mid-March to May; and mid-October to mid-December
(Kenya Meteorological Department, 2001). The streams are most active during these months of the
year when rainfall is heavy, and headward erosion of gullies and tributaries is common. The
transported load is mainly a result of erosion of the thick soil cover, and this frequently gives rise to
flowing streams of red mud (Survey of Kenya, 1990).
The study area is characterised by two main drainage basins. The Nairobi river and its tributary
valleys dissect and drain the northern and north-western parts; while the tributaries of the Athi river
drain the southern and south-western parts of the study area, respectively (Fig. 1.1). The main
tributaries of the Athi river include the Sosian, Mokoyeti, Donga and Mbagathi valleys; which are
generally medium – wide rivers, with gentle side-slopes and flat bottoms. Aerial photo -
interpretation and topographic surveys show a generally wide drainage pattern over the study area,
with distances of 1 to 6 km between adjacent channels. The drainage and flow direction are
generally consequent upon the local topography and slope direction, with the Nairobi river basin
draining generally eastwards, while the flow in the Athi river basin is mainly to the south and southeast.
A smaller drainage basin occurs mid-way across the study area, and consists of the Ngong
river and its tributary of Motoine valley, both of which flow generally eastwards. However, apart
from the Nairobi river and Athi river and their main tributaries, most of the other stream channels
are seasonal and dry during the hot months of the year (Saggerson, 1991; Survey of Kenya, 1990).

11
1.6 Climate
The Nairobi area and its immediate environments, including the current study area, experience a
type of climate characterised by temperatures and rainfall patterns that vary according to location,
altitude and season of the year (Survey of Kenya, 1990). The generally high altitude (about 1700 m
above sea level) results in a climate that is not quite typically tropical, despite the closeness of these
areas to the Equator. Generally, temperatures are neither uncomfortably high during the days nor
low at nights (Kenya Meteorological Department, 2001). However, exceptional weather conditions
are also experienced during the hot, dry period shortly before the start of rains in March, and is
characterised by relatively high temperatures (up to 28 °C at mid-day), low relative humidity (down
to 10%), and occasional dust-storms caused by the moderately strong easterly wind blowing across
the area at this time of the year. The weather is also affected by the occurrance of the south-east
winds in the coastal region of Kenya during the period of June to September. The latter winds
frequently result in the formation of a cap of cloud over the high ground immediately east of the
Great Rift Valley (including Nairobi and the study area). The persistance of the cloud cover causes
low temperatures during the day (up to a maximum of 18° C); and also at nights as well as during
early morning hours (as low as 6° to 8° C). Very cold nights may also occur in the months of
January and February when the sky is clear (Kenya Meteorological Department, 2001).
Table 1.1. Mean maximum and minimum temperatures for each month of the year over a period of
50 years, for Nairobi city and the present study area (Kenya Meteorological Department, 2001).
Mean
Mean
Months
maximum minimum Mean range
(°C)
(°C)
(°C)
January 26,8 13,1 13,7
February 28 13,4 14,6
March 27,4 14,4 13
April 24,6 14,3 10,3
May 24,1 14,2 9,9
June 23,1 12,6 10,5
July 22,3 11,5 10,8
August 22,7 11,8 10,9
September 25,3 12,2 13,1
October 26,2 13,7 12,5
November 23,6 14,4 9,2
December 25,1 13,8 11,6
Year 24,9 13,3 11,6
Nairobi and the current study area are characterised by mean minimum temperatures of 11,5° to
14,4 °C; and mean maximum temperatures ranging from 22,3° to 28 °C. Low temperatures are
registered during the cool, dry and rather cloudy months of June to mid - October; with mean
maximum temperatures of 22,3° to 25,3 °C. High temperatures are recorded in the months of mid -
December to mid - March, with mean maximum temperatures in the range of 26,8° to 28 °C. The
weather could be generally described as warm to hot, and dry with sunny days during this latter
period of the year. The coldest month is July with an average monthly temperature of 16,9 °C;
while the hottest month is March with an average monthly temperature of 20,9 °C. The mean
monthly range across the year is from 9,2 °C in November to 14,6 °C in February. A mean annual
temperature of 19,5 °C is recorded for the study area; with a mean annual range of 11,6 °C. No
large seasonal changes in temperature therefore occur. The highest and lowest temperatures

12
recorded for Nairobi and its adjacent areas over a period of 25 years are 32,8° C and 3,9° C,
respectively. A summary of temperature statistics over the last 50 years for Nairobi and adjacent
areas is presented in Table 1.1 (Kenya Meteorological Department, 2001).
Nairobi and the present study area are also characterised by wet and dry seasons which show
marked differences in the amounts of precipitation. The average annual rainfall is about 900 mm;
but the actual amount in any one year may vary from less than 500 mm to more than 1300 mm per
year (Kenya Meteorological Department, 2001). The distribution of rainfall is bimodal, with long
rains being registered in the months of mid - March to May ( main rainy season, with 137 to 195
mm per month); and short rains from mid - October to mid - December (secondary rainy season,
with 77 to 114 mm per month). The dates when the rainy seasons actually start and end are very
variable, i.e. the beginning and end of a wet season are therefore usually not well defined. The two
rainy seasons coincide approximately with the time of changeover of the south-west monsoon and
the north-east monsoon currents which affect eastern Africa (including Kenya) in April and
November, respectively. The dry season is experienced from mid - December to mid - March. The
amount of rainfall generally tends to increase with increasing altitude, from about 500 mm per year
around Kitengela (1520 m above sea level) located about 8 km to the south of the study area; to just
over 900 mm per year around the Nairobi city centre (1660 m above sea level) in the northern part
of the study area, respectively. The precipitation is mainly in the form of convectional rainfall,
commonly occurring as late afternoon showers (Survey of Kenya, 1990; Kenya Meteorological
Department, 2001). A summary of average rainfall (mm) for each month of the year, based on
records for 50 years is given in Table 1.2.
Average
Rainfall
(mm)
Mean relative humidity
values (%)
9:00 am 3:00 pm
Bright sunshine
(average hours
per day)
Months
January 48 79 45 8,9
February 48 74 37 9,5
March 115 82 43 8,7
April 195 86 53 7,0
May 137 85 55 5,7
June 42 85 59 5,8
July 15 83 53 4,4
August 21 85 53 4,2
September 24 82 50 5,9
October 52 80 47 7,0
November 114 81 57 7,0
December 77 83 54 7,9
Year 74 78 51 7
Table 1.2. The average rainfall (mm) based on records for 50 years, mean relative humidity values
(%) and amount of bright sunshine (average hours per day); for each month of the year, for Nairobi
city and the present study area (Kenya Meteorological Department, 2001).
Nairobi and its environs are also located within the path of predominantly easterly winds. The
winds are near the ground and blow across the area , generally between north-east and east from
October to April; and between east and south-east from May to September. Strong winds with
speeds of 30 to 40 km/h are common during the dry season, just before the start of the long rains in
mid - March; and especially from mid-morning to early afternoon. Wind speeds of 15 to 25 km/h
are also recorded during other times of the year; while nights are commonly characterised by light
winds.
Nairobi city and its adjacent areas are some 400 km away from the sea and do not therefore
experience the rather unpleasant humid heat that usually characterises tropical coastal towns. A very

13
marked daily range of relative humidity is not uncommon, with the air in the early mornings at/ or
very close to saturation. The afternoons are characterised by a relative humidity of about 50%,
although this may sometimes be as low as 10% on clear sunny days especially in February and
March (Kenya Meteorological Department, 2001). Table 1.2 shows a summary of mean relative
humidity values (%) for each month in a year, both in the mornings (9:00 am) and in the afternoons
(3:00 pm).
Nairobi and its surroundings receive a considerable amount of sunshine, averaged at about 7 hours
of bright sunshine per day, throughout the year. The quality of ultra-Violet radiation has also been
shown to be very high (Kenya Meteorological Department, 2001). The early mornings are often
cloudy. There is therefore 30% more sunshine in the afternoon than in the morning, with westerly
exposures receiving more insolation than those facing east. There is also considerably more
sunshine during the 6 months that the sun is in the southern hemisphere, than when it is in the north.
However, days with no sunshine at all occasionally occur; and this especially during the rainy
season (mid-March to May), and/ or in the months of June, July and August. Average number of
hours per day of bright sunshine for each month of the year are summarised in Table 1.2 (Kenya
Meteorological Department, 2001).
1.7 Method of survey
The criteria used in demarcating the current project area and selection of sampling sites were in
such a way as to accomplish the following:
(i)
(ii)
(iii)
(iv)
(v)
Include areas of future residential, industrial and civil engineering structural
developments
Include areas currently subjected to expansive ground movements, pronounced soil
reactivity (strong shrinkage cracking and potentially high expansive/ swelling character)
and with deteriorating and/ or damaged engineering structures
Include a range of terrain, vegetation and site development conditions
Cover fluvial and swampy soil deposits
Ensure site accessibility during the whole project period
In any case, the project area was selected after consulting with relevant government ministries
(Ministry of Environment and Natural Resources, Ministry of Public works), local councils and
authorities (Nairobi City Council, Kenya Airports Authority), as well as private companies and
landowners.
A desk study phase was undertaken before work in the field commenced. This phase involved the
study and interpretation of aerial photographs under a stereoscope (Allum, 1966; Colwell, 1983;
Verstappen, 1980), by drawing boundary lines round the various shade tones, vegetation and relief
patterns (steep slopes, valleys, plains, rocky outcrops). These features were later investigated during
the field study phase by following planned field profiles and road traverses. The patterns which
were found to be related to soil had their boundaries marked/ traced on the photos. The information
was then transferred from the photos to the 1:25 000/ 50,000 base maps on return from the field.
The aerial photographs covering the present study area and its environs were provided by the
Survey of Kenya, and were printed in 1990.
Observations made during the field survey showed that soil boundaries could easily be delineated
on the aerial photos in areas of natural vegetation, perennial crops and wetlands. Varying shade
tones on the photos were found to correspond to different vegetation types and patterns which in
turn served to reflect soil changes across the study area in terms of differing texture, depths as well

14
as degree of drainage and wetness. The black cotton soils could easily be distinguished and
separated from intercalated patches of the dark grey mottled clays through the presence of a
characteristic shrubby vegetation which appears as specks on the photos in areas of the former soil
type.
A field investigation plan and/ or map showing sampling sites is presented in a later chapter (Fig.
4.1). A total of 336 survey points and 36 sampling sites were covered along planned field profiles
within black clays in the study area. Geotechnical logging of soil profiles was effected at
representative soil sections obtained by hand-excavation as well as those exposed at cut-sections of
stream courses, roads and quarries. The soil sections facilitated correlation of soil profiles across the
study area. They also enabled determination of soil thickness as well as depth position and nature of
underlying bedrock. A standard description of the nature of underlying bedrock (in accordance with
ISRM, 1978a/ b) is included in a later chapter dealing with the geology of the study area.
Undisturbed U100 and disturbed soil sampling was done in the excavated pits. Besides excavated
and / or exposed soil sections, the nature and lithological variation of soils were also investigated by
employing soil augering techniques. Cone penetration sounding was also used to determine depth
variation of soils across the study area. In situ shear strength characteristics of soils were
investigated by means of vane shear testing.
The red soils were also investigated in this study (but to a lesser extent than the black clays). Their
inclusion in this study was mainly for comparison purposes. A total of 6 excavation, soil sampling
and/ or field investigation sites were covered for these soils.

15
Chapter 2
Previous works
2.1 Summary
A German professor, Suess (1892) summarised the geology and structure of the Eastern Rift
Valley, to the west of Nairobi city and the present study area. The Eastern Rift Valley runs
across Kenya from Malawi in the south to the Dead Sea in the north.
Gregory traversed the country of Kenya in the period of 1896 to 1919. He made important
early contributions by studying and establishing the geology of some selected areas. He also
studied and described the major volcanic series underlying Nairobi and the study area; and
which include the Kapiti phonolites, trachytes, agglomerates, the Nairobi building stone and
the basic lavas of the Ngong volcanic centre. He named and designated the Kapiti phonolite
as the oldest unit of the series. He published his works and findings two years later (Gregory,
1921) as well as a number of papers dealing with various aspects of the Rift Valley and its
environs.
Prior, of the Mineral Department of the British Museum, later examined a large number of the
rock specimens collected by Gregory from around the country and which included Kapiti
phonolite and phonolitic quartz trachytes from Nairobi and the current study area. In his
published works (Prior, 1903), he recognised various subdivisions of the phonolitic series.
Kunzli (1901) gave an account of the volcanic rocks of the Eastern Rift Valley that included a
description of the riebeckite trachytes and phonolitic trachytes of the Kikuyu highlands and
north – western parts of the present study area.
In 1905, Maufe, then of the Geological Survey of Great Britain, conducted a geological
survey along the railway from Mombasa (in the south) to Kisumu (in the west), through
Nairobi. He later published his works and findings (Maufe, 1908) in which he gave a brief
summary of the geology between Nairobi and Kijabe (to the north of the study area) by
describing the principal rock types found. He also commented on the nature of faulting on the
east flank of the Rift Valley.
Earlier works and brief comments on the geology of Nairobi and its surroundings were also
done by Walker (1903), Collie, (1912) and Krenkel (1925). Krenkel described the volcanic
succession found by making reference to specific rock types, based on the earlier works of
Maufe (1908) and Gregory (1921).
Sikes, then Director of Public Works Department, Kenya, conducted underground water
surveys in the Nairobi area in the period of 1920 to 1925. He also described the grid – fault
structures caused by late faulting in the areas to the west of Nairobi city and in the Rift region.
The results of his observations and findings were published in subsequent reports (Sikes 1926,
1934 and 1939). Additional hydrological investigations were undertaken in the years that
followed by staff of the Ministry of Public Works, Nairobi, who examined in detail boreholes
drilled at the time in the rapidly – growing Nairobi metropolitan area. The results of this work
were incorporated in a technical report by Gevaerts (1964).
Wayland, formerly Director of Geological Survey Uganda, made a survey of materials in
Kenya, including Nairobi and the present study area. He investigated the materials for their

16
possible use as building stone and in the manufacture of bricks and tiles, limes and cements.
His findings were published in a report issued by the Imperial Institute (1926).
Bailey Willis (1936) described the Kikuyu scarp to the west of Nairobi, the warped peneplain
around Nairobi and the grid – faulting that has affected the volcanic rocks of the Rift valley
region and western parts of Nairobi area. Saggerson and Baker (1965) also investigated and
described, among others, the Kikuyu scarp and the warped peneplain in the neighbourhood of
Nairobi area.
Campbell Smith (1931) described and classified the trachytes and phonolites of Nairobi and
its surroundings after examining and analysing a number of rock specimens collected from the
areas. His findings were published and incorporated in a report.
Bowen visited Kenya in the 1930´s and made an excursion into the Rift Valley. He also
collected specimens of volcanic rocks from other parts of the country, including Nairobi and
the present study area. In 1937, he published the results of his studies in a paper in which he
illustrated his experimental work with personal observations on the lavas.
The nature of alkali – feldspars in certain Kenyan rocks, including the Kapiti phonolite was
investigated by Häkli (1960). The specimens of Kapiti phonolite had been previously
collected by a Finnish Expedition in 1952 from the Stony Athi River and Nairobi, including
some parts of the study area.
Williams (1967) outlined the geology of Nairobi and its surroundings, in its regional setting.
Various lavas have also been collected from the Nairobi succession and dated (Baker et al,
1988). Curtiss, Evernden and Miller of the University of California, USA, carried out K/Ar –
age dating on rock specimens collected from the Nairobi region. The volcanic rocks were
found to be Upper Miocene to Pleistocene in age (Evernden et al, 1964).
Other studies involving Nairobi and covering the present study area include palaeomagnetic
studies by staff of the Department of Physics, University of Nairobi (Mussett et al, 1964-
1965); and the monitoring and detection of seismic activities and possible associated earth
movements, by the Seismology Section, Department of Geology, University of Nairobi
(1990-2002).
A number of other geological investigations have been carried out in Nairobi and its environs,
including the study area; but they are recorded as unpublished works by the Mines and
Geological Department, Nairobi. Some of the unpublished works include those of Binge,
Joubert, Pulfrey and Saggerson (Mines and Geological Department, Nairobi, 2001).
Early works regarding detailed survey and study of soils of the Nairobi area and its environs
were undertaken by Jones in 1952 and published in 1953. His works were later carried
forward to completion by Scott, of the then East African Agriculture and Forestry Research
Organisation. In his works, he described a variety of soils found as having developed under a
wide range of climatic conditions (Scott, 1963).
Other works in the study of soils were undertaken by Stephen, Bellis and Muir (1956), who
investigated the nature of the black cotton soils overlying Nairobi phonolite in the Athi
plains, with the purpose of establishing the cause for uneven growth of crops. They also noted
that soils from Embakasi area, Nairobi, contained some minerals typical of metamorphic
rocks.

17
Table 2.1 Stratigraphic correlation of rocks in the Nairobi region (Saggerson, 1991).
SOUTH CENTRAL NORTH
Kajiado Nairobi Kijabe
(Matheson, 1966) (Saggerson, 1968) (Thompson, 1964)
Soils, ashes, alluvium,
Soils and ashes
RECENT Soils loess, gypsiferous beds
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Longonot trachytes
Trachytes
U
Lacustrine deposists
xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxx
Sediments (Kandizi Valley)
Pyroclastics
PLEIST- Olorgesailie Upper Trachyte Division of xxxxxxxxxxxxxxxxx
OCENE M Lake Beds rhyolites, trachyrhyolites Trachytes, Kijabe
and obsidians ( with basalts, welded tuffs
interbedded diatomites)
_ _ _ _ _ _ _ _ _ _ __ _ _
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Upper Trachyte Division of:
Pyroclastics and
Welded tuffs & pyroclastics, Laikipian basalts
(Sattima Series/
L
Laikipian lavas)
Orthophrye trachyte
Orthophrye trachyte, and
Limuru trachytes
Alkali trachyte
xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Middle Trachyte Division:
Ol Keju Nero basalts
Tigoni & Karura trachyte
xxxxxxxxxxxxxxxxxxxxxxxxx Ruiru Dam trachyte
Ol Esayeti volcanics Kabete trachyte Sattima Series
Kerichwa valley tuffs
Kerichwa Valley tuffs
PLIOCENE and Ol Doinyo Narok Ol Esayeti volcanics and
agglomerate
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Lower Trachyte Division:
Olorgesailie
Ngong volcanics
Volcanic Series Ol Doinyo Narok agglomerate Laikipian lavas
Ololua trachyte
Nairobi / Kiambu trachytes
Mbagathi trachyte
Sediments & tuffs
Nairobi & Kandizi phonolites
Mbagathi phonolitic trachytes
x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-
Upper Athi tuffs
Athi tuffs and Lake Beds
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
Lower Trachyte Division:
Simbara basalt
MIOCENE Kapiti phonolite Kapiti phonolite Simbara Series
_________________________ _____________________________ _____________________
PRECAMB- Basement System
Metamorphic rocks of the
RIAN Mozambique Belt not exposed
xxx Faulting _ _ _ Erosion surface ___ Major unconformity

18
Nevill (1961), carried out investigations into the suitability of soils from Kabete and Sasumua
areas (located to the north and north-east of the present area, respectively) for stabilisation
with Portland cement and hydrated lime.
In 1968, Saggerson reconstructed the stratigraphical sequence of formations of Nairobi,
including the present study area. In the sequence, loose surface deposists (soils, ashes,
alluvium, loess, gypsiferous beds) are the youngest, having been formed in recent times;
while the Kapiti phonolite, which extruded in the area in Miocene, is the oldest unit of the
volcanic succession. These phonolites overly older Precambrian metamorphic rocks of the
Mozambique Belt. He also correlated the strata sequence found at Nairobi with those of
neighbouring areas of Magadi, Kajiado and Kijabe (Table 2.1). The results of his works were
published later by the Mines and Geological Department, Nairobi (Saggerson, 1991).

19
Chapter 3
Geology
3.1 Introduction
Geological records show that much of central Kenya, of which Nairobi and the present study
area are part, underwent peneplanation on more than one occasion in the period of late
Precambrian to Tertiary (Saggerson, 1991). Doming, linear warping and erosion of the then
sub-Miocene erosion surface was succeeded by the extrusion of the first lava flows which
flooded the eastern plains, including Nairobi and the present area, during mid-Miocene time.
This was followed by intermittent volcanicity which continued until Recent time. The
volcanicity was genetically associated with tectonic movements and the development of the
Rift Valley to the west of these areas. The western parts of Nairobi and sections of the present
study area have also been affected by faulting on at least three occasions, resulting in
spectacular topography. The lavas and pyroclastics originated mainly from fissure eruptions
of the Rift region. Two central volcanoes, Ngong and Ol Esayeiti, were also active during this
period and contributed their share of lavas and pyroclastic material to the eastern plains. The
volcanoes are located to the south-west of Nairobi city.
The geology of Nairobi and the present study area is, therefore, characterised mainly by a
succession of volcanic rocks and pyroclastics of Cainozoic age (Saggerson, 1991). Underlying
the volcanic rocks is a foundation of folded crystalline metamorphic rocks (gneisses and
schists) of Precambrian age which belongs to the Mozambique Belt. The metamorphic rocks
are only occasionally exposed and some of their fragments have been noted in agglomerates
previously derived from the former Ngong volcano, to the west of the area.
The volcanic rocks of Nairobi and its environs, including the study area, are a part of a wider
East African alkaline suite characterised by a dominance of soda over potash. The rocks are
distinguished into two groups; the first of which is a strongly alkaline series represented by
feldspathoid-bearing phonolites, basanites, tephrites and more basic varieties. The second
group is a mildly alkaline series and includes feldspathoid-free rocks containing soda-rich
amphiboles and pyroxenes. Differentiation of members of the two series is accompanied by
an increase in silica content, giving rise to trachytes, rhyolites and obsidians. On the contrary,
however, volcanic rocks associated with the Western Rift Valley (to the west, outside Kenya)
belong to the potash-rich alkaline series (Saggerson, 1991).
The rock formations are covered and overlain by thick deposits of soils and gravels of
Quaternary age. However, areas within the Rift are characterised by loess and lacustrine
deposits, some containing diatomaceous beds; and all of which serve to reflect major changes
in climatic conditions (Saggerson, 1991).
A number of authors have assigned various ages to the volcanic rocks of Nairobi region. A
majority of the formations are suggested to be Tertiary to Recent. Gregory, on the other hand,
was of the view that the earliest lavas of the Kapitian series were Cretaceous (Gregory, 1921).
Potassium-argon age-dating of the rocks was done by Curtiss, Evernden and Miller, of the
University of California. Their findings indicate volcanicity as having began in Upper
Miocene, and had almost completely ceased by the end of Pleistocene (Evernden et al, 1964).
The dating was performed on whole rock specimens as well as specific minerals
(anorthoclase, sanidine, feldspar, nepheline), and shows the ages of the rocks as varying from
0,8 Ma (Plateau trachyte series, Magadi road) to 13,40 Ma (Kapiti phonolite, Stony Athi

20
River). The dating also involved rock specimens from Nairobi metropolitan and present study
areas, and included Nairobi trachyte (3,17-3,45 Ma), Nairobi stone (4,84-5,67 Ma), Nairobi
phonolite (5,2-10,22 Ma) and Kapiti phonolite (12,90-13,40 Ma). There is also the likelihood
that volcanic activity continued into Recent times in areas to the west of Nairobi. This is
evidenced by the occurrence of weathered tuffs and ash layers within soil profiles,
representing the eruptive periods of the later times (Saggerson, 1991).
A summarised discussion of the major rock types underlying the present study area is given in
the following sections.
3.2 Precambrian rocks of the Mozambique Belt
The crystalline metamorphic rocks of Precambrian age outcrop in areas where the volcanic
cover has been removed by erosion. They are especially noticeable in the southern and southwestern
parts of the study area where they occur as isolated exposures in shallow stream
courses, or as float on the stream edges. They are generally small highly weathered outcrops
consisting mainly of layered fine-grained schists and coarse gneisses that have been invaded
in some places by pink quartzo-feldspathic pegmatites. Also found but to a lesser extent are
biotite-garnet-epidote gneisses, hornblende gneisses and quartz-feldspar gneisses.
The structures revealed in the metamorphic rocks generally indicate north-south trending
folds. However, the rocks also show some variations in strike and dip due to generally smallscale
folds. They also exhibit lineations which plunge at approximately 14° - 21° to the southwest;
and which are most probably related to the north-east to south-west fold trends that have
been recognised in neighbouring areas (Saggerson et al, 1960). The north-south and north-east
to south-west fold trends are typical of much of southern Kenya, including the Nairobi area,
and serve to point to the fact that the underlying Precambrian rocks have been subjected to a
minimum of two phases of deformation (Saggerson, 1991).
In other areas, the metamorphic rocks are unexposed and separated from the overlying
volcanic rocks by a plane of unconformity which represents an erosional surface formed in
early Miocene. Evidences of the erosional surface occur in the form of thin layers of
weathered metamorphic materials separating the Precambrian and volcanic rocks, and
exposed in some of the streams in the southern part of the study area. However, cases of soil
materials (clays, sands, detritus) directly overlying the Precambrian rocks also occur, and is
evidenced by a number of borehole records in the present area. According to Sikes (1939), the
presence of a former topography across the study area is also revealed by borehole records,
which show the crystalline rocks as occurring at different elevations above sea-level.
Borehole records of the Nairobi Station (BH 1937c; 37° 00`E, 1° 24`S), for instance, show the
erosional surface to have been penetrated at an elevation of about 838 m above sea-level. In
other places, the crystalline rocks were intercepted at elevations of 1519 m above sea-level
(BH 2655) in the south-eastern part of the area; 91 m depth below ground surface at Wattle
Blossom Farm, southern part of the area; and 1509 m above sea-level in the area around
Nairobi National Park. Records also reveal the crystalline metamorphic rocks to have
undergone cyclic peneplanations in early Tertiary. Evidence of earlier sedimentation is,
however, absent. The latest metamorphic events to affect the rocks are dated at 480-530 Ma,
based on the age of Phlogopite in crystalline limestone from Turoka, in southern Nairobi area
(Saggerson, 1991).
Early studies on soil samples collected from around Embakasi in the south-eastern part of the
present area (Saggerson et al, 1968; Stephen et al, 1956) showed them to contain typical

21
metamorphic minerals sillimanite, kyanite, staurolite and garnet. This serves to suggest that
some of the volcanic material of the Nairobi region, which on weathering gave rise to Recent
soils, had during their eruption incorporated pieces of the crystalline metamorphic rocks.
3.3 Volcanic rocks
The Nairobi region, including the present study area, are covered by a thick succession of
alkaline volcanic rocks and associated tuffs, derived from the Rift Valley region (Saggerson,
1991).The rocks accumulated in the period of mid-Miocene to atleast Upper Pleistocene,
during which time volcanic activity dominated the geological history of the Nairobi region.
According to Saggerson (1991), the volcanic rocks of Nairobi region are estimated to amount
in volume to more than 1042 km³, and cover an area of about 3000 km². The rocks are a part
of a more extensive late Cainozoic (Pliocene) to Recent petrographic province that extends
outside Kenya, from Ethiopia in the north to Tanzania in the south.
The earliest volcanic activity began with the extrusion of the Kapiti phonolite (dated at 13
Ma) onto a dissected erosion surface, which had developed across nearly the entire East
African region, in pre-Miocene times (Saggerson and Baker, 1965).Weathered pyroclastic
materials are also found associated in the Nairobi area, occurring as interstratifications within
the lava succession and in the deep red soils. They were originally fragmentary deposits, a
result of violent volcanic activity from a number of centres within the Rift region, during
Pleistocene (Saggerson, 1991). The only identified vents include those of the Ngong volcano
and its satellites, Ol Esayeiti and Ngoroi, all of which occur to the south – west of Nairobi
city. Much of the volcanic material must have, therefore, derived from fissure eruptions from
dispersing centres located in the Kikuyu-Limuru area, to the west of Nairobi city and the
present study area. Figure 3.1(a) shows the extent of early lava flows in the Nairobi region,
including the study area; while Table 2.1 (in previous chapter) gives a corresponding
stratigraphic classification. The approximate ages of rock formations immediately underlying
the soils of the area are given in Fig. 3.1(b).
There is a general thinning of the lava flows in an easterly direction across the areas covered.
The eastern limits of the flows are characterised by erosional scarps, which form terminal
walls of the more resistant lava sheets. Studies have also shown the volcanic material to be
mainly phonolitic and trachytic in character; while the pyroclastics are dominantly trachytic
(Saggerson and Baker,1965).

24
The following section includes a summary of the major volcanic rocks underlying the present
study area.
3.3.1 Kapiti phonolite
Borehole records from Nairobi city and the current study area show the Kapiti phonolites as
underlying associated volcanic rocks. They are also observed to overly a sub-volcanic floor of
erosion surface previously cut into Precambriam metamorphic rocks. These rocks therefore
represent the oldest unit of a succession of lavas present in the study area (Gregory, 1921).
Exposures of Kapiti phonolite are observable in the south-eastern part of the study area,
generally in the form of small outcrops confined along some of the stream courses found; and
in the southern part of the area in the tributary valleys of the Athi river. Farther south, outside
the present area, small outcrops also occur in the valley of the Athi river and in the tributary
valleys of Kitengela river; while extensive outcrops are in the valley of the Stony Athi River,
which forms the north-western border of the Kapiti plains. The rock unit descends with a
slope nearly identical to that of the sub-volcanic floor, from 1585 m above sea-level in the
south-eastern part, to1280 m above sea-level in the northern part of the study area and the
Nairobi City centre (Saggerson, 1991).
The lava originated from an area to the south of the present study area and was most probably
derived from vents located on the flanks of the Rift Valley and/ or from the site of the former
Olorgasailie central volcano (Baker, 1958). It was a part of the first Miocene flood eruptions
and flowed generally northwards onto the eroded surface, which was covered in places by
Tertiary conglomerates and grits (Fairburn, 1963). The Kapiti lava also extends westwards up
to Lion Drift in the Nairobi National Park by thinning against the higher ground. The Mua
hills and the Kanzalu range located to the east of the study area acted as a barrier to the
northerly lava flow, and instead, encouraged a north-westerly flow across the area. A further
contribution originated from areas to the north of the study area, where the flows had also
given rise to the formation of the Yatta plateau phonolite.
Borehole records from Nairobi metropolitan area, the study area and around Athi River
township (south of the study area) show the presence of sedimentary strata in the form of
clays and other sediments occurring intercalated between and separating several lava flows
(Gevaerts, 1964).
Exposures show parts of the phonolite to be vesicular in nature, commonly giving rise to
formation of small patches of amygdales filled with calcite and zeolite (mainly natrolite). The
elongated feldspar phenocrysts ( up to 76 mm long) and vesicles frequently display a
preferred orientation (Gregory, 1921; Saggerson, 1991).
Hand specimens of Kapiti phonolite are distinct, and show large crystals of feldspar and
waxy-looking nephelines occurring as phenocrysts set in a fine-grained dark green to black or
dark bluish-grey groundmass. The presence of phenocrysts of these two minerals is
diagnostic, and serves to identify and distinguish the Kapiti phonolite from other lavas across
Kenya (Gregory, 1921).
Thin sections of phonolite show phenocrysts of anorthoclase and nepheline set in an
interstitial groundmass (Häkli, 1960). Serpentinised pseudomorphs and patches of chloritic
material found in the groundmass are probably secondary after olivine, while opaque iron ore

25
and prismatic apatite occur as accessories (Saggerson, 1991). Composition of the groundmass
is also summarised in Table (3.4).
Table 3.1 Chemical analyses of alkali-feldspars from the Kapiti phonolite and similar rocks.
1 2 3 4
SiO2 65,61 62,79 64,33 64,80
Al2O3 20,48 22,12 20,94 21,14
Fe2O3 0,20 0,36 0,2 0,68
FeO 0,20 0,41 0,58 -
MgO 0,00 - - 0,10
CaO 7,03 3,76 2,01 1,18
Na2O 6,26 7,35 7,22 6,94
K2O 0,32 2,98 4,71 5,26
H2O+ 0,03 0,19 0,27 0,20
H2O- 0,00 0,07 0,10 0,20
Totals 99,93 100,03 100,36 100,00
1. Alkali-feldspar phenocryst (anorthoclase). From Kapiti phonolite, Stony Athi River (Häkli, 1960).
2. Loose crystals of anorthoclase (potash-oligoclase). Crater of Mount Erebus, Antarctica (Mountain, 1925).
3. Anorthoclase. Crater slopes, Mount Kenya (Mountain, 1925).
4. Anorthoclase phenocrysts. From Kenyte, Mount Kenya (Spencer, 1937).
The thin section descriptions and results of chemical analysis of Kapiti phonolite from the
study area and its environs, has been shown by various authors to be similar (Baker, 1954;
Bowen, 1937; Smith Campbell,1950; Saggerson, 1991; Prior, 1903). The results of chemical
analyses are summarised in Table 3.1, which also shows a comparison of the composition of
alkali-feldspar phenocrysts from the Kapiti phonolite; with those of anorthoclase crystals from
Mount Kenya and Antarctica. From the table, specimens from the Kapiti phonolite are found
to show a marked rise in the ratio of alkalis, Na2O/K2O, with increase in lime (CaO) content.
Specimens from Mount Kenya and Antarctica, however, exhibit a steady increase in CaO with
increasing Na2O/K2O ratio.
3.3.2 Nairobi phonolite
The Nairobi phonolite covers a large part of the Athi plains. It extends from Nairobi National
Park located in the western and south-western parts of the study area, and northwards across
the Nairobi city centre to Kiambu township, located north of /and outside the present study
area. It overlies the Athi Series with a probable disconformity. Outcrops of the phonolite are
observable in the southern and south-western parts of the study area in a prominent scarp,
which also shows the contact between the rock and the underlying Athi Series (Saggerson,
1991).

26
The Nairobi phonolite consists of a number of flows. This is evidenced by age determinations
made on feldspars from the rock, and which range from 5,2 to 10,2 Ma, suggesting the
possibility of more than one age for the lava. Observations in quarries have shown the various
flows to be generally between 30-46 m thick. Further evidence is provided by borehole
records, which show the thickness of the rock unit to be 79 m (BH 2023) at Embakasi in the
south –eastern part of the area; and 65 m (BH C201) at Wilson Airport in the western part of
the area. The base of the flow commonly consists of pumiceous and dribbly lava overlying a
slightly uneven surface of agglomeratic tuff (Saggerson, 1991).
Borehole evidence and flow directions recorded in the field suggest the lavas of Nairobi
phonolite having derived from the Muguga area, to the west of Nairobi city and present area.
The lavas flowed in a south-easterly stream across Nairobi and the present area, before
consolidation. Maximum thicknesses of the lavas therefore occur at/ and around the Jomo
Kenyatta International Airport located in the eastern part of the study area. The lavas
generally thin out north-westwards, as well as farther south-eastwards and southwards up to
Athi River township, located beyond and outside the present area (Saggerson, 1991).
Aerial photographs of the plain at Jomo Kenyatta International Airport reveal sub-circular
patches with crudely linear arrangements beneath the black cotton soil cover. These patches
probably represent the areas of maximum weathering at intersections of master joint-planes
(Scott, 1963).
Parts of the Nairobi National Park between Bushy Vale and Rocky Valley show the Nairobi
phonolite separated from the underlying Mbagathi phonolitic trachytes by some thickness of a
few feet of dark grey tuff, which belongs to the Athi Tuffs and Lake Beds Series. Outcrops
generally reveal minor differences in the Nairobi phonolites. They are characterised by a
marked fissility and platy texture developed locally at and/ or near the surface, and which is
attributed to flow phenomena. They also reveal the vesicular character of the upper portion of
a number of flows. However, the rocks are seldom amygdaloidal (Saggerson, 1991).
Fresh hand specimens of the rock are black or blue-grey to blue in colour. However, deep
weathering has altered much of the upper portions of the rock located near the ground surface.
The rock is also lacking in large phenocrysts, the phonolite having undoubtedly resulted from
the rapid cooling of an already viscous lava.
Weathering usually converts the Nairobi phonolites to pale brown and reddish ferricrete
deposits (murram), which are observed to overly the rocks in much of the study area. In other
parts, however, especially in the eastern and south-eastern sections of the present area, the
lava surface has been autobrecciated to form a boulder-bed consisting of rounded and angular
fragments of phonolite cemented in a light brown, weathered base of the same material
(Saggerson, 1991).
Thin sections of Nairobi phonolite exhibit long tabular phenocrysts of sanidine in a finegrained
groundmass. The groundmass consists of patches of soda-amphibole which include
cossyrite and kataphorite, while bright green aegirine and/ or aegirine-augite are found
associated. Nepheline is found occurring as small micro-phenocrysts, or as crystals in the
groundmass; larger crystals of nepheline commonly alter into yellowish zeolite, sodalite,
natrolite, analcime and calcite. Strongly pleochroic biotite with magnetite inclusions, orangebrown
mica flakes as well as occasional zeolite-filled vesicles may also be found (Fairburn,
1963). The matrix composition is also summarised in Table (3.4). Table 3.2 gives the results
of chemical analyses of Nairobi phonolite, as well as other rocks for comparison purposes.

27
Table 3.2 Chemical analyses of strongly alkaline lavas of Nairobi phonolite
and other rocks.
186 157 135 132 118 W64 112 99 108
SiO2 38,80 45,65 49,62 51,59 54,81 56,56 57,05 57,94 59,14
Al2O3 14,22 16,12 21,23 14,48 19,10 16,77 19,73 17,00 15,75
Fe2O3 7,28 3,07 3,77 7,77 1,57 3,17 1,85 4,03 6,65
FeO 6,34 9,08 2,02 - 3,52 2,74 4,31 1,94 1,35
MgO 5,65 7,38 0,65 2,90 0,96 1,36 0,34 1,22 0,56
CaO 12,35 9,70 3,40 4,53 1,81 1,18 1,56 0,99 1,42
Na2O 5,00 2,74 9,00 - 6,88 7,53 7,54 7,63 7,23
K2O 2,19 1,42 5,45 - 5,41 5,69 4,85 5,13 4,66
H2O+ 1,80 0,33 1,81 3,75 3,63 1,70 0,54 1,15 1,78
H2O- 1,40 0,68 1,75 3,75 0,57 1,55 0,68 0,94 0,25
TiO2 4,30 3,30 0,83 0,72 0,82 1,07 0,82 1,08 0,50
P2O5 0,72 0,43 0,11 - 0,22 0,13 0,23 0,18 0,09
MnO 0,14 0,13 0,18 - 0,27 0,33 0,35 0,30 0,35
CO2 - - - - 0,14 0,05 - 0,09 0,11
SO3 - - - - - - Tr - -
Cl - - 0,21 - 0,01 - - 0,13 0,02
F - - - - 0,09 - - 0,11 -
S - - Nil - 0,02 - - 0,02 -
Loss on - - - 1,23 - - - - -
ignition
-O eqv - - 0,05 - 0,04 - - 0,08 0,00
Total 100,10 100,03 99,88 88,97 99,79 99,83 99,85 99,80 99,86
186. Nephelinite (augitite). West of Ngong Bazaar (W. C. Smith, 1931).
157. Alkali basalt (basanite). Nyeri road, Aberdare range (W. C. Smith, 1931).
135. Phonolite. Ol Esakut (Tilley, 1956).
132. Nairobi phonolite. Nairobi (Analysed by E.A.I.R.B.).
118. Phonolite (Kapiti-type). Nairobi-Nyeri road, Nairobi (Bowen, 1937).
W64. Nairobi phonolite. Nairobi (Analysed by H. LIoyd).
112. Fayalite-bearing phonolite. Nyeri hill (W. C. Smith, 1931).
99. Phonolitic trachyte. Athi plains (Bowen, 1937).
108. Phonolite (Kenya-type). North slope, Suswa (McCall, 1964).
3.3.3 Nairobi trachytes
The Nairobi trachytes exhibit a wide distribution, extending from the Dagoretti – Karen area
located to the west of the study area, and eastwards to underly Nairobi city and the present
area. They also extend farther northwards, beyond the present area, to underly areas of
Kiambu and south Githunguri ( Figure 1.1).
A stratigraphic relationship is revealed by a number of borehole records and the many
outcrops in stream courses found in the immediate vicinity of Nairobi city.The Nairobi
trachytes are shown to underly tuffs and a number of other trachytes, while they overly the
Kiambu trachyte, Mbagathi phonolitic trachyte and Nairobi phonolite (Saggerson, 1991).
Outcrops of the rock are particularly observable in the south-western part of the present area,
where the trachytes form a flat-topped plateau terminating in a low but prominent escarpment

28
overlooking the Nairobi National Park. Outcrops of individual flows are also represented by
small steps in the escarpment immediately to the west of/ and overlooking Nairobi city and
the present study area (Saggerson, 1991).
Borehole records show penetrated rocks as generally occurring in a continous sequence of
lava flows, varying in thickness from 91 m around Nairobi city centre to 61 m at Ruaraka (to
the north of present study area). Records also show the flows as having vesicular upper
contact surfaces and/ or to be separated from one another by sediments or tuffs (BH 3001,
Mbagathi). In addition, stratigraphic information obtained from boreholes and exposures in
quarries in the Nairobi municipality and the present study area, show the Nairobi trachyte
separated from the underlying Nairobi phonolite by a few feet thickness of agglomeratic tuff
(Saggerson, 1991).
Table 3.3 Chemical analyses of mildly alkaline lavas of Nairobi trachyte & other volcanic rocks.
170 151 203 95 W15 87 202
SiO2 43,50 46,74 57,42 59,38 62,56 62,54 59,70
Al2O3 18,65 18,88 17,92 10,84 14,96 15,85 19,20
Fe2O3 1,70 3,11 7,50 3,92 3,25 3,66 8,00
FeO 10,48 7,15 7,50 4,28 3,36 2,62 8,00
MgO 7,42 3,16 1,17 0,89 0,36 0,36 0,30
CaO 10,26 9,30 Tr 3,31 1,21 1,46 2,40
Na2O 2,63 4,17 - 5,60 6,42 6,12 6,40
K2O 0,94 1,67 - 5,01 5,64 5,07 2,80
H2O+ 0,54 0,63 - 3,14 0,54 0,59 -
H2O- 0,80 1,07 - 3,14 0,73 0,45 -
TiO2 2,40 2,44 0,60 0,81 0,66 1,07 1,2
P2O5 0,62 1,29 Nd 1,00 0,10 0,13 -
MnO 0,12 0,18 - 0,35 0,19 - -
CO2 - 0,16 3,42 1,30 Nil - -
SO3 - - 0,15 0,04 0,00 0,08 -
Cl - - - 0,01 0,04 0,04 -
F - 0,11 - - - - -
S - - - - - - -
-O eqv - 0,05 - - 0,02 0,01 -
Totals 100,06 100,01 - 99,88 100,00 100,03 100,00
170. Alkali basalt, Rogati river, Kikuyu ( W. C. Smith, 1931).
151. Olivine basalt, Kijabe Hill (Shand, 1937).
203. Tuff, junction Ngong-Dagoretti roads, Impala Institute, Nairobi (W. C. Smith, 1931).
95. Alkali trachyte (Plateau Trachyte Series), Magadi Hospital (Baker, 1958).
202. Welded tuff/ Nairobi building stone (E.A.I.R.B).
87. Trachyte phonolitique (soda-trachyte), Kikuyu escarpment (Lacroix, 1923).
W15. Nairobi trachyte, Kenyatta Avenue, Nairobi (E.A.I.R.B).
Hand specimens of Nairobi trachyte are greenish-grey in colour, and occasionally porphyritic
exhibiting tabular phenocrysts of feldspar in a predominantly fine-grained groundmass. Dark
streaks, patches and bands may also be found and serve to indicate segregation of the mafic
constituents. The more homogeneous varieties of the rock also show a trachytic texture
characterised by a typical silvery lustre. Lamination and banding are also characteristic and
are a result of flow and flattening within the lava body. The flow is further emphasised by
fluxionally arranged feldspar laths, as well as variations in rock composition and colour

29
caused by partial segregation of the dark and light-coloured constituents present, during
crystallisation (Saggerson, 1991).
Thin sections show small tabular sanidine phenocrysts (exhibiting Carlsbad twinning) set in a
fine-grained groundmass. The groundmass shows a marked trachytic texture and consists of
orthoclase associated with soda-amphiboles and pyroxenes, as well as rare iron ore. The sodarich
minerals consist of deep brown cossyrite and green aegirine-augite, and occur as
prominent moss-like aggregates of prismatic crystals. The alternating brown and green
patches serve to impart a mottled appearance to the rock in hand specimens.
Table 3.3 gives results of chemical analyses of Nairobi trachyte, as well as other rocks for
comparison purposes. Results of mineralogical analyses of Nairobi trachyte, and a number of
other rocks underlying Nairobi and the present study area, are summarised in Table 3.4
(Saggerson, 1991).
Table 3.4 Mineral compositions of trachytes and phonolites from Nairobi and the study area
(after Saggerson, 1991).
Rock type:
Nairobi Kiambu Nairobi Kandizi Mbagathi Kapiti
trachyte trachyte phonolite phonolite phonolitic phonolite
trachyte
Sanidine X X R Ac
Anorthoclase R X X
Nepheline
X
Titanaugite R X
Olivine
R
Phenocrysts
Matrix
Biotite
X
Alkali feldspar X X X X X X
Nepheline X X X X
Quartz
Aegirine
Aegirine-augite
Ok X X On X X
Cossyrite X X X X X
α-kataphorite X X I X X
β-kataphorite
X
Riebeckite
X
Arfvedsonite
Biotite
X
Barkevikite
R
α-kataphorite – colourless to pale brownish yellow, smoky-brown to rose-red and/ or
reddish yellow
- cleavage 124°
β-kataphorite – pale smoky-brown to greenish yellow, deep purplish brown to opaque
X - present; R - rare; A - altered; Ac – accessory
Ok – overgrowths on kataphorite
On – overgrowths on nepheline
I – including large crystals

30
Chapter 4
Field methods
4.1 Introduction
The principal objectives of the field study were to:
(i)
(ii)
(iii)
(iv)
Provide information on the nature and distribution of black cotton soils (expansive
and reactive soils) and red soils of the study area.
Acquire site specific information on environmental characteristics which impact
on in situ soil behaviour (groundwater conditions, climate, vegetation).
Investigate and model depth variation of soils across the study area.
Compare in situ soil properties, behaviour and derived parameters with those
predicted and/ or derived from laboratory studies.
The cone penetration sounding, soil augering, in-situ description of soils and underlying rock,
tests for possible carbonate content, as well as undisturbed and disturbed soil sampling were
carried out, among others. Documentation of results was done according to known standards
(BGR, 1996; DIN 4021/ 4096, 1980; IAEG, 1981; ISRM, 1978; NLfB & BGR, 1991).
The sampling technique was selected and planned in such a way as to facilitate collection and
provision of high quality and detailed data for validating models and assessing variation of
engineering soil properties across the project area.
4.2 Investigation and sampling scheme
A number of site investigation and sampling schemes have been devised by practical
statisticians, and are available for use by geologists (Koch and Link, 1970; Krumbein and
Graybill, 1965; Meyer, 1975; Wetherill, 1966; ). According to Cheeney (1983), the principal
motivation for the diversity of available procedures is the need to achieve maximum
efficiency and precision within constraints of cost, time, effort, available facilities, manpower
skills and specimen accessibility. However, the application of these schemes may be
hampered and/ or limited by the fact that, geological materials and observations are frequently
hard won under difficult circumstances.
A site investigation/ sampling plan is a fundamental part of the study of soils, especially as
may regard the assessment, analysis and evaluation of the spatial distribution of important
engineering geological characteristics and/ or parameters. The use of an unsuitable and
inadequate procedure could lead to bias in the observation and selection of specimens, with a
consequent distortion of the outcome of important soil studies and evaluations such as
hypothesis testing and statistical tests. It is therefore essential that the field investigation/
sampling method adopted is such as to allow for the acquisition of necessary information and
soil samples which are free as much as possible from bias. This is because a biassed set of
observations and/ or sample would only serve to give a biassed impression of the parent
population from which it was drawn (Cochran, 1977).
In the present study, field investigations and sampling were carried out with the aim of
achieving both descriptive and analytical objectives of soils. Among the analytical objectives
is hypothesis testing, the methods of which are based on samples. Attempts were therefore
made to avoid choosing and applying defective site investigation and sampling practices that

32
could otherwise lead to erroneous decisions regarding assessments and estimation of
parameters of parent populations (of selected soil types). The boundaries/ limits of the areas to
be site investigated and sampled were clearly defined and marked. The investigation and
sampling points were then arranged and distributed in such a way that they covered the
population (or projected area of interest) more or less uniformly.
A systematic site investigation and/ or sampling technique was employed in the current study
as the standard procedure for the black clays (Fig. 4.1). The procedure generally involves
dividing the population or the projected area of investigation and sampling into a number of
mutually exclusive subpopulations or strata, over the spatial range of interest. In this way,
there would be as many strata as there are to be specimens in the projected sample. During
site investigation and sampling, the position of the first specimen taken from the first stratum,
is decided at random. Succeeding specimens would then be taken from the same/ similar
position but in their respective strata. According to Cheeney (1983), systematic investigation
and sampling is straightforward, easy to execute, fast; and involves relatively low labour
input. In addition, this site investigation/ sampling scheme is usually adopted as an effort
towards improving on the precision of estimation of some population parameter.
Figure 4.1 illustrates the field arrangement of systematic site investigation and sampling as
effected for the black clays in this study. Site investigation (soil augering & cone penetration)
points were located at equal intervals of 250 m distance along planned field profiles. A total
of five parallel field profiles (A, B, C, D, E) in an east-west direction, as well as another five
similar profiles (F, G, H, I, J) in a north-south direction; were used over the extent of
investigation interest of black clays across the study area. The interprofile separation distance
was 1000 m for the east-west running profiles and 2000 m for the north-south profiles.
Disturbed and undisturbed soil sampling were effected in hand-dug excavation pits (Plates 4.1
& 4.2).
Plates 4.1(a) & (b) Hand-dug excavation pits and undisturbed sampling in black clays.

33
Plates 4.2(a) & (b) Hand-dug excavation pits and undisturbed sampling in red soils.
The nature and depth of soils between adjacent excavation pits and/ or sampling points was
investigated and confirmed by employing soil augering (Plate 4.3) and cone penetration
sounding (Plate 4.4) techniques.
Plate 4.3 Augering of red soils.
Plate 4.4 Cone penetration sounding of soil
depths.

34
The red soils in this study occur mainly within the built-up areas of Nairobi metropolitan.
Extensively open grounds to facilitate more systematic site investigation and sampling were
therefore hard to come by. As a result, only random site studies and sampling of red soils
were effected in this study. However, in an effort to achieve precise assessments in the
variations of soil depths and engineering properties within these soils, results obtained from
current investigation have been supplemented with some results of previous works (Otando,
1996) covering the same aspects; and this especially in the western and northern sections of
the project area.
4.3 Cone penetration depth sounding
Cone penetration sounding technique was used to determine soil depths across the project
area. A combined presentation of the sounding plan and obtained results for both red soils
and black clays is shown in Fig. (4.2) in which values at site investigation points refer to
respective depths. The red soils are mainly confined to the central-western and north-western
parts of the project area, and show depths of just over 4m to 18m. The black clays cover the
rest of the project area and commonly vary in their depths from about 1m to around 4m. In
general, however, both red soils and black clays show progressively decreased soil depths
eastwards and south-eastwards across the study area, just as indicated by the direction of
arrows in Fig. (4.2).
Depth variations of red soils and black clays are also summarised in contour form (Fig. 4.3),
block diagram (Fig. 4.4), as well as contour overlays on a base map (Appendix D: Fig. D1).

35
36°58'E
1°24'S
North
18 17 16 14 13 12 11 1 1 1 1 1 1 4 4
16 15 14 13 12 4 4 4
1 1 1 1 1 1 1
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000
15 14 13 12 11 1 1 1 1 2 2 11 1 11 1 11 1 11 1
14 14 12 1 1 1 2 2 2 1 1 1 1 1 1 1 1 1
13 12 11 1 1 1 4 2
3322
332 21 1 1 1 1 1 1 1 1 1 1
12 11 10 1 1 1 1 1 1 1 1 1 1 331 1 1 1 1 1
11 10 9 1 1 2 2 1 1 1 1 1 1 1 1
10 9 8 8 1 1 2 2 2 1 1 1 1 1 1 3331 1 1 1
9 9 8 7 2 2 1 1 11 3332 1 1 1 1 1 1 1 1
8 8 8 0 1 2 2 0 1 3 1 1 1 1 2 3 1 1 1 1
6 5 6 5 1 1 1 2 1 1 1 2 1 1 1 1 1
1 0 0 0 1 1 0 1 1 1 1 2 1 1 1
1 2 2 2 1 0 1 0 2 2 2 0 1 1 0 1 1 0 1 1
2 2 1 0 1
2 1 2 0 0 1 0 0 0 0
Distance (m)
Figure 4.2. Depth sounding results showing values in metres.
15000
East
1°15'S
36°45'E
10000
5000
0
Distance (m)

36
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
Black clays Red soils
36°58'E
1°24'S
1°15'S
15000
Distance (m)
10000
36°45'E
West
5000
0
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000
South
Distance (m)
Figure 4.3. Depth variations in red soils and black clays.

4
3
2
1
0
37
Depth (m)
15
10
5
1°15'S
Distance (m)
36°58'E
West
1°24'S
36°45'E
5
17
16
15
14
13
12
11
10
9
8
7
6
South Distance (m)
Black clays Red soils
Depth (m)
Figure 4.4. Block diagram showing depth variations in red soils and black clays.

38
4.4 Vane test
4.4.1 Introduction
The field vane allows for the in situ measurement of the shear strength of very soft and
sensitive clay soils (up to 20 kPa or less), from which it would be otherwise very difficult to
prepare undisturbed specimens for other types of test (Head, 1988; Richwien & Lesny, 2000).
The vane shear tests therefore serve to determine the overall shear strength of undrained soils
through a relatively faster shearing process. It is, however, not possible to separate the shear
strength so determined into the components and/ or shear parameters of friction and cohesion,
since the effective shear strength in the shear surface is unknown (DIN 4096, 1980). Design
and related calculations involving projected works in soft undrained soils therefore assume an
angle of internal friction (φ) of zero.
In the present area, isolated sections of very soft soil materials occur in the form of peaty and
swampy environs, and/ or areas of impeded drainage in those parts of black cotton soils; as
well as in some relatively dense forested areas of red soils.
4.4.2 Testing procedure
The principle of the vane test is shown in Figure 4.2. This study adopted the British Standard
(BS 1377: 1975, Test 18 and/ or DIN 4096: 1980) field vane testing procedure, which
specifies a rate of rotation of the vane of 6-12° /minute. The apparatus was designed to have a
capacity of up to 240 Nm in terms of applied external torque, thereby capable of measuring
shear strength of soft to stiff soils of up to 100 kN/m², with an accuracy of 0,2 kN/m².
The apparatus used in this investigation consisted of a four-blade cruciform vane of diameter
(D) of 50 mm and height (H) of 100mm, attached to the lower end of a rod of small diameter
(Plate 4.5). The test involved pushing the cruciform vane into the soil followed by its rotation.
Figure 4.5 Principle of vane shear test showing vane blades and cylinder of soil rotated by
vane (after Head, 1988).

39
The upper ends of the vanes were operated at a penetration depth of at least 300 mm from the
ground surface, to avoid possibly disturbed upper soil zones (DIN 4096, 1980). The torque
required to cause rotation of the cylinder of soil enclosing the vane was measured and/ or read
off, and this enabled the undrained shear strength of the soft soil to be calculated using the
equation below (DIN 4096, 1980; Head, 1988; Richwien & Lesny, 2000) i.e.,
where c = vane shear strength (kN/m²)
T = applied torque (kN.m)
D = diameter of vane (m)
and H = 2D for apparatus used in this study (m)
c = 6T/(7πD³) kN/m² (4.1)
Plates 4.5(a) & (b) Vane shear testing in soft black clays.
A general guide to the description of soils based on results of vane shear tests is summarised
in Table 4.1 (BS 1377: 1975).

40
Table 4.1 Classification of soft soils based on vane shear strength (BS 1377: 1975).
General descriptive term
Maximum shear stress
for strength
(kN/m²)
Very soft 20
Soft 40
Soft to firm 60
Firm 90
4.4.3 Results of study
Results of field vane shear tests performed in this study are summarised in Tables 4.2, 4.3 and
4.4 as shown below. The variations of vane shear strength with soil depth are presented in
Figure 4.4.
Table 4.2 Results of field vane shear tests in soft black clays at Madaraka West, Nairobi.
Date: 14.01.2001 Spring = Tr/ θf i.e. K = 1333,33
Location: Madaraka West Constant, K Nmm/deg.
Soil type: Black clays
Depth, Moisture Bulk Angle of Torque, Vane shear Soil
Dp (m) Content, density Rotation, θf Tr (Nmm) Strength, strength
Wn (%) (g/cm³) (degrees) c (kPa) description
0,3 40,53 1,56 10,6 14166,67 30,94 Soft
0,6 40,46 1,74 14,3 19000,00 41,50 soft to firm
1,0 24,68 1,78 45,6 60833,33 132,86 firm to stiff
1,5 28,23 1,78 60,6 80833,33 176,53 Stiff
The soft and sensitive clay soils are characterised by a very soft to firm consistency, up to
depths of 1 m for black clays and 3 m for red soils. The black clay variants exhibit undrained
vane shear strength of 30,94 – 41,50 kPa; while the red soil equivalents have values of 10,92
– 87,36 kPa. There is a general increase in the undrained shear strength with depth, in both
soil types. This is probably due to preconsolidation of deeper soil horizons by overburden
material above, so that bulk density of the soils generally increases with depth (Tables 4.2, 4.3
& 4.4).
Table 4.3 Results of field vane shear tests in soft red soils at Arboretum, Nairobi.
Date: 11.01.2001 Spring = Tr/ θf i.e. K = 1333,33
Location: Arboretum Constant, K Nmm/deg.
Soil type: Red friable clays
Depth, Moisture Bulk Angle of Torque, Vane shear Soil
Dp (m) Content, density Rotation, θf Tr (Nmm) Strength, c strength
Wn (%) (g/cm³) (degrees) (kPa) description
0,3 39,44 1,37 3,8 5000,00 10,92 very soft
1,0 24,03 1,41 30,0 40000,00 87,36 Firm
2,0 24,25 1,41 36,5 48616,47 106,18 firm to stiff
3,0 25,07 1,52 38,3 51020,39 111,43 firm to stiff

41
Table 4.4 Results of field vane shear tests in soft red soils at Kenya High School, Nairobi.
Date: 11.01.2001 Spring = Tr/ θf i.e. K = 1333,33
Location: Kenya High Constant, K Nmm/deg.
Soil type: Red friable clays
Depth, Dp Moisture Bulk Angle of Torque, Vane shear Soil
(m) Content, density Rotation, θf Tr (Nmm) strength, c strength
Wn (%) (g/cm³) (degrees) (kPa) description
0,3 37,85 1,36 5,7 7555,19 16,50 very soft
1,0 26,86 1,40 15,1 20147,16 44,00 soft to firm
2,0 27,05 1,43 21,4 28558,60 62,37 Firm
3,0 27,78 1,68 22,7 30220,74 66,00 Firm
The variation of vane shear strength with soil depth tends to fit closely to a logarithmic
relationship (Fig. 4.6), with relatively strong correlation, i.e.R = 0,94 for black clays; and R =
0,97 – 1,0 for red soils.
Vane shear strength (kPa)/ depth (m)
200,00
180,00
c = 95,496Ln(Dp) + 126,71
R 2 = 0,8766
Vane shear strength c (kPa)
160,00
140,00
120,00
100,00
80,00
60,00
40,00
c = 44,727Ln(Dp) + 72,397;
R 2 = 0,9397
c = 22,33Ln(Dp) + 43,936;
R 2 = 0,9901
Red soil,
Kenya High
Red soil,
Arboretum
Black clays,
Madaraka
Logarithmic
(Kenya High)
Logarithmic
(Arboretum)
Logarithmic
(Madaraka)
20,00
0,00
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5
Depth Dp (m)
Figure 4.6 Variation of field vane shear strength with soil depth for the soft red friable clays
and soft black clays.

42
The variation of vane shear strength with natural moisture content of soils closely
approximates a polynomial relationship with strong correlation for both soil types, giving R =
1.0 for black clays and R = 0,91 - 0,99 for red soils (Fig. 4.7).
Vane shear strength (kPa)/ natural moisture content (%)
200,00
180,00
c = -1,5051Wn 2 + 91,987Wn - 1220,7
R 2 = 0,9976
160,00
140,00
Red soil, Kenya
High
Vane shear strength c (kPa)
120,00
100,00
80,00
c = -1,6845Wn 2 + 101,55Wn - 1373,9
R 2 = 0,9834
Red soil,
Arboretum
Black clays,
Madaraka
Polynomial
(Madaraka)
Polynomial
(Kenya High)
60,00
c = 0,3359Wn^2 - 24,801Wn + 473,93
R^2 = 0,8292
40,00
20,00
0,00
0 5 10 15 20 25 30 35 40 45
Natural moisture content Wn (%)
Figure 4.7 Variation of field vane shear strength with natural moisture content of soils for the
soft red friable clays and soft black clays.
Vane shear strength and bulk density were observed to generally increase with increased soil
depths (Tables 4.2, 4.3, 4.4). So far, the variation of vane shear strength with bulk density is
best described by a polynomial relationship (Fig. 4.8) with a very strong correlation (R = 0,97
for black clays; R = 0,99-1,0 for red soils).

43
Vane shear strength (kPa)/ bulk density (g/cm³)
200,00
c = 12597ρ 2 - 41512ρ + 34133
180,00
R 2 = 0,9366
Vane shear strength c (kPa)
160,00
140,00
120,00
100,00
80,00
60,00
40,00
c = -4406,1ρ 2 + 13695ρ - 10461
R 2 = 0,9714
c = -3406,9ρ 2 + 10178ρ - 7533,1
R 2 = 0,9996
Kenya High
School
Arboretum
Madaraka West
Polynomial
(Madaraka West)
Polynomial
(Arboretum)
Polynomial
(Kenya High
School)
20,00
0,00
1,2 1,4 1,6 1,8 2
Bulk density ρ (g/cm³)
Figure 4.8 Variation of field vane shear strength with bulk density of soils for the
soft red friable clays and soft black clays.
4.4.4 Applications
In practice, determination and knowledge of the values of low shear strength of soft and/ or
weak soils to a reasonable degree of accuracy is essential, and some of the applications are as
mentioned below.
First of all, the determined low shear strength could be useful in indicating the maximum safe
bearing pressure which a stratum of very soft soil could sustain initially, when external loads
of constructed engineering structures are placed and/ or imposed on them. This would in turn
determine and influence the designed thickness of such structures (embankments, etc.) which
could be built and placed as a first stage. Thereafter, subsequent consolidation would serve to
increase the shear strength of the soil so that construction could proceed in stages, based on
the shear strength criterion (Head, 1988).
Secondly, the shear strength of soft strata near the ground surface is usually used for the
estimation of “negative skin friction” resulting from adhesion on piles driven down to a firmer
stratum below (Lambe and Whitman, 1979). Infact, soft soil deposits which have been
extensively disturbed by the effect of driving of piles commonly end up exhibiting significant
amounts of remoulded strength.

44
And thirdly, the field vane apparatus can be used to investigate and provide the relationship
between undrained shear strength and moisture content in cohesive soils; and/ or the
relationship between shear strength and soil moisture suction (Lewis and Ross, 1955).

45
Chapter 5
Types of soil
5.1 Superficial deposits
Superficial deposits of Recent age consist of alluvia and conglomerates exposed in the
principal river courses and their tributaries, especially in the southern and south-eastern
sections of the study area. Accumulations of alluvium occur mainly in the form of old river
sediments. Also included are new sediments currently being added to the flood plain without
developed morphology but have a more humic surface horizon. On the Athi plains, these soils
have also developed from the drying up of papyrus swamps, either as a result of canalisation
of the rivers found and/ or drier climatic conditions. Exposures of a conglomerate band
composed of boulders and pebbles of local lava occur on the north bank of the Mbagathi river,
within the Nairobi National Park. The boulders and pebbles are partly indurated while, on the
other hand, the more modern gravels also found exposed along the course of the Mbagathi
river are largely uncemented.
Deposits of shallow stony soils associated with rock outcrops characterise the courses of most
of the streams draining the high ground east of the Rift, especially in the northern and northwestern
sections of the present area. The soils are variously developed and originated from
steep slope areas where they derived by processes of accelerated erosion of the various soils
found, and which have lost their original characteristics (Saggerson, 1991). They are very
shallow and occur on the main valley sides in the form of pockets on slight shelves and
between boulders.
Isolated areas of impeded drainage also occur across the study area, and are characterised by
development of swamps with black peaty soils as well as dark greyish-brown mottled clays,
both of which form accumulations of poorly-drained soils. The peaty swamps constitute a
very minor soil group in the study area and usually occur in the form of narrow stretches
confined to the banks of streams and/ or rivers flowing through them. The peaty soils found
associated are characterised by the growth of papyrus plants and a typical grass vegetation
cover. These soils also exhibit a high humus content and are usually subjected to seasonal or
permanent water table. They are usually covered by water during flood times (Saggerson,
1991). The swampy areas clearly show up as dark tones on aerial photographs.
The mottled clays commonly exhibit a dark grey to greyish-brown humic (2-3% carbon)
topsoil overlying a greyish-brown angular blocky mottled subsoil. These ill-drained soils
generally occur in the main valley bottoms of old drainage lines and/ or depressions where the
greyish-brown mottled clays commonly fill in areas of fault troughs and/ or occupy the flat
ground at the foot of some fault scarps where drainage is locally impeded. The depressions
are most probably a result of minor fissuring which caused the formation of sinkholes at the
junctions of such fissures. Later on, the sinkholes and old drainage lines could have been
filled in with deposited materials, which also acted as the parent material for the development
and formation of the mottled clays under conditions of impeded drainage. According to Scott
(1963), the mottled clays most probably owe their origin and development from the
weathering and alteration of previously deposited volcanic ash and colluvium, as well as other
relatively soft Pleistocene materials. Seepage of water from the surrounding soil into the
mottled clays occurs during the rainy season, causing the latter soils to be water-logged for a
period of 2 to 3 months, commonly with a water depth of up to 30 cm lying on the soil
surface. The edges of the mottled clays are usually characterised by the formation of a laterite

46
horizon, most probably a result of base accumulations from the seepage waters coming from
the surrounding soils. The mottling character of these clays serves to distinguish and separate
them from the otherwise black to dark grey clays (black cotton soils) of the Athi and Kapiti
plains. The mottled clays are usually found uncultivated in their areas of occurrence. They
therefore exhibit and are recognised by a uniform light colour tone on aerial photos given by
the grass vegetation cover.
Deposits of pale brown or reddish ferruginous soil and ferricrete (murram) are also found
occurring, especially in the southern half of the present area. These materials are products of
weathering and alteration of underlying lava and tuff. They are generally hard, of sufficient
thickness and commonly quarried for use as road surfacing material. The south-western part
of the study area, along Langata road and around Nairobi National Park, shows the Nairobi
trachyte escarpment to be sufficiently weathered, giving rise to considerable deposits of
ferricrete. This material is extracted from a number of quarries located at the base of the
scarp, and utilised for surfacing earth roads and tracks.
5.2 Soils
The Nairobi region, which includes the present study area, is covered by various thicknesses
of soils that are products of weathering of mainly volcanic rocks under relatively high
temperature and rainfall conditions. Good drainage conditions prevailing in the Kikuyu
highlands to the west of Nairobi city as well as in the north-western section of the present
study area, have facilitated the weathering processes to give rise to red soils of up to 15 m
thickness. On the other hand, poorer drainage conditions typifying the Athi and Kapiti plains
covering much of the present area to the south and east of the Nairobi city, have favoured the
development and formation of black cotton soils.
Early works in Nairobi and the present study area classified the soils found by separating
them into major world groups, based on the soil map in the Atlas of Kenya (Survey of Kenya,
1991). Scott (1963), modified the classification to facilitate a survey carried out as an exercise
in the interpretation of aerial photographs. In the present study, initial separation of soils was
also largely based on actual observations made on aerial photographs. A further subdivision
of the soils involved their classification according to climatic regions, topography and/ or
slope, and drainage. Thus soils could be placed into one of three classes of well-drained soils,
soils with slight seasonal impeded drainage or soils with impeded drainage. The well-drained
soils could be further subdivided into soils of humid, sub-humid or semi-arid climatic regions
and/ or conditions. Humid regions have rainfall amounts of over 1000 mm per year; while
sub-humid and semi-arid regions are characterised by rainfall amounts of 760-1000 mm and
500-760 mm per year, respectively. The distribution of major soil types found in Nairobi and
the present study area is presented in Figure 5.1. The soil map serves to give a good general
picture of the study area and its surroundings. It also serves as a guide to where more detailed
soil studies could be undertaken, as may be required. A geotechnical soil map (Appendix E)
has also been prepared; and apart from showing the distribution, extent, boundaries and depth
variation of red soils and black clays, it also presents a summary of the index and engineering
properties of the soils (details discussed in a later chapter).

48
5.2.1 Red friable clays
The red soils are found overlying the trachytic rocks in the higher altitude sections of the
study area, located mainly to the north-west. This part of the study area is sub-humid and
characterised by a high rainfall of 760 – 1000 mm per year (Kenya Meteorological
Department, 2001). The soils are well-drained and exhibit a dark reddish-brown (Sample No.
Rd-30cm) and humic (up to 2,5% carbon) topsoil overlying a red subsoil (Sample Nos. Rd-
100cm; Rd-200cm; Rd-400cm) of weak subangular blocky friable clay. Stone lines are
sometimes found associated, occurring just above the metamorphic rocks.
The soils are most probably polygenetic in that, the humid conditions must have favoured
their development by weathering and alteration from the volcanic rocks, tuff and ash found.
Later episodes of violent volcanicity in the Rift region to the west must have contributed
additional showers of volcanic ash that covered already developed red soils of the underlying
land surface (Scott, 1963). Current field observations show the red soils as having profiles
consisting of burried humic horizons; and this serves to indicate that sufficient time must have
elapsed between subsequent ash showers, thereby permitting the development and formation
of a new soil from the previously deposited ash.
The red soils extend further westwards and north-westwards, beyond the present area and
across the Kikuyu highlands, to cover and include Kiambu township. The soils occupy
principally the tops /summits as well as upper and middle slopes of hills and ridges found, and
this serves to indicate the irregular character of the land surface over which the volcanic ash
fell. On the lower slopes of the ridges, the topography becomes generally flatter, with
accompanying poorer drainage conditions, resulting in the formation of shallow yellow-brown
to yellow-red friable clays overlying a laterite horizon or rock. The transition zone between
the red friable clays and the shallow yellow-brown to yellow-red friable clays is characterised
by narrow bands of an intermediate red friable clay with iron concretions, merging into
massive laterite.
Commonly found associated with the red friable clays are intercalations of poorly drained
dark greyish-brown mottled clays observable in parts of the Nairobi municipality; and which
also occur in depressions between ridges, as well as the generally flat land surface overlying
the trachytes to the north and north-west of Nairobi city. The mottled clays are partly a result
of the development of subsidiary drainage in the lowlying land and/ or depressions between
the undulations of ridges.
Further north-westwards, beyond the present area of study, the red soils give way to strong
brown to yellow-red friable clays and dark red friable clays in a humid region covering the
high ground flanking the Rift Valley, and where a rainfall amount of over 1000 mm per year
is characteristic (Kenya Meteorological Department, 2001). These latter types of soil also
overly the trachytic rocks found, and have been derived from the volcanic rocks, tuff and ash
through processes of weathering and alteration. The soils are well-drained and also
characterised by a dark reddish-brown high humic (3-7% carbon) topsoil overlying a dark red
subsoil of subangular blocky friable clay with clay skins. The red friable clays are
comparatively less humic, have a lower total exchangeable base content and are less saturated
than the dark red friable clays. They are also characterised by a much weaker structure and
less marked development of clay skins. The brighter red colour and weaker structure of these
soils may be partly a result of the lower humus status.

49
The red friable clays on the whole are relatively deeper than the black clays, mottled clays and
shallow yellow-brown to yellow-red friable clays. This may possibly be attributed to the
higher rainfall received in areas of red soils as well as better drainage conditions of these soils
both of which favour a relatively faster weathering of the underlying rock. However,
development of the red soils on the concave slopes of the ridges is limited by the formation of
a laterite cap over parts of the rock surface; and this would protect the rock from further
weathering. The formation of laterite is favoured by the generally concave shape of the slopes
of the ridges that results in changing drainage conditions downslope.
Table 5.1. Profile description of a red friable clay.
DEPTH
(M)
SAMPLE
NO.
DESCRIPTION
0,0-0,30 Rd1-30cm Dark reddish brown, slightly
humic, weak crumbly friable
clay
0,30-1,0 Rd1-100cm Dark red to red weak
subangular blocky friable clay
1,0-2,0 Rd1-200cm Red weak subangular blocky
friable clay with slight clay skin
development
2,0-4,0 Rd1-400cm Red weak subangular blocky
friable clay with slight clay skin
development
SILT
(%)
CLAY
(%)
C (%)
73 18 2,47
74 20 0,73
69 24 0,83
72 20 0,45
The boundary lines between the red clays and other clay soils mentioned above were first
fixed along roads and/ or planned traverses by ground inspection; and then extended and
joined by interpolation. On the aerial photographs, the red clays show up as darker tones
relative to the other adjacent soil types mentioned above. A typical red friable clay from the
present study area is described in Table 5.1. The amount of total carbon was found to decrease
with depth, and this is most likely also indicative of decreasing organic matter (humus)
content of the soils with depth.
Table 5.2. Soluble base content in soils of the study area.
Type of soil Depth
(m)
Sample
No.
MgO
(%)
CaO
(%)
MnO
(%)
Na2O
(%)
K2O
(%)
Total
(%)
Red friable 0,0-0,30 Rd1-30cm 0,12 0,26 0,96 - 0,96 2,30
clays 0,30-1,0 Rd1-100cm 0,097 0,24 0,53 - 0,66 1,53
1,0-2,0 Rd1-200cm 0,14 0,16 0,61 - 0,68 1,59
2,0-4,0 Rd1-400cm 0,13 0,26 0,52 - 0,60 1,51
Shallow yellow 0,0-0,30 SC17-30cm 1,17 1,28 0,529 0,74 1,33 5,05
- brown clays 0,30-0,50 SC17-50cm 1,28 1,47 0,488 0,82 1,41 5,47
on laterite >0,50 - - - - - - -
0,0-0,30 SA2-30cm 1,07 1,25 0,298 0,63 1,30 4,55
Black clays 0,30-0,70 SA2-70cm 1,25 1,28 0,312 1,06 1,78 5,68
0,70-1,05 SA2-105cm 1,22 4,29 0,363 1,04 1,73 8,64
Black clays 0,0-0,30 SB1-30cm 1,02 1,26 0,44 0,70 1,35 4,77
0,30-0,50 SB1-50cm 1,16 1,54 0,455 0,71 1,27 5,14
0,50-0,70 SB1-70cm 1,23 2,01 0,439 0,76 1,34 5,78

50
The variation in the content of total soluble bases in red soils is presented in Table 5.2
(sample Rd1), alongside that for black clays for comparison purposes. The amount of total
soluble base was also observed to decrease with increasing depth of the red soils. This could
be explained by the good drainage conditions of the soils which serve to facilitate removal of
the basic substances in solution during rainfall.
Further eastwards across the study area, the red soils pass into a composite soil group
consisting of shallow yellow-brown friable clays overlying a laterite horizon; and shallow
yellow-red friable clays on rock. This is because of the difficulty in distinguishing between
and separating these two soil units on aerial photographs. However, the two soil components
are genetically different. These soils commonly exhibit a brown (Sample No. SC17-30cm)
relatively low humic (up to 1,5% carbon) topsoil overlying a yellow-brown (Sample No.
SC17-50cm) subangular blocky friable clay with iron concretions passing downwards into
massive laterite (ferricrete). In other places, the soils consist of a brown low humic (0,5-1,5%
carbon) topsoil underlain by a yellow-red subangular blocky friable clay passing into rock.
The soils are usually characterised by a slight seasonal impeded drainage. They also support a
scrub grass vegetation which appears as a light tone on aerial photographs. The shallow soils
over rock are generally redder than their lateritic counterparts. They are found occurring
mainly on the flat lands adjoining and/ or at the edges of the black clays in the plains. They
are apparently youthful soils which have developed most probably after the removal of the
black clays by erosive processes. The shallow soils over laterite occur mainly on the lower
slopes of ridges. According to Scott (1963), they are most probably a result of seepage water
from higher ground whose downslope flow is checked by the change of slope and poorer
drainage conditions at the foot of the slope, causing the deposition of iron and aluminium
compounds from the seepage waters to form a laterite sheet. The development of the laterite
sheet also serves to increase the drainage impedence , thereby favouring further development
and formation of the shallow lateritic soils which extend upslope.
Table 5.3. Profile description of a shallow yellow-brown clay overlying laterite.
Depth (m) Sample No. Description Silt Clay C (%)
(%) (%)
0,0-0,30 SC17-30cm Brown slightly humic,
crumbly friable clay with
occasional rounded murram
concretion
59 29 1,54
0,30-0,50 SC17-50cm Yellow brown subangular
blocky friable clay with
occasional rounded murram
concretion
>0,50 - Mainly murram concretion
passing into massive murram
59 27 1,29
- - -
A typical yellow-brown friable clay overlying laterite in the current study area is described in
Table 5.3. The amount of total carbon is observed to decrease with increasing depth of soils,
and this could as well serve to reflect possible decrease of organic matter content of the soils
with depth. However, the soils exhibit a slight increase in the amount of total soluble base
with increased depths (Table 5.2, Sample SC17), and this is most probably due to the slightly
impeded drainage character of the soils which serves to limit and/ or inhibit removal of the
basic substances in solution.

51
Results of chemical analyses of the red soils performed in the current study (Table 5.4) show
them as lacking and deficient in magnesia (MgO: 0,097-0,14 %). Chemical analyses of the
rocks from which these soils are derived also indicate their relatively very low magnesia
content (MgO: 0,30-0,89 %), compared with the basic lavas and other volcanic rocks of the
Nairobi region (Table 3.4). Farmers in the coffee-growing estates found in the areas of red
soils tackle this problem through frequent artificial application of magnesite-rich fertilisers.
The red soils generally exhibit lower contents of SiO2, MgO, CaO, Na2O and K2O than the
volcanic materials from which they are derived (Tables 3.3 and 5.4). This could also be a
result of the good drainage conditions of the soils as well as the high rainfall which favoured
leaching and removal of the soluble bases and silica, thereby leaving the soils relatively
enriched in Fe2O3 and Al2O3 minerals.
Table 5.4. Results of chemical analyses of red soils obtained in this study.
% content of: Rd1-30cm Rd1-100cm Rd1-200cm Rd1-400cm
SiO2 47,50 47,10 47,20 47,6
Al2O3 32,20 33,40 33,20 33,30
Fe2O3 15,30 15,50 15,50 15,20
FeO - - - -
BaO 0,062 0,061 0,071 0,064
MgO 0,12 0,097 0,14 0,13
CaO 0,26 0,24 0,16 0,26
Na2O ‹ ‹ ‹ ‹
K2O 0,96 0,66 0,68 0,60
P2O5 0,19 0,086 0,096 0,062
ZrO2 0,37 0,37 0,37 0,36
TiO2 1,57 1,51 1,51 1,46
MnO 0,96 0,53 0,61 0,52
Loss on ignition - - - -
Total 99,49 99,55 99,54 99,56
Rd1-30cm, Rd1-100cm, Rd1-200cm, Rd1-400cm. Samples of red soils collected from Arboretum (Nairobi), at
depths of 30cm, 100cm, 200cm and 400cm, respectively.
‹: Concentration less than 0,04 % (400 ppm).
Similar results of previous chemical analyses of a number of clays and red soils of Nairobi
area are also given in Table 5.5, for comparison purposes. A relation of the results of Tables
5.4 and 5.5 reveals a depletion in the amounts of soluble bases (MgO, CaO, NaO) to have
taken place with time, most probably due to effects of leaching in the red soils.
Correspondingly, the proportionate amounts of Fe2O3 (free iron oxide, haematite, goethite)
and Al2O3 (clay minerals) have increased with time.
The more easily leached components from areas of red soils persist or accumulate on lower
ground of depressions between ridges and/ or the flat plains, where they contribute to the
formation of black soils.

52
Table 5.5. Earlier results of chemical analyses of clays and red soil.
% content of: 1 2 3 4
SiO2 40,65 34,16 38,69 35,60
Al2O3 20,98 25,55 32,74 30,75
Fe2O3 11,48 14,15 13,56 15,55
FeO - 0,90 - -
MgO 1,50 0,37 0,26 -
CaO 2,00 1,09 Tr -
Na2O - 0,09 Nd -
K2O - 0,34 Nd -
H2O+ 9,62 9,24 - -
H2O- 12,51 11,10 - -
TiO2 - 3,12 1,07 1,40
MnO - 0,38 0,19 -
Loss on ignition - - 13,14 15,03
Total 98,74 100,49 99,65 98,33
1. Clay. Fairview Estate, Kiambu. .
2. Red clay. Escarpment, Kiambu (Maufe, 1908)
3. Red soil, Kiambu (EAIRB, 1953. Technical Pamphlet 61)
4. Clay. Sasamua (Terzaghi, 1958).
According to Rösler (1980), weathering and alteration of feldspar-rich volcanic materials
under humid conditions of rainfall and in the presence of carbon dioxide usually results in the
conversion of aluminiumsilicates contained, and formation of kaolinite-rich soils. The red
soils in the present area must have therefore mainly formed by weathering and alteration of
the underlying Nairobi trachytes. This is evidenced by chemical/ mineralogical studies of the
soils which show them as being predominantly kaolinite (80-81%) and iron oxide (haematite;
15-16%) in their composition (see next chapter). The mineral components rich in alkali (Na,
K) and alkali-earth metals (Mg, Ca), as well as silica (SiO2) were therefore removed in
solution, leaving behind the altered volcanic rocks rich in quartz, aluminium and other stable
minerals in the form of kaolinite-rich red soils. These findings are further supported by
previous studies made by staff of the Ministry of Agriculture on the red clay soils of Nairobi,
which revealed that leaching of the soils had the effect of removing the soluble bases and
silica, leaving the soils rich in iron oxide and/or haematite; as well as aluminium in the form
of clay minerals, metahalloysite and hydrated halloysite (Dumbleton, 1967; Sherwood, 1967).
The free iron oxide has the effect of cementing the clay particles together. This is a possible
explanation for the anomalous results obtained in the present study by subjecting the red soils
to British Standard soil classification tests (Table 5.6).
Table 5.6. Some results of soil classification tests performed on red soils in the present study.
LOCATION SAMPLE
NO.
CLAY
CONTENT
(BS 1377)
(%)
LIQUID
LIMIT
(%)
PLASTIC
LIMIT
(%)
PLASTICITY
INDEX
(%)
Arboretum Rd1-30cm 18 49 31 18
(Nairobi)
Arboretum Rd1-100cm 20 51 31 20
Arboretum Rd1-200cm 24 50 30 20
Arboretum Rd1-400cm 20 48 30 18

53
Results of previous soil classification tests carried out on a number of clays and red soils in
the neighbourhood of the present study area are summarised in Table 5.7, for comparison
purposes.
It can be observed from the results of Tables 5.6 and 5.7 that the cementing effect of clay
particles by free iron oxide must have progressively reduced the proportion of free active clay
particles in the red soils through geological time. As a result, values of the Atterberg limits
(plastic and liquid limits, plasticity indices) of the soils have also decreased with time.
Table 5.7. Earlier results of soil classification tests on red clays from the Nairobi area (After
Sherwood, 1967).
LOCATION
SOIL
NO.
CLAY
CONTENT
(BS1377)
(%)
LIQUID
LIMIT
(%)
PLASTIC
LIMIT
(%)
PLASTICITY
INDEX
(%)
Kabete 816 82 76 39 37
Limuru 830 83 87 44 43
Nairobi-Limuru
road
639 88 87 48 39
5.2.2 Black to dark grey clays (black cotton soils)
The black to dark grey clays cover much of the present study area that forms a part of the Athi
and Kapiti plains. The soils are characterised by impeded drainage, and are underlain mainly
by the Nairobi and Kapiti phonolites which form a practically impermeable strata, favouring
further development and occurrence of the ill-drained soils. Contributions to the formation of
the black clays also comes from eroded kaolinite-bearing materials as well as leached
components of soluble bases and silica derived from areas of red soils occurring on the higher
grounds forming the Kikuyu highlands to the west and north-west of the present area. This is
evidenced by chemical/ mineralogical results from current studies, which show the black
clays as containing relatively elevated amounts of MgO, CaO, Na2O, K2O and SiO2 (Table
5.8), as well as traces of finely dispersed kaolinite (Table 5.9).
The black to dark grey clays are also known as black cotton soils which are characterised by
both calcareous and non-calcareous variants. The calcareous types occur on a much lesser
extent in the south-eastern sections of the study area, while the non-calcareous varieties cover
the remaining portion of the plains in the present study area.
Commonly found associated are local swampy environments and alluvium, which occur as
isolated patches within the black cotton soils. A limited intercalated occurrence of the dark
greyish-brown mottled clays is also recognised, while shallow stony soils with rock outcrops
are concentrated along the generally easterly/ south-easterly trending stream valleys found.

54
Table 5.8. Results of chemical analyses of black clays obtained in this study.
% SA2- SA2- SA41- SB1- SB1- SB41- SB41- SC17-
content
of:
70cm 105cm 50cm 50cm 70cm 30cm 50cm 50cm
SC41-
30cm
SC41-
50cm
SiO2 52,50 51,40 58,35 52,79 53,44 52,64 53,21 56,38 52,06 53,71
Al2O3 14,71 14,93 13,37 12,74 12,84 12,14 12,20 11,87 12,06 11,77
Fe2O3 8,66 8,85 6,98 6,90 6,96 6,67 6,75 6,20 6,49 6,10
FeO - - - - - - - - - -
BaO 0,052 0,052 0,031 0,062 0,062 0,064 0,062 0,085 0,123 0,104
MgO 1,25 1,22 1,13 1,16 1,23 1,43 1,42 1,28 1,34 1,31
CaO 1,28 4,29 1,28 1,54 2,01 2,61 2,32 1,47 1,64 1,48
Na2O 1,06 1,04 0,69 0,71 0,76 0,50 0,56 0,82 0,90 0,89
K2O 1,78 1,73 1,25 1,27 1,34 1,02 1,04 1,41 1,32 1,31
P2O5 0,026 0,024 0,037 0,032 0,038 0,038 0,038 0,047 0,036 0,04
ZrO2 0,092 0,091 0,085 0,091 0,092 0,078 0,078 0,072 0,073 0,072
TiO2 0,96 0,95 0,90 0,76 0,75 0,77 0,77 0,75 0,79 0,77
MnO 0,312 0,363 0,249 0,455 0,439 0,497 0,505 0,488 0,469 0,511
SO3 - - - - - - - 0,011 0,028 0,016
Cl 0,065 0,039 - - - - - - 0,015 0,015
F 0,262 0,362 0,247 0,294 0,325 0,281 0,267 0,311 0,300 0,293
Loss on - - - - - - - - - -
ignition
Total 83,01 85,34 84,60 78,80 80,29 78,74 79,22 81,19 77,64 78,39
SA2-70cm, SA2-105cm, SA41-50cm, SB1-50cm, SB1-70cm, SB41-30cm, SB41-50cm, SC17-50cm, SC41-
30cm and SC41-50cm are black clay samples.
The black to dark grey clays are characterised by a number of distinctive properties which
serve to point to their possible genetic origin, as discussed below.
Results of chemical analyses performed on these soils in this study (Table 5.8) show the
common exchangeable bases of Ca, Mg, K and Na to be all of the same order irrespective of
the type of rock underlying the soil. This serves to show that the black clays are most
probably not derived from the underlying rocks by weathering. This is further supported by
results of chemical analyses of the soils, which generally do not agree with and/ or are not of
the same order as those for the main underlying rocks of Nairobi and Kapiti phonolites
(Tables 3.1, 3.2 and 3.3).
The black clays exhibit a generally uniform depth, varying from 0,90 to 1,50m. Depths of up
to 1,80 to 2,10m are also registered in the swampy areas of Madaraka West and environs. The
clays generally show pronounced cracking in the form of strong shrinkage cracks on drying
from a wet condition.
The black clays were also observed harbouring occasional sandier variants in some places.
Results of X-ray diffraction analyses performed in this study as well as earlier results of
chemical/ mineralogical studies by Stephen, Bellis and Muir (1956) show the black clays as
containing a number of heavy minerals usually characteristic of metamorphic rocks of the
gneissic type. The heavy minerals include sillimanite, kyanite, garnet and staurolite; and were
traced in soil samples collected from Embakasi area and its environs which are underlain by
Nairobi phonolite. The presence of the sandier variants and heavy minerals serves to point to
the possibility of portions of materials forming the black clays as having originated from

55
neighbouring metamorphic areas and deposited by rivers and/ or surface run-off into the
present area. Alteration of kaolinitic red soils deficient in alkali and alkali earth metals (Na,
K, Mg, Ca) could have also caused the formation of the respective aluminiumsilicates of the
heavy minerals. The altered soils were then eroded, transported and deposited in the lower
areas of black clays.
The black clays generally exhibit sharp to fairly sharp contacts with the underlying volcanic
rocks. Cases of patches of black clays overlying red soils were observed in the northern and
north-western sections of the study area. In other parts of the study area, laterite was
commonly found underlying the black clays. In addition, areas with black clays having been
removed by erosion show the underlying rock weathering through red. The above four
observations contradict the suggestion that the black clays could have been derived from the
underlying volcanic rocks by weathering.
The neighbouring areas further eastwards are underlain by metamorphic rocks, and commonly
exhibit a line of quartz stones between the black soils and rock (Scott, 1963; Saggerson,
1991). However, no quartz veins have been observed within the black clays; and this is
contrary to what would be expected if the soils had actually developed in situ, suggesting
once again that the black clays may have not derived from the weathering of the underlying
rocks. In the present study area underlain by volcanic rocks, pieces of quartz stones are found
randomly imbedded in the black clays. This could point to the possibility of the quartz stones
and other materials as having been transported from the surrounding areas into the present
area by erosive and depositional processes.
The general morphology of the area of occurrence of black clays in the present study area and
neighbouring areas further eastwards is roughly basin-like. The eastern and western margins
of this basin tend to follow the 5200 feet contour, while the northern margin is at an altitude
of about 4800 feet. The altitude of the southern boundary of the soils is up to 6200 feet in the
vicinity of Ngong hills. The soils are found occurring on both flat sites and appreciable slopes.
However, isolated patches of black clays occurring on high grounds are also found; and are
possibly part of the same earlier and larger formation, which later underwent erosion leaving
behind only relics of the soils.
The properties and characteristics of black clays discussed above serve to conclusively
suggest that these soils are not wholly developed in situ. A more plausible explanation as to
the genetic origin of the black clays would be based on the existance of a large basin in their
present areas of occurrance, prior to the formation of the soils. The basin was bounded by the
Machakos hills to the east, the Ngong hills to the south and the high ground of the eastern
flanks of the Rift Valley to the west. The northern margin of the basin was most probably
associated with the Ithanga-Kakuzi hills. The lowest point of the basin was most probably
around Athi River Township. The basin was filled with water during a wet cycle, forming a
lake. Materials of colluvium and alluvium from the surrounding hills were later deposited
into the lake. Portions of the basin within and/ or in the vicinity of metamorphic rock areas
were therefore characterised by more sandier variants of black clays than areas of the basin
within volcanic rock areas and/ or next to volcanic hill masses. This is also a possible
explanation for the presence of sandier variants of black clays in the current study area, which
is underlain by volcanic rocks. In addition, there is a high possibility of volcanic ash having
also been deposited into the lake during this period; and this could account for the high
presence of smectites in the black clays, as detected during mineralogical (X-ray diffraction)
analyses performed on the soils in this study (Table 5.9).

56
Table 5.9. Clay mineral composition of black clays.
Sample No. Smectites
(montmorillonite)
(%)
Illite
(%)
Kaolinite
(%)
Quartz
(%)
K-feldspars
(sanidine,
orthoclase)
SA2-70cm >90 - 90 Trace 95 - 95 - 95 - 95 Trace 95 - 95 - >5 4 trace
SC33-30cm >95 - 95 Trace

57
Table 5.10. Profile description of black clay.
Depth (m) Sample No. Description Silt (%) Clay (%) C (%)
0,0-0,30 SA2-30cm Very dark grey humic, angular 64 26 0,98
blocky plastic clay, cracks
when dry
0,30-0,7 SA2-70cm Dark grey angular blocky 62 26 0,50
plastic clay
0,70-1,05 SA2-105cm Grey structureless plastic clay 61 24 0,96
with occasional CaCO3
concretions
>1,05 - Murram and/ or weathered tuff - -
passing into phonolite rock
0,0-0,30 SB1-30cm Very dark grey humic, angular
blocky plastic clay, cracks
when dry
0,30-0,6 SB1-50cm Dark grey angular blocky
plastic clay
0,60-0,90 SB1-70cm Grey plastic clay with some
CaCO3 concretions
>0,90 - Weathered tuff passing into
phonolite rock
60 30 -
65 26 1,13
66 25 1,19
- - -
The black to dark grey clays appeared as dark tones on the aerial photos, with lighter toned
mounds found scattered within being very prominent.
The numbering of soils in this study follows the legend of maps to this thesis work.

58
Chapter 6
Chemical and mineralogical analyses
6.1 Introduction
A number of soil samples previously collected from the field were subjected to laboratory
chemical and mineralogical analyses and/ or studies using the X-ray fluorescence (XRF), X-
ray diffraction (XRD) and scanning electron microscope (SEM) techniques.
6.2 X-ray fluorescence (XRF) studies
6.2.1 Scope and method
Chemical analyses were carried out using X-ray fluorescence technique to determine the
percentage composition of major oxides in the clay soils, and include SiO2, Al2O3, Fe2O3,
FeO, MgO, CaO, MnO, Na2O, K2O, TiO2, P2O5 and SO4. Percentage content of F, Cl, S
and Cr, as well as losses on ignition were also determined. The analysis results have been
useful in computing mineral contents in the tested soils (according to Olphen and Fripiat,
1979), especially where direct measurement of mineralogical compositions, such as kaolinite
in red soils, has proved difficult.
A PW-1480 (Rh 100kV LiF220 Ge111 T1AP) X-ray spectrometer and a shale (claystone)
standard were employed in the XRF analysis. Preparation of test specimens involved mixing
of Hoechst wax (as diluent) with soil sample at a ratio of 1:5 to make a homogenous mixture.
6.2.2 Results
Results of chemical analyses obtained for the soils in this study are presented in Table 6.1 for
red soils and Table 6.2 for black clays.
Table 6.1. Results of chemical analyses of red soils obtained in this study.
% content of: Rd1-30cm Rd1-100cm Rd1-200cm Rd1-400cm
SiO2 47,50 47,10 47,20 47,6
Al2O3 32,20 33,40 33,20 33,30
Fe2O3 15,30 15,50 15,50 15,20
FeO - - - -
BaO 0,062 0,061 0,071 0,064
MgO 0,12 0,097 0,14 0,13
CaO 0,26 0,24 0,16 0,26
Na2O ‹ ‹ ‹ ‹
K2O 0,96 0,66 0,68 0,60
P2O5 0,19 0,086 0,096 0,062
ZrO2 0,37 0,37 0,37 0,36
TiO2 1,57 1,51 1,51 1,46
MnO 0,96 0,53 0,61 0,52
Loss on ignition - - - -
Total 99,49 99,55 99,54 99,56
Rd1-30cm, Rd1-100cm, Rd1-200cm & Rd1-400cm are red soil samples.

59
The presence of BaO in the red and black soils is most likely a result of contributions in the
form of BaCO3 and/ or BaSO4 as contaminants from industrial effluents and/ or wastes.
Fertilizers and industrial water could also account for the presence of traces of P2O5 in both
types of soil.
Table 6.2. Results of chemical analyses of black clays obtained in this study.
Content SA2- SA2- SA41- SB1- SB1- SB41- SB41- SC17- SC41- SC41-
of (%): 70cm 105cm 50cm 50cm 70cm 30cm 50cm 50cm 30cm 50cm
SiO2 52,50 51,40 58,35 52,79 53,44 52,64 53,21 56,38 52,06 53,71
Al2O3 14,71 14,93 13,37 12,74 12,84 12,14 12,20 11,87 12,06 11,77
Fe2O3 8,66 8,85 6,98 6,90 6,96 6,67 6,75 6,20 6,49 6,10
FeO - - - - - - - - - -
BaO 0,052 0,052 0,031 0,062 0,062 0,064 0,062 0,085 0,123 0,104
MgO 1,25 1,22 1,13 1,16 1,23 1,43 1,42 1,28 1,34 1,31
CaO 1,28 4,29 1,28 1,54 2,01 2,61 2,32 1,47 1,64 1,48
Na2O 1,06 1,04 0,69 0,71 0,76 0,50 0,56 0,82 0,90 0,89
K2O 1,78 1,73 1,25 1,27 1,34 1,02 1,04 1,41 1,32 1,31
P2O5 0,026 0,024 0,037 0,032 0,038 0,038 0,038 0,047 0,036 0,04
ZrO2 0,092 0,091 0,085 0,091 0,092 0,078 0,078 0,072 0,073 0,072
TiO2 0,96 0,95 0,90 0,76 0,75 0,77 0,77 0,75 0,79 0,77
MnO 0,312 0,363 0,249 0,455 0,439 0,497 0,505 0,488 0,469 0,511
SO3 - - - - - - - 0,011 0,028 0,016
Cl 0,065 0,039 - - - - - - 0,015 0,015
F 0,262 0,362 0,247 0,294 0,325 0,281 0,267 0,311 0,300 0,293
Loss on - - - - - - - - - -
ignition
Total 83,01 85,34 84,60 78,80 80,29 78,74 79,22 81,19 77,64 78,39
SA2-70cm, SA2-105cm, SA41-50cm, SB1-50cm, SB1-70cm, SB41-30cm, SB41-50cm, SC17-50cm, SC41-
30cm and SC41-50cm are black clay samples.
The total sum of the various chemical components of the black clays add up to only 77,64 –
85,34% (Table 6.2), and not 100%. This is a relatively large difference which could be partly
accounted for by the presence in the soils of accessory amounts of heavy metal compounds,
light and noble elements; and partly by the loss on ignition of organic matter components. The
difference is comparatively small and negligible in the red soils (Table 6.1) in which the total
sum of the chemical components is virtually 100%, probably due to the relatively low and/ or
negligible organic content (Table 6.7).
6.3 X-ray diffraction (XRD) studies
6.3.1 Scope and method
X-ray diffraction studies and analyses were carried out in the present work to determine the
mineralogical composition of the black clays and red soils. Clay minerals present were
identified by their characteristic diffraction patterns.
A diffractometer (Type PW1710-Basis) was used to determine the presence of clay minerals
which include smectites ( montmorillonite), kaolinite and illite. This was done for selected
conditions of continuous scanning rate of 0,020 (° 2θ/second), chart speed 0,10 (mm/ second)
and using Cu-Kα radiation. The clay minerals were X-rayed from prepared clay fractions (

60
2µm size) filtered both by means of a programmable centrifuge as well as through a filter
membrane. The original soil sample had been previously and moderately ground to reduce
strong textural effects usually caused by presence of quartz and feldspars. Treatment with
hydrogen peroxide (3-10% H2O2) to oxidise any organic matter found had also been done,
followed by treatment with ammonia solution (0,01 N NH3-water) to remove the carbonate
components and prevent Ca ions from flocculating the clay particles in prepared suspensions.
Calcium fluoride standard was adopted where upon preparations of 200mg CaF2 mixed with
1g of soil sample were used.
The presence of swelling clay minerals (smectites) was investigated by measuring and
comparing diffractograms from filtered air-dried specimens with those from filtered and
glycolised specimens. Glycolisation involved heating specimens to 40 °C to collapse the
layers of any montmorillonite present, followed by heating in a chamber of glycol
atmosphere. Usually glycol has the same effect as water and, in the presence of smectites,
would go in-between the layers and move them apart, thereby causing pronounced expansion.
The difference in the amount of impulse (reflex intensity) between the resulting
diffractograms and / or smectite peaks and those from air-dried specimens would therefore be
significant (Reynolds and Moore, 1989; Sattler, 2000).
The clay minerals were identified from basal reflections obtained from their sheet-like
structure and by matching the obtained diffraction patterns with calculated one – dimensional
diffraction profiles ( Reynolds and Hower, 1970; Reynolds and Moore, 1989; Sattler, 2000).
Primary non-clay minerals were also identified by scanning the preparations at selected
ranges; and include quartz, calcite, dolomite and iron oxides. The minerals were identified by
matching their characteristic diffraction peaks with known standards.
6.3.2 Results
The results of percentage composition of clay minerals in the black clays are summarised in
Table 6.3, while the corresponding diffractograms are given in Figures 6.1 to 6.6. The
diffractograms obtained for the red soils are presented in Figures 6.7 to 6.10. However, it was
not possible to determine the percentage clay mineral composition of the red soils. This is
most probably due to the high contents of iron in these soils which caused formation of very
thin layers of specimens on glass slides, thus making it difficult to prepare oriented samples
necessary for drop analysis and/ or pipette analysis methods.
The results of the analyses show the black clays to be composed of over 90% smectites
(montmorillonite: CaNaMgFeAlSiOOH.HO) and less than 10% kaolinite (Al2Si2O5(OH)4).
Illite (K-Na-Mg-Fe-Al-Si-O-H2O) occasionally occurs in trace form, and is probably an
alteration product of the feldspars and / or kaolinite. Also found contained are quartz (4-9%)
and accessories of K-feldspars [sanidine (K,Na)AlSi3O8), orthoclase (Na, K)Si3O8),
microcline (KAlSi3O8)], haematite (Fe2O3) and carbonates, i.e.calcite (CaCO3), dolomite
(CaMg(CO3)2, ankerite (Ca(Fe,Mg)(CO3)2, siderite (FeCO3)].
According to Coduto (1994), montmorillonite often results from weathering of
ferromagnesian minerals, calcic feldspars and volcanic materials. In addition, sodium
montmorillonite is often formed from weathering of volcanic ash, while other
montmorillonites form in environments of alkaline conditions characterised by a supply of
magnesium ions and a lack of leaching. The montmorillonite in black clays of the present
study must have resulted from weathering and alteration of volcanic ash previously deposited
in a basin-like lake environment. Additional contribution in montmorillonite formation

61
resulted most probably from supplied magnesium and other cations leached down from
surrounding higher areas and deposited under conditions of impeded drainage and high
alkalinity in the basin. The alkalinity (contributed by seepage waters containing soluble bases
of Mg, Ca, Na, K leached down from higher areas of red soils) served to facilitate the Si/Al
build-up and properties characteristic of montmorillonite. The impeded drainage further
serves to retain most of the above soluble bases necessary in the build-up of montmorillonite
structure. Presence of accessories of haematite could be attributed to transported components
from areas of red soils.
Table 6.3. Clay mineral composition of black clays.
Sample No. Smectites
(montmorillonite)
(%)
Il lite
(%)
Kaolinite
(%)
Quartz
(%)
K-feldspars
(sanidine,
orthoclase)
SA2-70cm >90 - 90 trace 95 - 95 - 95 - 95 trace 95 - 95 - 95 - 95 trace

62
800
Impulse
700
600
500
SA 2 - 105cm + Standard
Q: Quartz
Sm: Smectites
K: Kaolinite
Or: Orthoclase
Sn: Sanidine
Mc: Microcline
H: Haematite
Cc: Calcite
Ak: Ankerite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
Sn
F
232
F
F
400
300
200
100
Quartz: 4%
Calcite: 3% (s.a. Scheibler)
Accessories: K - Feldspar, Haematite,
Dolomite, Ankerite
Rest: Clay minerals
(S. Texture preparation)
Sm
Mc
K Sn Sn
57
K
K Or
Sm
Q Sn
Sn
Sn
Or
Mc
Q
Cc
32
Sn
Tr
K
Sn
Cc
Sm Cc
Q K H Sn
Q
Cc QAk
Q
H
Cc
Q
Sm
H
Cc
Q
F
0.0
0 20 40 60 [ ° 2 ]
Figure 6.2. X-ray diffraction diagram of black clays collected at 1,05m depth.
800
Impulse
700
600
500
SB 1 -70cm + Standard
Q: Quartz
Sm: Smectites
I: Illite
K: Kaolinite
Or: Orthoclase
Sn: Sanidine
Mc: Microcline
H: Haematite
Sd: Siderite
Ak: Ankerite
D: Dolomite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
Mc
Sn
F
275
F
F
400
300
Quartz: 4%
Accessories: K -Feldspars, Haematite
Carbonates
Rest: Clay minerals
(S. Texture preparation)
I
Q
Tr
200
100
Sm
I
K
Mc
Sn Sn
I
K
Sm
56
Or Or
Sn K
D
Sn
Sn
Sm
Q H Or
Sd Q Sn
Q
Sd
Ak
Q
Or
Sm
Q
D
H
Q
0.0
0 20 40 60 [ ° 2 ]
Figure 6.3. X-ray diffraction diagram of black clays collected at 0,70m depth.

63
800
Impulse
700
600
500
SC 17 -50cm + Standard
Q: Quartz Td: Tridymite
Sm: Smectites
K: Kaolinite Cr: Cristobalite
Or: Orthoclase
Sn: Sanidine
Mc: Microcline
H: Haematite
Sd: Siderite
Ak: Ankerite
D: Dolomite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
F
256
F
F
400
300
Quartz: 4%
Accessories: K -Feldspars, Haematite,
Carbonates
Rest: Clay minerals
Q
(S. Texture preparation)
Sn
66 Mc
Tr
200
100
Sm
Or
K SnOr
K
Q
Sm Or
Q Sn
Sm
Td Sn
Cr Or Mc
K
Sn
Sd Sn
H Q Q
Q
Sd
Ak
Q
H
Sm
Q
D
H
Q
0.0
0 20 40 60 [ ° 2 ]
Figure 6.4 X-ray diffraction diagram of yellow-brown to black clays collected at 0,50m depth.
800
Impulse
700
600
500
SC 33 -30cm + Standard
Q: Quartz Td: Tridymite
Sm: Smectites Cr: Cristobalite
Or: Orthoclase
Sn: Sanidine
Mc: Microcline
H: Haematite
Sd: Siderite
Ak: Ankerite
D: Dolomite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
Q
F
252
F
F
400
Quartz: 9%
Accessories: K -Feldspars, Haematite,
Mc
Carbonates
I Sn
Rest: Clay minerals
(S. Texture preparation)
Tr
300
125
200
100
Sm
Or
Or
Sn Sn
Sm Or
Q
Sn TdCr
Sn
Or
Ak
Sm
Q H Sd
Q Q
Sd Sn
Sn
D
Q
Sd
Ak
Q
AkSm
D
Q D
H H
Q
Sd
0.0
0 20 40 60 [ ° 2 ]
Figure 6.5. X-ray diffraction diagram of black clays collected at 0,30m depth.

64
800
Impulse
700
600
500
SC 33 -50cm + Standard
Q: Quartz Td: Tridymite
Sm: Smectites Cr: Cristobalite
Or: Orthoclase
Sn: Sanidine
Mc: Microcline
H: Haematite
Sd: Siderite
Ak: Ankerite
D: Dolomite
Q
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
F
254
F
F
400
Quartz: 7%
Accessories: K -Feldspars, Haematite,
Carbonates
Rest: Clay minerals
(S. Texture preparation)
300
200
100
Sm
Or Or
Sn Sn
Sm
Q
Td
Cr
Or
98 Sn
Sn Mc
Mc
Sm
Ak Q Sd
D
Sd Q
Or
Sn
H Q Sn
Mc
Tr
Mc
Q
Q
Sd
Ak
Q
H
Sm
Q
D
H
Q
0.0
0 20 40 60 [ ° 2 ]
Figure 6.6. X-ray diffraction diagram of black clays collected at 0,50m depth.
The diffractograms for the red soils show them as containing mainly kaolinite and haematite
with accessories of quartz. According to Day (2001) and Holtz & Kovacs (1981), kaolinite
belongs to kaolin minerals, i.e. a group of clay minerals consisting of hydrous aluminium
silicates. Approximate mineralogical compositions of red soils could therefore be estimated
from results of chemical analyses (Table 6.1) obtained for the soils. This was done by
assuming that the kaolinite portion is constituted mainly by SiO2 and Al2O3; and haematite
by Fe2O3. The resulting estimated mineralogical compositions for the red soils are presented
in Table 6.4, and show the red soils as being mainly kaolinite (80-81%) and haematite (15-
16%) sediments; with accessories of quartz (1-3%) as well as minerals of titanium (about 2%)
and manganese (about 1%). The kaolinite must have resulted from weathering and alteration
of aluminiumsilicate-rich feldspars in the underlying Nairobi trachytes and other volcanic
materials, under humid conditions (rainfall) and in the presence of carbon dioxide gas.
Table 6.4. Estimated mineralogical composition of red soils.
Sample No. Kaolinite
(%)
Haematite
(%)
Quartz
(%)
Titanium
(TiO2)
Manganese
(MnO2)
(%)
Rd1-30cm 80 15 1 2 1
Rd1-100cm 81 16 3 2 1
Rd1-200cm 80 16 2 2 1
Rd1-400cm 81 15 1 2 1
In addition, hydrothermal alteration of the aluminiumsilicates could have formed
pseudomorphs of kaolinite from feldspars and/or muscovite, topas, leucite, andalusite, and
pyrophyllite contained in the rocks. Kaolinite-rich red soils containing alkali and alkali-earth
metals (Na, K, Mg, Ca) usually alter during favourable conditions into secondary feldspars,

65
sericite and chlorite (see next section on scanning electron microscope studies); while those
deficient in the alkali and alkali-earth metals alter into aluminiumsilicates of andalusite,
sillimanite and kyanite.
Theoretical chemical compositions of the minerals kaolinite, illite and montmorillonite are
given in Table 6.5. A comparison of the results in this table with those of chemical analyses in
Table 6.1 suggest the kaolinite in red soils to be depleted in Al2O3. The depletion is most
probably a result of low PH conditions caused by formation of weak carbonic acids in the
soils through organic matter decomposition and/ or hydrothermal alteration at significant
temperatures.
Table 6.5. Theoretical compositions of kaolinite, illite and montmorillonite
(Jasmund & Lagaly, 1993; Moore & Reynolds, 1989; Rösler, 1980).
% content of: Smectites
Il lite Kaolinite
(montmorillonite)
SiO2 48-56 51-54 46,55
Al2O3 11-22 21-29 39,5
H2O 12-24 - 10,0-13,95
Fe2O3 ≥5 0,5-5,3 Trace
FeO - 0,9-1,2 -
BaO - - Trace
MgO 4-9 2,8-3,5 Trace
CaO 0,8-3,3 ,02-,05 Trace
Na2O trace ,08-,10 Trace
K2O trace 2-9 Trace
P2O5 - 0,6-0,82 -
ZrO2 - - -
TiO2 - 0,60-0,82 Trace
MnO - - -
Loss on ignition - - -
Total >80 >79,5 96,0-100,0
800
Impulse
RD 1 - 30cm + Standard
F
F
700
600
Q: Quartz (1%)
H: Haematite
K: Kaolinite
F: Fluorite (CaF2- Standard)
Tr: Specimen holder
500
F
400
214
300
Tr
200
100
K
K
Q
Q
18
K
H
H
K
Q
K
H
Q
H Q
H
H
Q
H
K
H
Q
F
0
0 20 40 60 [ ° 2 ]
Figure 6.7. X-ray diffraction diagram of red soils collected at 0,30m depth.

66
800
Impulse
700
600
RD 1 - 100cm + Standard
Q: Quartz (3%)
H: Haematite
K: Kaolinite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
F
216
F
500
F
400
Tr
300
200
100
K
K K Q
Q
35
K
H
H
H
K
K
Q
K
H
Q
H
H
K
Q
H
K
H
F
Q
0
0 20 40 60 [ ° 2 ]
Figure 6.8. X-ray diffraction diagram of red soils collected at 1,0m depth.
800
Impulse
700
600
500
400
300
200
100
RD 1 - 200cm + Standard
Q: Quartz (2%)
H: Haematite
K: Kaolinite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
K
Q H
Q
24
F
211
H
K
H K
Q K
H
Tr
F
H Q
H
F
Q
H
K
H
F
H
0
0 20 40 60 [ ° 2 ]
Figure 6.9. X-ray diffraction diagram of red soils collected at 2,0m depth.

67
Q: Quartz (1%)
H: Haematite
K: Kaolinite
F: Fluorite (CaF2-Standard)
Tr: Specimen holder
204
16
H
0.0
Figure 6.10. X-ray diffraction diagram of red soils collected at 4,0m depth.
6.4 Scanning electron microscope (SEM) analysis
6.4.1 Scope and method
The scanning electron microscope technique was applied on fresh undisturbed soil specimens
sliced into suitable volume (2cm*2cm*2cm). The analysis was also performed on prepared
specimens of the underlying phonolitic and trachytic rocks. The analysis involved establishing
the morphology, mode of occurrence relative to clayey matrix and diagenetic features of clay
and other minerals. Possible alterations of clay minerals to form other minerals (and vice
versa) were also sought and identified; while relative abundance and type of texture and/ or
fabric (flocculation, randomness, orientations, lamination) of minerals were also investigated
and established. In addition, the porosity, interconnection of pores and permeability
(possibility of water movement/ percolation) of the soils were assessed.
The analysis employed a Leiz Scanning electron microscope (Modell: ISI Super 40/3A)
manufactured by International Scientific Instruments (ISI, 1972). The microscope
incorporated a Eumex detector having Lithium fitted with an ultra-thin Beryllium window to
facilitate element measurements in the range of carbon (C) to Uranium (U). Use was also
made of Hardware for High Performance X-ray Micro-analysis (EDX: WINEDS) as well as
Wineds 3.0 & Edisson 32 Software for element analysis (Thomson Scientific Instruments Pty
Ltd. & Fa.Getac, 1995). In addition, Hardware for digital photo-processing with
Rasterelektronenmikroskop (REM PC Einsteckkarte); as well as WinDISS 2.0 Software for
digital photo-taking, were employed (Fa.Point Elektronic GmbH, 1997).

68
6.4.2 Results
6.4.2.1 Nairobi phonolite
Results of scanning electron microscope analysis show Nairobi phonolite to exhibit a
generally massive, homogeneous and compact structure (Plate 6.1). Texture is mainly of large
phenocrysts embedded in a fine-grained matrix (Plate 6.3). The phenocrysts show presence of
O, Al, Si, K implying K-feldspars; and some Fe and Ti probably due to inclusions of iron and
titanium ore. The phenocrysts show partly corroded and/ or leached and altered surfaces
(Plate 6.2). The matrix also shows an increased presence of Si, K and Fe, as well as some O
and Al, implying a predominance of K-feldspars with some iron oxides and/ or ores. The iron
oxides are most likely a result of weathering and/ or chemical decomposition of phonolite to
form tuffs as well as ferricrete and / or iron concretions (Plates 6.4/6.5). Occasional presence
of Ca in the matrix could be attributed to weathering of phonolitic tuff to form secondary
limestone and/or calcite (Plate 6.5).
Plate 6.1. Massive and compact structure
Plate 6.2. Corroded, leached K-feldspar

69
Plate 6.3. K-feldspar phenocryst in fine matrix (diagram: K-feldspar phenocryst).
Plate 6.4. Alteration of rock forming ferricrete
(diagram: composition of matrix).
Plate 6.5.Alteration of rock/tuff forming
ferricrete and secondary calcite.
(diagram: matrix).

70
6.4.2.2 Nairobi trachytes
Plate 6.6. Less compact structure formed by
plates of fine K-feldspar phenocrysts.
Plate 6.7. Relatively more porous structure
formed by tabular K-feldspar crystals.
Plate 6.8. Large tabular K-feldspar phenocrysts,
with Fe and Ti-rich crusts as part of matrix.
Plate 6.9. K-feldspar crystals with leaching
surfaces and solution pores.

71
The trachytes generally show a relatively less compact and more porous structure (Plates 6.6/
6.7), and are characterised by plates of tabular K-feldspar phenocrysts showing patched
overgrowths of Fe and Ca-rich crusts (Plate 6.8). The phenocrysts show a composition of O,
Al, Si, K and occasionally Na, and this is suggestive of sanidine. The feldspars show effects
of weathering and alteration through leached surfaces and solution pores (Plate 6.9). The
matrix is predominated by K-feldspars showing presence of O, Al, Si, K suggestive of
sanidine and/or orthoclase; while crusts rich in Fe and Ca also occur as weathering and
alteration products of the rock.
6.4.2.3 Black clays
The black clays exhibit a generally heavy, dense and massive appearance with solution pores,
cavities and / or cracks found within them (Plates 6.10, 6.11 & 6.24). Quartz grains and K-
feldspar crystals show overgrowths of Fe, Ca and Mg-rich crusts (Plates 6.12, 6.15); and
occur in a fine, weathered and generally smectitic clay matrix (Plate 6.23). The generally
smooth sub-rounded quartz grains (Plates 6.19, 6.20) indicate water transport, and this serves
to support the hypothesis that part of the materials forming black clays have been contributed
by erosional processes from the surrounding high areas. The feldspars show alteration effects
through corroded and leached solution surfaces (Plate 6.18). Apparent detrital components
showing presence of O, Ca, Fe, K, Si, Ti also occur and suggest biotite (Plate 6.14), a
probable result of weathered phonolitic and/ or metamorphic rock inclusions. Iron concretions
(Plates 6.17, 6.21) and blocks of ankerite (Plate 6.22) showing elevated Fe content are also
found embedded in the clay matrix. Traces of organic material (Plate 6.16) are also found.
Occasional weathered plates of kaolinite also form part of the clay matrix (Plate 6.13). The
presence of Fe and Ca-rich crusts could be attributed to underlying phonolitic rocks
weathering through tuff and ferricrete thereby forming secondary limestone and iron
concretions (murram); and partly to weathering of the smectites in the clay matrix. The
solution pores are a probable result of the iron, limestone and/ or other carbonate components
going into reaction/ solution, while the partings and cracks are most likely due to shrinkage
effects in the smectitic clay matrix in a dry condition. Presence of Ti and Zn in the matrix
could be attributed to heavy mineral inclusions transported from surrounding areas.
Plate 6.10. Massive, dense, heavy clays with
solution pores.
Plate 6.11. Partings, cracks due to shrinkage;
and solution pores.

72
Plate 6.12 Quartz grain with overgrowth of
Fe, Mg and Ca-rich crusts.
Plate 6.13. Weathered kaolinite (K) in
smectitic clay matrix with Fe and Ca crusts.
Plate 6.14. Inclusions of detrital biotite in
weathered clay matrix.
Plate 6.15. K-feldspar phenocryst in smectitic
clay matrix.

73
Plate 6.16. Organic matter (remains of plant roots) in a weathering smectitic clay matrix.
Plate 6.17. Iron concretion. Al, Si, K, Ti due
to contribution from clay matrix.
Plate 6.18. Corroded/ leached K-feldspar.
Fe, Ti and Zn due to clay matrix.

74
Plate 6.19. Sub-rounded quartz grains in
weathering smectitic matrix.
Plate 6.20. Sub-rounded quartz grain in clay
matrix.
Plate 6.21. Iron concretions (o) in smectitic
clay matrix (s).
Plate 6.22. Blocky ankerite in clay matrix.
Plate 6.23. Smectitic clay matrix exhibiting Al, Ca, Fe, and high Si content. Crusts with K,
Ti, Zn found associated.

75
Plate 6.24. Solution pores/ cavities in weathering smectitic clay matrix with Al, Ca, Fe,
and high Si content; and associated K, Ti crusts.
6.4.2.4 Red soils
The red soils show a relatively loose, porous and friable structure of weakly bound or
cemented lumps of clay matrix (Plate 6.25). Large grains of iron concretions and quartz are
embedded in a fine weathering clay matrix. The iron concretions show an elevated Fe content,
up to 70% by weight (Plate 6.29); and weather through leaching to form solution pores (Plate
6.30). Quartz grains are slightly abraded and rounded to sub-rounded (Plate 6.28), and are
probably detrital, having been derived by erosion and water transport. The relatively high
porosity of the soil structure (Plate 6.31) could therefore be partly attributed to binding and
cementation of clay matrix by iron oxide to form detached lumps; and partly to leaching of
soluble bases of Ca, Mg, K, Na; as well as solution effects caused by Fe going into reaction.
Plate 6.25. Loose, porous and friable structure
of red soils.
Plate 6.26. Lumps of cemented clay
matrix with Fe-rich crusts.
The fine matrix shows presence of Si and Al mainly, suggesting kaolinite; as well as Fe,
implying heamatite. Fine crusts bearing Ti, Mn and K also occur (Plate 6.27) . Kaolinite
generally occurs as fine weathered and altered plates which have lost the original six-sided
form and/ or characteristic crystal structure. The plates are commonly weakly bound
/cemented together and covered by fine Fe-rich crusts of matrix (Plate 6.28). Weathering and

76
alteration of kaolinite is most likely caused by low pH conditions that result in Aluminium
depletion (Table 6.1). Remains of organic matter in the form of plant roots are also found
(Plate 6.27). On the whole, presence of quartz is limited, while that of feldspar is not evident
and this could be attributed to possible weathering and alteration.
Plate 6.27. Organic matter intermingled with Fe-rich lumps of clay matrix
(matrix composition in diagram).
Plate 6.28. Abraded sub-rounded to rounded quartz grain in clay matrix of obscure fine
weathered kaolinite plates covered with Fe-rich crust (composition of matrix in diagram).

77
Plate 6.29. Iron concretion with elevated Fe content.
Plate 6.30. Solution pores/ cavities in
weathered iron concretion.
Plate 6.31. Porous structure of red soils.
6.5 Importance and influence of clay minerals on engineering properties of black clays
and red soils
Clay minerals are usually platy with a surface negative charge (Johnson and Degraff, 1988).
In the presence of water, electrical attraction of opposing charges on water molecules causes
the clay particles to form clusters and/ or flocculate and settle down in the suspension.
According to Day (2001), clay minerals serve to influence soil engineering properties such as
plasticity, swelling, shrinkage, shear strength, consolidation and permeability. They therefore
account for the increase in volume and/ or swelling which occurs when clay soils are allowed
free access to water. Different clay minerals exhibit stronger or weaker negative charges and
this in turn determine the relative degree and ease with which they flocculate and cause the
clay soils involved to swell/ expand, i.e. the reactivity of the clay soil.
Clay mineralogy is an important controlling factor of the plasticity of clay soils (Johnson and
Degraff, 1988). In practice, kaolinite and illite are classified as nonexpansive clay minerals
and are characterised by relatively lower values of plasticity indices (PI). According to Holtz
and Kovacs (1981), kaolinite is a relatively inactive clay mineral so that even though it is

78
technically a clay, it behaves more as a silt material. In this study, the red soils were found to
show relatively lower plasticity indices (18-22%) as well as lower swelling capabilities (free
swell: 15-20%) and activity values (0,83-1,03); and this could be attributed to the higher
kaolinite content (80% and over) which characterises these soils.
On the other hand, montmorillonite belongs to a group of clay minerals that are characterised
by weakly bonded layers (Day, 2001). Each layer consists of two silica sheets with an
aluminium (gibbsite) sheet in the middle. Water and exchangeable cations (Na, Ca, etc.) can
enter and separate the layers, creating a very small crystal that has a strong attraction for
water. As a result, montmorillonite has the highest activity (A = 4-7) and usually exhibits the
highest water content, greatest compressibility, and lowest shear strength of all the clay
minerals.
Montmorillonite usually harbours a relatively larger negative surface charge and readily
attracts water molecules by adsorption (Johnson and Degraff, 1988). It is therefore classified
as expansive and would usually exhibit a wide range of PI values depending on adsorbed
cations available to satisfy its charge deficiencies. The charge deficiencies are better satisfied
by calcium-rich pore water (due to higher calcium valence of 2) which also results in
improved interparticle bonding so that the amount of water needed for complete charge
neutralisation would be considerably reduced. As a result, calcium-rich montmorillonite
would exhibit relatively lower plasticity indices. On the other hand, sodium-rich pore water
would result in sodium-rich montmorillonite which usually requires more water (due to lower
valence of 1) for complete charge neutralisation so that the plasticity indices are also
comparatively higher. The black clays involved in this study are characterised by
comparatively higher plasticity indices (39-55%), swelling capabilities (free swell: 100-
145%) and activity values (1,10-3,38); and this could be related to the higher content of
smectites (90% and over) in these soils. Results of mineralogical analyses obtained in this
study suggest the black clays to be more of calcium-rich montmorillonite than sodium-rich
montmorillonite.
Illite has a structure similar to that of montmorillonite, but the layers are more strongly
bonded together (Day, 2001; Holtz & Kovacs, 1981). It is usually intermediate in activity (A
= 0,5-1,3) between kaolinite and montmorillonite in terms of its cation exchange capacity,
ability to absorb and retain water, as well as physical characteristics such as plasticity index;
and often plots just above the A-line on the plasticity chart. In this study, traces of illite were
noted in black clays, and are most probably a result of alteration of feldspars and / or
kaolinite.
6.6 Carbon and sulphur content of soils
6.6.1 Method of analysis
Determination of carbon and sulphur contents of soil was carried out using the Leco CS-225
apparatus under conditions of pressurised gas (99,5% oxygen). Two standards were employed
in the analysis, i.e.
Leco-Iron filings 3,32%C + 0,063%S, and
Leco-Steel-ring specimen 0,358%C + 0,0178%S.

79
The CS-225 is a micro-processor analysis system for determination of carbon and sulphur
contents through infrared absorption. Iron and tungsten filings acting as catalysts were added
to the soil specimen which was then heated in a high frequency induction oven. The
combustion/ heating gases were dried and then directed to the infrared sulphur cell for the
measurement of the sulphur dioxide content. The gases were further passed through a
catalysing oven where carbon monoxide (CO) and sulphur dioxide (SO2) were converted into
carbon dioxide (CO2) and sulphur trioxide SO3, respectively. The SO3 was collected in a
sulphur-trap, thereby permitting measurement of the total carbon content in terms of CO2 in
the infrared carbon cell. The results of the analysis were calculated by the microprocessor
system with the aid of calibration factors and after applying relevant weight compesations.
The organic carbon content was determined by first pretreating the soil specimen with
concentrated hydrochloric acid to remove the carbonate part. The excess acid was evaporated
by heating on a hot plate and finally removed by passing over potassium iodide and antimony
metal.
The sulphur content of both the red soils and black clays was found to be negligible. The
results are therefore not reported.
6.6.2 Results
Results of carbon content determinations of soils performed in this study are given in Table
6.6. The black clays exhibit 0,50-1,48% total carbon (mean: 1,17%) and 0,22-1,19% organic
carbon (mean: 0,93%); and therefore 0,04-0,74% inorganic carbon (mean: 0,24%). The red
soils have 0,45-2,47% total carbon (mean: 1,12%), 0,35-1,90 organic carbon (mean: 0,85%)
and 0,10-0,57% inorganic carbon (mean: 0,27%). The total carbon content in both types of
soil is generally low (less than 2,5%) so that difficulties were encountered in trying to
separate and determine the organic and inorganic components during laboratory analysis. In
this study, therefore, the total carbon and organic carbon contents were determined; and their
numerical difference adopted as the inorganic carbon content, i.e.
Total carbon (%) = organic carbon (%) + inorganic carbon (%), or
Ctotal (%) = Corganic (%) + Cdiff. (%),
where the numerical difference, Cdiff., corresponds to the inorganic carbon component.
The organic carbon component serves to represent the organic matter content of the soils. The
inorganic carbon component represents the carbonate content occurring in the soils in the
form of calcite, dolomite, ankerite, siderite and/or other inorganic carbonates. The Scheibler
method for determination of carbonate content of soils could not be applied in the present
analysis due to the generally low carbonate content (less than 5%) of the black clays and red
soils. This analysis method depends on atmospheric conditions of temperature and pressure,
and is therefore only applicable to soils with carbonate contents of over 5%.

80
Table 6.6. Carbon content of soils of the study area.
Sample No. Total carbon;
Ctotal (%)
Organic carbon;
Corganic (%)
Inorganic carbon;
Cdiff. (%)
SA1-30cm 1,16 1,04 0,12
SA1-50cm 1,48 0,81 0,67
SA2-70cm 0,50 0,46 0,04
Black clays SA2-105cm 0,96 0,22 0,74
SA41-50cm 1,14 1,02 0,12
SB1-50cm 1,13 1,00 0,13
SB1-70cm 1,19 1,08 0,11
SB42-30cm 1,34 1,19 0,15
SB42-50cm 1,30 1,17 0,13
SC17-50cm 1,29 1,08 0,21
SC33-30cm 1,39 1,19 0,20
Rd1-30cm 2,47 1,90 0,57
Red soils Rd1-100cm 0,73 0,54 0,19
Rd1-200cm 0,83 0,61 0,22
Rd1-400cm 0,45 0,35 0,10
6.6.3 Analysis of results
The variation of carbon content of soils with depth, Dp, is represented diagrammatically in
Figures 6.11, 6.12 and 6.13.
Red soils: carbon content/ depth
3,00
carbon (%)
2,50
2,00
1,50
1,00
0,50
Ctotal = 1,0356Dp -0,6115
R 2 = 0,8831
Corg = 0,7821Dp -0,6116
R 2 = 0,8766
total carbon
organic carbon
inorganic carbon
Potential (total carbon)
Potential (organic
carbon)
Potential (inorganic
carbon)
Cdiff = 0,2526Dp -0,6128
0,00
R 2 = 0,8871
0,00 1,00 2,00 3,00 4,00 5,00
depth Dp (m)
Figure 6.11. Variation of carbon content of red soils with depth.

81
The red soils generally exhibit a decrease in their carbon contents with depth (Fig. 6.11). The
decrease of the organic carbon is indicative of a general decrease of the organic matter content
of the soils with depth. The decrease of the inorganic carbon could be related to the good
drainage conditions of the soils which favour leaching and removal of carbonates as well as
other soluble components of Mg, Ca, Na and K. The variation of total carbon, organic carbon
and inorganic carbon contents of these soils with depth is best described by a potential
relationship with strong correlations (R = 0,94; R = 0,94; and R=0,94, respectively).
Generally, however, the soils exhibit higher contents of organic carbon than inorganic carbon.
The total carbon and organic carbon contents of black clays generally decrease with depth
(Fig. 6.12); and this is most probably due to the decrease of organic matter content of the soils
with depth. On the contrary, the inorganic carbon component is found to generally increase
with depth, and this could be attributed to the weathering of the underlying volcanic tuffs
which usually results in the formation of secondary limestone in situ. The variation of total
carbon with depth could be described by a polynomial relationship, but with a relatively
weaker correlation (R = 0,56); while the variation of organic carbon with depth is best
approximated by an exponetial relationship with a strong correlation (R = 0,85). On the other
hand, variation of inorganic carbon content with depth tends to fit a polynomial relationship,
with a moderately strong correlation (R = 0,63).
Black clays: carbon content/ depth
1,60
1,40
1,20
Ctotal = 0,5235Dp 2 - 1,3301Dp + 1,7069
total carbon
carbon (%)
1,00
0,80
0,60
0,40
R 2 = 0,3146
Corg = 2,4918e -2,0216Dp
R 2 = 0,7303
CDiff = 1,5224Dp 2 - 1,4352Dp + 0,5028
organic carbon
inorganic carbon
Polynomial (total
carbon)
Exponential
(organic carbon)
Polynomial
(inorganic carbon)
0,20
R 2 = 0,3943
0,00
0,00 0,20 0,40 0,60 0,80 1,00 1,20
depth Dp (m)
Figure 6.12. Variation of carbon content of black clays with depth.
Combined results of black clays and red soils also show a general decrease of total carbon as
well as organic carbon with depth (Fig. 6.13). The results also reveal approximating potential
relationships with strong correlations in the variation of total carbon (R = 0,77) and organic
carbon (R = 0,75) with depth. However, no well defined relationship occurs to describe the
variation of combined data of inorganic carbon of the two types of soil with depth.

82
Combined black and red soils: carbon content/ depth
3,00
2,50
2,00
carbon (%)
1,50
Ctotal = 0,8709Dp -0,4442
R 2 = 0,5916
total carbon
organic carbon
inorganic carbon
Potential (total carbon)
Potential (organic carbon)
1,00
0,50
Corg = 0,618Dp -0,5706
R 2 = 0,5645
0,00
0,00 1,00 2,00 3,00 4,00 5,00
depth Dp (m)
Figure 6.13. Variation of carbon content of both black clays and red soils with depth.

83
Chapter 7
Laboratory soils index and engineering properties determination
7.1 Moisture content and index tests
7.1.1 Introduction
The performance of soil index tests usually involves the measurement of moisture content of
soils, both in the natural state as well as under certain defined test conditions (i.e. consistency
limits). Index tests serve to investigate the way in which the amount of water in soils can
influence their behaviour. They also provide a useful method of classifying cohesive soils and
of assessing their engineering properties (Head, 1984).
The index properties of soils investigated in this study include natural moisture content,
Atterberg limits (liquid and plastic limits, plasticity index), swelling capability (free swell)
and linear shrinkage.
Natural moisture content and Atterberg limits usually serve to describe and provide a
sufficient understanding of the nature of clay soils, as may be required by many applications
in engineering practice; and this especially when coupled with a knowledge of the geological
history of the soils (Atterberg, 1911; Nelson and Miller, 1992).
7.1.2 Natural moisture content
Determination of natural moisture content of soils was carried out in this study by ovendrying
method and in accordance with British Standard (BS 1377; 1975, Test 1(A)) and
German Standard, i.e. Deutsches Institut für Normung (DIN 18 121, Teil 1). This testing
procedure specifies a standard drying temperature of 105-110 °C.
Soil samples used for natural moisture content determination had been previously collected
from the field, whereupon they were immediately well sealed in polythene bags to prevent
moisture loss. The moisture content so determined also represents the moisture content of
natural undisturbed soil in situ.
Natural moisture content, Wn, of a soil is usually expressed as a percentage of its dry mass
(Head, 1984); and is given by the equation
natural moisture content = (loss of moisture/dry mass) * 100%, or
Wn (%) = ((m2-m3)/(m3-m1)) * 100 (7.1)
where m1 = mass of empty container
m2 = mass of container + wet soil
m3 = mass of container + dry soil
In this study, up to three separate moisture content determinations were performed on each
soil sample and the average value calculated. Results of natural moisture content
determination of soils performed in this study are presented in Table (7.2); and are reported to
the nearest whole number or 1%.

84
7.1.3 Atterberg limits and consistency of clay soils
7.1.3.1 Scope
The Atterberg limits serve to define the range of moisture contents within which a clay soil
exhibits a plastic consistency by measuring and describing the plasticity range in numerical
terms (Nelson and Miller, 1992).
The Atterberg limits of measurement adopted in this study to describe the consistency of soils
include the liquid limit (LL) and the plastic limit (PL). The liquid limit is the moisture content
at which a soil passes from the plastic to the liquid state, while the plastic limit represents the
moisture content at which a soil passes from the plastic to the solid state and becomes too dry
to be in a plastic condition. The plasticity index (PI) is the moisture content range between the
plastic and liquid limits, and serves as a measure of the plasticity of the clay soil (Nelson and
Miller, 1992).
In this study, the state of consistency of a clay soil in its natural state was numerically
described by relating its natural moisture content to the plastic and liquid limits using two
parameters, i.e. the relative consistency (Cr) and the liquidity index (LI). This is because in
general, clay soils occur as a slurry (liquid state) when moisture contents exceed their liquid
limits, while they exhibit a firm and plastic consistency with moisture contents lying within
the plasticity range, i.e. between plastic and liquid limits. On the other hand, clay soils with
moisture contents below their plastic limits would usually exhibit a hard to stiff consistency
(Head, 1984). According to Terzaghi and Peck (1948), relative consistency, Cr, is the ratio of
the difference between liquid limit and moisture content to the plasticity index, i.e.
Cr = (LL-W)/(LL-PL), or
Cr = (LL-W)/PI (7.2)
where W = moisture content
Lambe and Whitman (1969) defined the liquidity index, LI, as the ratio of the difference
between moisture content and plastic limit to the plasticity index, i.e.
LI = W-PL/PI (7.3)
According to Skempton and Northey (1953) and Wood and Wroth (1978), clay soils usually
exhibit increasing shear strength values as their state of consistency varies with progressive
decrease of moisture content, starting from the liquid limit down to the plastic limit. This
study tries to investigate and establish any possible consistent form of relationship between
shear strength parameters and liquidity index and/ or relative consistency for the clay soils
found.
Skempton (1953) showed that the Atterberg limits of clays are related to the particle size and
clay mineralogical composition of the soils. He noted that the the plasticity index, PI, is
dependent on the proportion of clay fraction, and defined colloidal activity (A) of a clay soil
as the ratio between these two parameters, i.e.
activity A = PI/ clay fraction (7.4)

85
According to Johnson and Degraff (1988), clay minerals are usually platy with a negative
surface charge which causes them to readily adsorp water. The presence of clay minerals in a
soil therefore causes it to increase in volume or expand in the presence of water. Different
clay minerals exhibit stronger or weaker negative charges which in turn serve as a measure of
the activity of the clay soil in terms of the relative degree with which the negative charge
would influence opposing charges in adsorped water to cause the clay particles to form
clusters or flocculate and settle down in a suspension of water. As a result, the amount and
type of clay minerals present in a soil have a significant effect on soil engineering properties
such as plasticity, swelling, shrinkage, shear strength, consolidation and permeability.
Table 7.1. Activity of clay soils and clay minerals (After Skempton, 1953;
Mitchell, 1976; and Day, 2001).
CLAY SOILS
ACTIVITY
Inactive clays < 0,75
Normal clays 0,75-1,25
Active clays 1,25-2,0
Highly active clays > 2,0
Extremely active clays, e.g. bentonite 6 or more
CLAY MINERALS
APPROX. ACTIVITY
Kaolinite 0,3-0,5
Il lite 0,5-1,3
Na-montmorillonite 4,0-7,0
Ca-montmorillonite 1,5
Granular soils, quartz 0
THIS STUDY
ACTIVITY
Black clays 1,1 – 3,38 (mean value: 1,87)
Red soils 0,83 – 1,03 (mean value: 0,94)
The colloidal activity is usually more or less constant for a particular type of clay soil. A
suggested classification of clay soils based on colloidal activity is given in Table (7.1). A
similar classification of common clay minerals is also included in the table. Ranges of activity
values for black clays and red soils of this study are also given for comparison purposes. A
whole list of calculated activity values from results of plasticity indices and grain size analysis
obtained in this study is included in Table (7.3). An activity classification of the soils in this
study is also presented alongside. The activity state and/ or activity level of black clays and
red soils of this study is illustrated diagrammatically in the activity chart (Fig. 7.2, after
NAVFAC, 1982).

86
7.1.3.2 Plastic limit
Plastic limit tests were carried out in this study to determine the lowest moisture content at
which the clay soils found are plastic. The tests were carried out according to British Standard
(BS 1377: 1975, Test 3) and German Standard (DIN 18 122, Teil 1); by using a suitable
amount of disturbed soil previously oven-dried (105 °C over a 12-24 hour period) and sieved
to provide a homogeneous material.
The testing method is shown in Plate (7.1) in which a uniform pressure was used to roll
threads of a homogeneous soil paste between the fingers of one hand and a glass plate from
about 6 mm diameter size until they just crumbled at about 3 mm diameter. The moisture
content of the threads at the first crumbling point was determined. In this study, a total of
three separate moisture content determinations of the threads was carried out for each soil
sample and an average value calculated and taken as the plastic limit (PL) of the soil. Results
of plastic limit of soils obtained in this study are reported to the nearest whole number (1%)
alongside those of liquid limit and plasticity index in Table (7.2).
Plate 7.1. Plastic limit determination in this study.
7.1.3.3 Liquid limit
Liquid limit determination of soils in this study was carried out using the Casagrande
apparatus (Casagrande, 1958), and in accordance with British Standard (BS 1377: 1975, Test
2(B)) and German Standard (DIN 18 122, Teil 1). The apparatus used in this study is shown
in Plate (7.2). The test involved applying bumps or blows to the cup of the apparatus so as to
close a V-shaped groove cut in the middle of a homogeneous soil paste contained in the cup.
The moisture content of the paste at which a total of 25 bumps closed the groove along a
continuous distance of 10 mm was determined and recorded as the liquid limit (LL) of the
soil.

87
Table 7.2. Results of index tests performed on black clays and red soils in this study.
Profile Sample No. Wn (%) LL (%) PL (%) PI (%) Cr LI FS (%) LS (%)
PI/LL class
(BS 1377) Consistency
SA1-30cm 31,6 78,2 37,2 41,1 1,136 -0,14 135 25 MV semi-solid
A SA1-50cm 30 74,9 36,3 38,6 1,163 -0,16 100 23 MV semi-solid
SA2-30cm 33,7 92,6 38,8 53,9 1,094 -0,09 135 27 CE semi-solid
(black clays) SA2-70cm 34,1 90 36,3 53,7 1,04 -0,04 170 25 CV-CE semi-solid
SA2-105cm 29,7 88,1 32,9 55,2 1,059 -0,06 120 26 CV semi-solid
SA28-30cm 25,6 84,8 39,7 45,1 1,313 -0,31 125 25 MV-CV semi-solid
SA28-50cm 26,8 88,5 41,3 47,2 1,307 -0,31 135 25 MV-CV semi-solid
SA37-30cm 19,4 80,9 38,1 42,7 1,439 -0,44 115 24 CV semi-solid
SA37-50cm 21,8 83,3 39,2 44,2 1,393 -0,39 115 26 CV semi-solid
SA37-80cm 23,1 85,1 39,2 45,9 1,35 -0,35 115 26 CV semi-solid
SA41-30cm 23 88,5 41,3 47,2 1,387 -0,39 135 26 CV semi-solid
SA41-50cm 24,9 89,7 41,1 48,6 1,334 -0,33 120 24 CV semi-solid
SB1-30cm 23 81,7 34,3 47,4 1,237 -0,24 125 23 CV semi-solid
B SB1-50cm 26,7 83,7 36,1 47,6 1,197 -0,2 135 24 CV semi-solid
SB1-70cm 29,7 89,9 39,1 50,8 1,187 -0,19 145 25 CV semi-solid
(black clays) SB5-30cm 24 80,9 34 47 1,212 -0,21 120 23 CV semi-solid
SB5-50cm 26,8 84,6 36,6 48 1,205 -0,2 135 24 CV semi-solid
SB5-70cm 30,5 91,1 40,2 51 1,19 -0,19 145 26 CE semi-solid
SB7-30cm 23,2 82,9 34,6 48,3 1,235 -0,23 130 24 CV semi-solid
SB7-50cm 26,1 85,8 37 48,8 1,222 -0,22 135 24 CV semi-solid
SB7-70cm 29,6 87 38,3 48,7 1,179 -0,18 140 25 CV semi-solid
SB18-30cm 20 82,4 39,8 42,6 1,463 -0,46 120 23 MV-CV semi-solid
SB18-50cm 20,1 72,4 33,6 38,8 1,346 -0,35 120 21 CV semi-solid
SB27-30cm 21,6 85 38 47,1 1,348 -0,35 130 24 CV semi-solid
SB27-50cm 22,2 87,7 39,2 48,5 1,352 -0,35 130 25 CV semi-solid
SB31-30cm 23,4 86,9 38,1 48,8 1,3 -0,3 130 24 CV semi-solid
SB31-50cm 22,9 86,1 37,6 48,4 1,304 -0,3 130 25 CV semi-solid
SB41-30cm 23 88,8 44,6 44,1 1,491 -0,49 145 26 MV semi-solid
SB41-50cm 24 90,7 43,6 47,1 1,415 -0,42 145 25 MV semi-solid
SB42-30cm 22 87,6 43,8 43,8 1,499 -0,5 145 25 MV semi-solid
SB42-50cm 21,3 91,1 44,5 46,6 1,498 -0,5 145 26 ME-CE semi-solid
SC1-30cm 24,2 82 34,5 47,5 1,218 -0,22 125 23 CV semi-solid
C SC1-50cm 25,9 84,9 36,7 48,2 1,224 -0,22 135 24 CV semi-solid
SC5-30cm 23 83,1 34,7 48,4 1,24 -0,24 125 23 CV semi-solid
(black clays) SC5-50cm 25,1 83,5 36,2 47,2 1,236 -0,24 135 24 CV semi-solid
SC9-30cm 24,8 83,6 35,6 48 1,223 -0,22 130 24 CV semi-solid
SC9-50cm 26 84,6 37,1 47,5 1,233 -0,23 135 24 CV semi-solid
SC17-30cm 21,2 75,7 34,7 41 1,329 -0,33 105 25 CV semi-solid
SC17-50cm 22 80,7 32,5 48,2 1,219 -0,22 110 24 CV semi-solid
SC25-30cm 26,1 87,9 39 48,9 1,265 -0,27 125 25 CV semi-solid
SC25-50cm 25,8 88,6 41 47,6 1,321 -0,32 145 25 MV semi-solid
SC29-30cm 20,7 81,5 32,2 49,3 1,235 -0,23 110 25 CV semi-solid
SC29-50cm 20,4 80,2 36,8 43,4 1,379 -0,38 110 26 CV semi-solid
SC33-30cm 20,3 81,7 32,5 49,1 1,248 -0,25 120 25 CV semi-solid
SC33-50cm 21,4 82,9 33 49,9 1,233 -0,23 120 26 CV semi-solid
SC41-30cm 17,7 85,2 37,8 47,4 1,422 -0,42 125 29 CV semi-solid
SC41-50cm 17,2 83,6 36,6 47 1,412 -0,41 125 29 CV semi-solid

88
Table 7.2, continued. Results of index tests performed on black clays and red soils in this
study.
Profile Sample No. Wn (%) LL (%) PL (%) PI (%) Cr LI FS (%) LS (%)
PI/LL class
(BS 1377) Consistency
SD1-30cm 26,5 82,3 35,9 46,4 1,203 -0,2 125 26 CV semi-solid
D SD1-50cm 27,1 83 36,1 46,9 1,19 -0,19 130 27 MV-CV semi-solid
SD1-80cm 29,5 91,1 41,7 49,4 1,248 -0,25 140 26 ME-CE semi-solid
(black clays) SD5-30cm 26,2 83,5 36,4 47,1 1,217 -0,22 125 26 CV semi-solid
SD5-50cm 27,4 84,2 36,4 47,9 1,188 -0,19 130 27 CV semi-solid
SD5-80cm 29,7 92,3 42 50,4 1,243 -0,24 140 27 ME-CE semi-solid
SD7-30cm 26,9 84,3 36,4 48 1,198 -0,2 110 27 CV semi-solid
SD7-50cm 27,1 85,9 37,1 48,8 1,205 -0,2 140 26 CV semi-solid
SD7-70cm 29,5 90,8 38,7 52,1 1,177 -0,18 135 26 CV semi-solid
SD17-30cm 20,1 83,8 39,9 43,9 1,451 -0,45 120 24 CV semi-solid
SD17-50cm 21,5 83,9 34,5 49,4 1,263 -0,26 130 25 CV semi-solid
SD25-30cm 21,7 80,1 41,3 38,9 1,503 -0,5 125 25 MV semi-solid
SD25-50cm 21,1 84,4 39,8 44,6 1,419 -0,42 135 25 MV semi-solid
SD29-30cm 17,5 85,3 37,9 47,4 1,429 -0,43 125 27 CV semi-solid
SD29-50cm 21,5 86 35,2 50,8 1,268 -0,27 145 26 CV semi-solid
SD33-30cm 19,8 84,6 37,3 47,3 1,37 -0,37 125 27 CV semi-solid
SD33-50cm 21 85,6 35,4 50,2 1,287 -0,29 145 25 CV semi-solid
SD41-30cm 23,2 84,9 39,3 45,6 1,353 -0,35 130 26 CV semi-solid
SD41-50cm 24,7 90 41,2 48,8 1,338 -0,34 135 26 CV semi-solid
SE1-30cm 23,7 82,2 34,6 47,6 1,229 -0,23 125 23 CV semi-solid
E SE1-50cm 27,1 84,9 37,3 47,6 1,214 -0,21 135 24 CV semi-solid
SE1-70cm 29,9 90,8 39,3 51,5 1,181 -0,18 145 25 CE semi-solid
(black clays) SE5-30cm 24,6 84,7 37 47,8 1,258 -0,26 125 24 CV semi-solid
SE5-50cm 26,9 85,5 37,9 47,6 1,23 -0,23 130 25 CV semi-solid
SE5-70cm 29,1 92,2 42,1 50 1,261 -0,26 145 26 MV semi-solid
SE13-30cm 20,8 85,8 39,1 46,7 1,391 -0,39 125 24 CV semi-solid
SE13-50cm 23,5 88,6 39,9 48,7 1,338 -0,34 130 25 CV semi-solid
SE21-30cm 25,2 88,1 39 49,1 1,281 -0,28 125 25 CV semi-solid
SE21-50cm 25,7 90,1 40,7 49,4 1,304 -0,3 145 26 MV semi-solid
SE29-30cm 21,8 80,9 35,5 45,5 1,301 -0,3 120 25 CV semi-solid
SE29-50cm 23 82,5 36 46,5 1,279 -0,28 120 26 CV semi-solid
SE37-30cm 20,8 84,6 37 47,6 1,339 -0,34 125 24 CV semi-solid
SE37-50cm 21,8 85,4 37,8 47,6 1,337 -0,34 130 25 CV semi-solid
SE41-30cm 22 86,7 39,8 46,8 1,381 -0,38 135 25 MV semi-solid
SE41-50cm 20,9 87,5 40 47,5 1,403 -0,4 135 26 MV semi-solid
Rd1 -30cm 39,4 48,6 30,7 17,9 0,512 0,49 20 10 MI soft
Red soils Rd1-100cm 24,2 51,1 30,6 20,5 1,316 -0,32 15 11 MH semi-solid
Rd1-200cm 24,3 49,4 29,6 19,8 1,27 -0,27 20 11 MI-MH semi-solid
Rd1-400cm 25,1 47,7 30 17,7 1,278 -0,28 15 11 MI semi-solid
Rd2-30cm 37,9 53,3 34,7 18,6 0,826 0,17 20 10 MH stiff
Rd2-100cm 26 56,8 34,4 22,4 1,372 -0,37 15 11 MH semi-solid
Rd2-200cm 25,4 54,3 33,2 21,1 1,369 -0,37 15 11 MH semi-solid
Rd2-400cm 26,1 53,3 33,9 19,4 1,401 -0,4 15 11 MH semi-solid

89
Plate 7.2a. Apparatus for liquid limit
determination of soil.
Plate7.2b. Liquid limit determination
showing V-shaped groove in soil sample.
The numerical difference beween the liquid limit (LL) and plastic limit (PL) was calculated to
give the plasticity index (PI) of the soil, i.e.
PI = LL – PL (7.5)
Results of liquid limit tests obtained for the soils in this study are reported to the nearest
whole number (1%) alongside those of plastic limit and plasticity index in Table (7.2).
7.1.3.4 Evaluation and application of results
Results of Atterberg limits obtained in this study have been used for physical classification of
the clay soils found (Fig. 7.1), based on the standard plasticity chart or A-line chart (Draft
revision of CP 2001; BS 1377: 1975). The clay soils have been classified into one of five
categories of low plasticity (CL), medium plasticity (CI), high plasticity (CH), very high
plasticity (CV) or extremely high plasticity (CE). Similarly, the silty varieties of soils have
been correspondingly classified and denoted by ML, MI, MH, MV, or ME.
In Fig. (7.1), the black clays involved in this study have been classified as very high to
extremely high plasticity inorganic clays (CV, CE) and silty clays (MV, ME) while the red
soils wholly plot below the A-line and fall in the class of medium to high plasticity silts and
silty clays (MI, MH).

90
Plasticity Index (PI)
0,5
0,4
0,3
0,2
0,1
0,0
Soft
Sand
Low
plasticity
(L)
Clays
CL
ML
Mediu
m
plastici
y
High
plasticity
(H)
CH
CI
Silt
MI
MH
BS A-line: PI = 0,73(LL-20)
0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1
Liquid Limit (LL)
Very high
plasticity (V)
CV
MV
Organic Silt
ME
Black clays
Red soils
Figure 7.1. A plasticity chart classification of black clays and red soils of this study.
On the standard plasticity chart (BS 1377:1975), the A-line has been defined by the
relationship
PI = 0,73(LL-20) (7.6)
based on experience with British soils (Fig. 7.1).
It was originally drawn from experimental data and serves as a tentative boundary between
clayey and silty varieties of cohesive soils which generally plot above and below it,
respectively. For the black clays and red soils of the present study area, however, the
plasticity index is related to liquid limit through a new relationship derived in this study, i.e.,
PI = 0,79(LL-25) (7.7)
Classification of black clays and red soils based on their activity (after Skempton, 1953;
Mitchell, 1976) is presented in Table (7.3). The black clays fall in the class of normal active
to highly active clays, their relatively higher activity being attributed to the higher content of
smectites (montmorillonites). According to Holtz and Kovacs (1981), montmorillonite has
the highest activity of clay minerals and would often plot above the A-line of the plasticity
chart. The red soils are classified as essentially normal active clays, and this most probably
due to the predominant presence of kaolinite. Kaolinite is a low activity clay mineral and
would often plot below the A-line (Holtz and Kovacs, 1981).
The activity state/ level and classification of black clays and red soils is also summarised
diagrammatically in Fig. (7.2). From the activity chart, the black clays are found to harbour
medium to very high activity levels (i.e. normal to highly active) while the red soils occur in a
medium activity state ( i.e. normal active).

91
Activity chart
Plasticity index PI (%) of
whole sample
100
50
0
LOW
Activity = 2,0
HIGH
MEDIUM
VERY HIGH
Activity = 1,0
Activity= 0,5
0 50 100
Black clays
Red soils
% clay in whole sample
Figure 7.2. Activity level classification of black clays and red soils in this study (After,
NAVFAC, 1982).
Results of Atterberg limits and /or activity values have also been used to asses the probable
type of clay minerals present in the soils, based on the classification by Skempton (1953) and
Day (2001), i.e. Table (7.1). It is therefore anticipated that black clays which exhibit activity
values of 1,10–3,38 (mean: 1,87) are mainly smectites (Ca-montmorillonites mainly); while
the red soils with activity values of 0,83-1,03 (mean: 0,94) would be predominated by
kaolinite and / or illite (Table 7.3). This prediction has been confirmed by actual clay
mineralogical studies on the soils which showed the black clays to be over 90% smectites in
composition; while the red soils are mainly kaolinite (80% and over).
In addition, results of Atterberg limits obtained during first stage tests served to aid in the
selection of samples for chemical/ mineralogical analyses as well as more detailed soil studies
involving shear strength and consolidation-settlement tests.
7.1.4 Free swell test (after Gibbs & Holtz, 1956)
7.1.4.1 Scope
This test was carried out to determine the increase in volume of clay soils from a loose dry
powder-form condition when poured into water, expressed as a percentage of the original
volume.
According to Gibbs and Holtz (1956), the amount of free swell serves to be indicative of the
probable swelling and/ or expansive behaviour of clay soils, especially when wetted from
relatively dry conditions under light structural loads.
A classification of soils based on free swell is summarised in Table (7.4).

92
Table 7.3. Atterberg limits, activity/ free swell classification and assessment of possible clay
mineralogy of black clays and red soils of this study.
Profile Sample No. Wn (%) LL (%) PL (%) PI (%) FS (%) Clay (%) Activity
F. swell
class
Activity class
(BS 1377: 1975)
Possible
clay
minerals
SA1-30cm 32 78 37 41 135 18 2,28 high highly active
A SA1-50cm 30 75 36 39 100 16 2,41 high highly active
SA2-30cm 34 93 39 54 135 26 2,07 high highly active
(black
clays) SA2-70cm 34 90 36 54 170 26 2,07 high highly active smectites
SA2-105cm 30 88 33 55 120 24 2,30 high highly active
SA28-30cm 26 85 40 45 125 23 1,96 high active
SA28-50cm 27 89 41 47 135 18 2,62 high highly active
SA37-30cm 19 81 38 43 115 32 1,33 high active
SA37-50cm 22 83 39 44 115 31 1,43 high active
SA37-80cm 23 85 39 46 115 34 1,35 high active
SA41-30cm 23 89 41 47 135 37 1,28 high active
SA41-50cm 25 90 41 49 120 44 1,10 high normal
SB1-30cm 23 82 34 47 125 30 1,58 high active
B SB1-50cm 27 84 36 48 135 26 1,83 high active
SB1-70cm 30 90 39 51 145 25 2,03 high highly active
(black
clays) SB5-30cm 24 81 34 47 120 29 1,62 high active smectites
SB5-50cm 27 85 37 48 135 25 1,92 high active
SB5-70cm 31 91 40 51 145 26 1,96 high active
SB7-30cm 23 83 35 48 130 31 1,56 high active
SB7-50cm 26 86 37 49 135 27 1,81 high active
SB7-70cm 30 87 38 49 140 26 1,87 high active
SB18-30cm 20 82 40 43 120 13 3,28 high highly active
SB18-50cm 20 72 34 39 120 14 2,77 high highly active
SB27-30cm 22 85 38 47 130 32 1,47 high active
SB27-50cm 22 88 39 49 130 33 1,47 high active
SB31-30cm 23 87 38 49 130 35 1,39 high active
SB31-50cm 23 86 38 48 130 30 1,61 high active
SB41-30cm 23 89 45 44 145 20 2,21 high highly active
SB41-50cm 24 91 44 47 145 22 2,14 high highly active
SB42-30cm 22 88 44 44 145 23 1,90 high active
SB42-50cm 21 91 45 47 145 21 2,22 high highly active
SC1-30cm 24 82 35 48 125 30 1,58 high active
C SC1-50cm 26 85 37 48 135 27 1,79 high active
SC5-30cm 23 83 35 48 125 31 1,56 high active
(black
clays) SC5-50cm 25 84 36 47 135 27 1,75 high active smectites
SC9-30cm 25 84 36 48 130 31 1,55 high active
SC9-50cm 26 85 37 48 135 27 1,76 high active
SC17-30cm 21 76 35 41 105 29 1,41 high active
SC17-50cm 22 81 33 48 110 27 1,79 high active
SC25-30cm 26 88 39 49 125 23 2,13 high highly active
SC25-50cm 26 89 41 48 145 18 2,64 high highly active
SC29-30cm 21 82 32 49 110 28 1,76 high active
SC29-50cm 20 80 37 43 110 26 1,67 high active
SC33-30cm 20 82 33 49 120 27 1,82 high active
SC33-50cm 21 83 33 50 120 28 1,78 high active
SC41-30cm 18 85 38 47 125 16 2,96 high highly active
SC41-50cm 17 84 37 47 125 19 2,47 high highly active

93
Table 7.3 (continued). Atterberg limits, activity/ free swell classification and assessment of
possible clay mineralogy of black clays and red soils of this study.
Profile
Sample No. Wn (%) LL (%) PL (%) PI (%) FS (%) Clay (%) Activity
F. swell
class
Activity class
(BS 1377:1975)
Possible clay
minerals
SD1-30cm 27 82 36 46 125 32 1,45 high active
D SD1-50cm 27 83 36 47 130 26 1,80 high active
SD1-80cm 30 91 42 49 140 27 1,83 high active
(black clays) SD5-30cm 26 84 36 47 125 31 1,52 high active smectites
SD5-50cm 27 84 36 48 130 26 1,84 high active
SD5-80cm 30 92 42 50 140 28 1,80 high active
SD7-30cm 27 84 36 48 110 32 1,50 high active
SD7-50cm 27 86 37 49 140 26 1,88 high active
SD7-70cm 30 91 39 52 135 25 2,08 high
SD17-30cm 20 84 40 44 120 13 3,38 high
highly
active
highly
active
highly
active
SD17-50cm 22 84 35 49 130 21 2,35 high
SD25-30cm 22 80 41 39 125 24 1,62 high active
SD25-50cm 21 84 40 45 135 14 3,19 high
highly
active
SD29-30cm 18 85 38 47 125 27 1,76 high active
SD29-50cm 22 86 35 51 145 28 1,81 high active
SD33-30cm 20 85 37 47 125 29 1,63 high active
SD33-50cm 21 86 35 50 145 28 1,79 high active
SD41-30cm 23 85 39 46 130 35 1,30 high active
SD41-50cm 25 90 41 49 135 41 1,19 high active
SE1-30cm 24 82 35 48 125 31 1,54 high active
E SE1-50cm 27 85 37 48 135 27 1,76 high active
SE1-70cm 30 91 39 52 145 26 1,98 high active
(black clays) SE5-30cm 25 85 37 48 125 31 1,54 high active smectites
SE5-50cm 27 86 38 48 130 28 1,70 high active
SE5-70cm 29 92 42 50 145 26 1,92 high active
SE13-30cm 21 86 39 47 125 33 1,42 high active
SE13-50cm 24 89 40 49 130 34 1,43 high active
SE21-30cm 25 88 39 49 125 24 2,05 high
highly
active
SE21-50cm 26 90 41 49 145 20 2,47 high
highly
active
SE29-30cm 22 81 36 46 120 27 1,69 high active
SE29-50cm 23 83 36 47 120 29 1,60 high active
SE37-30cm 21 85 37 48 125 33 1,44 high active
SE37-50cm 22 85 38 48 130 31 1,54 high active
SE41-30cm 22 87 40 47 135 19 2,46 high
highly
active
SE41-50cm 21 88 40 48 135 19 2,50 high
highly
active
Rd1 -30cm 39 49 31 18 20 18 0,99 low normal
Red soils Rd1-100cm 24 51 31 21 15 20 1,03 low normal kaolinite
Rd1-200cm 24 49 30 20 20 24 0,83 low normal
Rd1-400cm 25 48 30 18 15 20 0,89 low normal and/ or
Rd2-30cm 38 53 35 19 20 low
Rd2-100cm 26 57 34 22 15 low illite
Rd2-200cm 25 54 33 21 15 low
Rd2-400cm 26 53 34 19 15 low

94
Table 7.4. Free swell classification of clay soils (Gibbs & Holtz, 1956).
FREE SWELL VALUE
FS (%)
FREE SWELL
CLASSIFICATION
POSSIBLE
EXPANSIVENESS
200 very high very high
Up to 2000 (e.g. bentonite) extremely high extremely high
7.1.4.2 Procedure
About 50g of a selected soil sample was oven-dried (105 °C) overnight, cooled and passed
through a 400 µm sieve. About 10 ml of the sieved dry soil powder was measured out into a
dry 25 ml measuring glass cylinder, without compacting and/ or shaking it down. The soil
powder was then steadily drizzled into 50 ml of distilled water contained in a 50 ml glass
measuring cylinder. The main part of the solid particles was allowed to come to rest for a
period of 30 minutes (Plate 7.3), when the total volume of settled solids was read off and
recorded.
Plate 7.3. Free swell determination of clay soils.
7.1.4.3 Calculation
Free swell is the change in volume of the dry soil expressed as a percentage of its original
volume; and is given by
free swell (%) = ((V-Vo)/Vo)*100, or
free swell (%) = ((V-10)/10)*100 (7.8)

95
where
7.1.4.4 Results
V = volume of settled solid particles
Vo = 10ml, i.e. original volume of dry soil
Results of free swell test (FS %) obtained from the present study are given in Table (7.2), and
are reported to the nearest whole number or 1%. Classification of soils based on swelling
capabilitiy is summarised in Table (7.3). Black clays have high swelling capabilities, being
characterised by free swell values of 100% and over. The red soils with free swell values of
less than 50% (i.e. 15-20%) are classified as low swelling capability clays. Potential influence
and/ or effects of these swelling capabilities on light constructed structures are discussed in
the next chapter.
7.1.5 Linear shrinkage
7.1.5.1 Scope and procedure
Linear shrinkage tests were carried out in this study according to British Standard testing
procedure (BS 1377: 1975, Test 5). The tests usually serve to determine the percentage linear
shrinkage of clays as well as soils of low plasticity such as silts (Head, 1984). In practice, the
results of the tests serve to point to possible volumetric changes soils would undergo in situ
during a change of seasons from rainy wet months to hot dry ones. In this study, the tests were
performed on both the highly plastic black clays as well as the low plasticity silty red soils;
and involved determination of linear or one-dimensional change in length of a semicylindrical
bar sample of clay soil when dried out, starting from near the liquid limit.
The apparatus and procedure are as summarised in Plate (7.4) in which a suitable amount of
selected soil, previously dried (105 °C) and conveniently prepared, was mixed with distilled
water to produce a homogeneous paste around its liquid limit. The paste, initially filled into a
semi-cylindrical standard metal (brass) mould of 140 mm length and 25 mm diameter, was
dried (105 °C) and the average length (Ld) of dried test specimen determined by measuring its
top and lower surface lengths. Linear shrinkage, LS, was calculated as a percentage of the
original length of the specimen, i.e.
LS (%) = ((Lo-Ld)/Lo)*100 (7.9)
where Lo = original length (140 mm) of semi-cylindrical bar sample of soil at about its liquid
limit
Ld = average length of dried specimen
Results of linear shrinkage tests obtained for the soils in this study are presented in Table
(7.2), and are reported to the nearest 1%. Black clays exhibit LS values of 21-29%, while red
soils show relatively lower values of 10-11%.

96
Plate 7.4a. Linear shrinkage determination
showing wet soil in apparatus.
Plate7.4b. Linear shrinkage test:
dried and shrunk soil specimens.
7.1.5.2 Evaluation of results
Experience with British soils has shown that the plasticity index, PI, of clay soils could be
approximately estimated from results of linear shrinkage, LS, through the relationship
PI = 2,13 * LS (7.10)
This relationship is especially useful in soils of low clay content, high mica content and low
plasticity; all in which determination of Atterberg limits and obtaining of reproducible results
are difficult. In such cases, results of linear shrinkage tests could be used to give more
consistent plasticity values (BS 1377: 1967).
The application of this relationship to results of linear shrinkage obtained in this study is
discussed in a later chapter. The relationship between laboratory measured values of linear
shrinkage and those of plasticity index as obtained for the soils in this study, is also included
for comparison purposes. A correlation of laboratory-measured plasticity indices with
calculated plasticity index values (Equation (7.10)) is also given.
Possible engineering implications of shrinkage behaviour of black clays and red soils on
construction practice are mentioned in the next chapter.
7.2 Grain/ particle size analysis
7.2.1 Scope
The purpose of grain size analysis of soils was to group the discrete particles found into
separate ranges of sizes, and thereby determine the relative proportions by dry weight of each
size range. This facilitated to determine whether the soils were predominantly gravelly, sandy,
silty or clayey; and therefore which of these size ranges would most likely control the
engineering properties of the soils found. The test therefore served to facilitate classification
of soils according to particle size, by placing them into one of the six arbitrary categories of
boulders, cobbles, gravel, sand, silt or clay, based on the dominating grain size. As a result,
the analysis served to provide a useful engineering classification system whereby soils were
separated into materials and/ or components of significantly differing engineering properties.

97
A standard classification of soils by particle size (Glossop and Skempton, 1945; British Code
of Practice CP 2001; BS 1377: 1975) is given in Table (7.5), for comparison purposes. In
addition, the clay fraction has been used as an index for correlating with other engineering
properties of soils (see Chapter 10).
Application of results of particle size analysis of soils is, however, usually limited by the fact
that engineering properties are also influenced by other factors such as mineral type, structure
and geological history, and which cannot be assessed from particle size tests alone (Head,
1984).
Table 7.5. Soil classification by particle size (BS 1377: 1975).
Particle size (mm) Designation Test procedure
> 200 Boulders Measurement of
200-60 Cobbles separate pieces
60-20 Coarse gravel
20-6 Medium gravel
6-2 Fine gravel Sieve analysis
2-0,6 Coarse sand
0,6-0,2 Medium sand
0,2-0,06 Fine sand
0,06-0,02 Coarse silt Sedimentation
0,02-0,006 Medium silt Analysis
0,006-0,002 Fine silt _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
0,002 and less Clay
7.2.2 Procedure and results
Particle size distribution of soils in this study was investigated by employing both the sieving
and sedimentation methods of analysis. Sieving involved use of a series and/ or set of 10
sieves of standard aperture openings (Plate 7.5). A wet sieving procedure in accordance with
British Standard (BS 1377:1975, Test 7(A)) was adopted to facilitate washing down and
separation of fines (silt and clay) from the coarse sand and gravel sizes, and thereby determine
the proportions of coarse as well as fine material present in the soils
Plate 7.5. Set of sieves used for
grain-size analysis of soils.
Plate 7.6. Sedimentation apparatus and grain-size
analysis of finer fraction of soils.

98
The percentage by mass of soil retained on each of the sieves was calculated, and from this
the cumulative percentage passing (P) each sieve size was then calculated by subtracting the
corresponding summation from 100, i.e.
Pi(%) = 100 - (Σmi/M1) *100 (7.11)
where M1 = total initial dry mass of soil
Σmi = cumulative mass retained on upper sieves, down to and including sieve
in question
Pi (%) = cumulative percentage passing sieve size in question
The total mass of fines (Mf) and corresponding percentage fines (Pf) representing clay and silt
sizes which pass through the bottom-most (0,063mm) sieve was given by
and
Mf = M1 – Σmi (7.12)
Pf (%)
= ((M1- Σmi)/M1 )*100, or
= 100 – ((Σmi/M1)*100) (7.13)
where M1, Mf and Σmi are as defined above.
The cumulative percentage passing (P%) of each sieve size was plotted against corresponding
sieve size (D mm) on a semi-logarithmic chart to give grain size distribution curves of
analysed soils in the coarse fraction range (sand, gravel).
The sedimentation analysis was carried out according to British Standard (BS 1377: 1975,
Test 7(D)), and aimed at investigating the distribution of particles in the finer fraction (silt
and clay) of soils, by using a suitably calibrated specific gravity hydrometer to monitor and
measure the density of a suspension of the fraction in water with time (Plate 7.6). The analysis
essentially facilitated the distribution of particles in the silt range (0,060 – 0,0020 mm) to be
assessed. The maximum diameter of particles remaining above a particular depth, H, in the
suspension at any time, t, from the start of the sedimentation test was calculated by applying
Stokes´ law (after Sir George Stokes, 1891), i.e.
D = 0,005 531* [ηH/(t(Gs- 1))]^½ (7.14)
where
D = particle diameter (mm)
η = dynamic viscosity of water (mPas)
g = acceleration due to gravity (i.e. 9,81m/s²)
H (mm) = effective depth
t (min) = elapsed time
ρw = mass density of liquid water (i.e. 1,000 Mg/m³)
Values of η and ρw for water temperatures of 0-40 °C are given in Table (7.6), i.e. after Kaye
and Laby (1973), while intermediate values may be obtained by either arithmetic or graphical
interpolation.

99
Table 7.6. Viscosity and density of water (Kaye and Laby, 1973).
Temperature
(°C)
Dynamic viscosity, η
(mPas)
Density, ρw
(Mg/m³)
0 1,7865 0,999 84
5 1,5138 0,999 95
10 1,3037 0,999 70
15 1,1369 0,999 09
20 1,0019 0,998 20
25 0,8909 0,997 04
30 0,7982 0,995 65
40 0,6540 0,992 22
The relationship in Equation (7.14) is based on the assumption that the particles are
approximately spherical, posses similar densities and exhibit a uniform distribution and small
terminal velocities within the fluid/ water used; and that the fluid/ water used is maintained at
constant temperature, has no turbulence and posseses viscous flow in its still state. It is
therefore implied that large soil particles in a suspension would settle more quickly than small
ones.
The percentage, K, by mass of particles smaller than the equivalent diameter, D, was given by
the equation
K (%) = [(Gs * R)/(m(Gs-1))] * 100 (7.15)
where Gs (dimensionless) = specific gravity of soil particles
R (dimensionless) = fully corrected hydrometer reading (i.e. ρ-1)*1000
m (g) = mass of dry soil; and ρ (g/cm³) = density of soil suspension
The values of K calculated for each hydrometer reading were plotted against the
corresponding particle size, D (logarithmic scale) to give a particle size distribution curve in
the silt range. The intersection of this curve with the 0,002 mm ordinate gave the percentage
corresponding to the clay fraction. The curve was plotted on the same semi-logarithmic sheet
as that used for the sieving analysis for the same sample. The two curves were joined to give a
single continouous curve representing the particle size distribution and grading characteristics
of the whole sample (Fig. 7.3a). It is observed that grading curves for both black clays and red
soils are generally steep and positioned higher up to the left side of the chart, i.e. largely
confined within the finer section of the chart (Fig. 7.3a), implying that silt and clay are the
major components of these soils. The percentages by mass of gravel, sand, silt and clay sizes
computed for the soils in this study are presented in Table (7.7).
To aid in classification, the percentage contribution of the three major components of clay, silt
and coarse fraction (sand + gravel) in each soil sample were plotted in a Triangular
classification chart (Fig. 7.3b; after BS 1377: 1975). In this chart, the percentage content of
the three components in any one soil sample would usually add up to 100%. It is evident in
Table (7.7) that sand is generally the predominant part of the coarse fraction in the tested
samples. As result, the terminology “sand” has been adopted in this chart to represent the
combined contribution by the coarse fraction. Classification of samples of black clays and red

100
Table 7.7. Results of grain size analysis for black clays and red soils in this study.
Profile
Sample No. Clay (%) Silt (%) Sand (%) Gravel (%) % fines
Class
(BS 1377: 1975)
SA1-30cm 18 72 8 2 90 clayey silt
A SA1-50cm 16 56 12 16 72 clayey silt
SA2-30cm 26 64 8 2 90 silty clay with sand
(black clays) SA2-70cm 26 62 9 3 88 silty clay with sand
SA2-105cm 24 61 9 6 85 silty clay with sand
SA28-30cm 23 71 5 1 94 silty clay with sand
SA28-50cm 18 70 6 6 88 clayey silt
SA37-30cm 32 57 9 2 89 silty clay
SA37-50cm 31 61 4 4 92 silty clay
SA37-80cm 34 56 8 2 90 silty clay
SA41-30cm 37 59 3 1 96 silty clay
SA41-50cm 44 45 6 5 89 clay
SB1-30cm 30 60 6 4 90 silty clay
B SB1-50cm 26 65 6 3 91 silty clay with sand
SB1-70cm 25 66 5 4 91 silty clay with sand
(black clays) SB5-30cm 29 60 7 4 89 silty clay with sand
SB5-50cm 25 66 6 3 91 silty clay with sand
SB5-70cm 26 64 6 4 90 silty clay with sand
SB7-30cm 31 58 7 4 89 silty clay
SB7-50cm 27 64 6 3 91 silty clay with sand
SB7-70cm 26 64 6 4 90 silty clay with sand
SB18-30cm 13 71 8 8 84 clayey silt
SB18-50cm 14 74 9 3 88 clayey silt
SB27-30cm 32 62 4 2 94 silty clay
SB27-50cm 33 59 5 3 92 silty clay
SB31-30cm 35 55 5 5 90 silty clay
SB31-50cm 30 63 5 2 93 silty clay
SB41-30cm 20 68 9 3 88 clayey silt
SB41-50cm 22 64 7 7 86 silty clay with sand
SB42-30cm 23 65 9 3 88 silty clay with sand
SB42-50cm 21 65 8 6 86 silty clay with sand
SC1-30cm 30 57 9 4 87 silty clay
C SC1-50cm 27 63 7 3 90 silty clay with sand
SC5-30cm 31 57 8 4 88 silty clay
(black clays) SC5-50cm 27 64 6 3 91 silty clay with sand
SC9-30cm 31 58 7 4 89 silty clay
SC9-50cm 27 62 8 3 89 silty clay with sand
SC17-30cm 29 59 8 4 88 silty clay with sand
SC17-50cm 27 59 6 8 86 silty clay with sand
SC25-30cm 23 71 5 1 94 silty clay with sand
SC25-50cm 18 71 5 6 89 clayey silt
SC29-30cm 28 63 8 1 91 silty clay with sand
SC29-50cm 26 66 7 1 92 silty clay with sand
SC33-30cm 27 64 8 1 91 silty clay with sand
SC33-50cm 28 63 8 1 91 silty clay with sand
SC41-30cm 16 58 8 18 74 clayey silt
SC41-50cm 19 66 8 7 85 clayey silt

101
Table 7.7, continued. Results of grain size analysis for black clays and red soils.
Profile
Sample No. Clay (%) Silt (%) Sand (%) Gravel (%) % fines
Class
(BS 1377: 1975)
SD1-30cm 32 59 7 2 91 silty clay
D SD1-50cm 26 62 8 4 88 silty clay with sand
SD1-80cm 27 65 6 2 92 silty clay with sand
(black clays) SD5-30cm 31 60 7 2 91 silty clay
SD5-50cm 26 62 8 4 88 silty clay with sand
SD5-80cm 28 64 6 2 92 silty clay with sand
SD7-30cm 32 58 8 2 90 silty clay
SD7-50cm 26 63 7 4 89 silty clay with sand
SD7-70cm 25 66 5 4 91 silty clay with sand
SD17-30cm 13 71 8 8 84 clayey silt
SD17-50cm 21 63 8 8 84 silty clay with sand
SD25-30cm 24 60 8 8 84 silty clay with sand
SD25-50cm 14 60 10 16 74 clayey silt
SD29-30cm 27 63 6 4 90 silty clay with sand
SD29-50cm 28 64 5 3 92 silty clay with sand
SD33-30cm 29 61 6 4 90 silty clay with sand
SD33-50cm 28 63 6 3 91 silty clay with sand
SD41-30cm 35 57 7 1 92 silty clay
SD41-50cm 41 48 6 5 89 clay
SE1-30cm 31 58 7 4 89 silty clay
E SE1-50cm 27 64 6 3 91 silty clay with sand
SE1-70cm 26 65 5 4 91 silty clay with sand
(black clays) SE5-30cm 31 58 7 4 89 silty clay
SE5-50cm 28 63 5 4 91 silty clay with sand
SE5-70cm 26 64 6 4 90 silty clay with sand
SE13-30cm 33 60 5 2 93 silty clay
SE13-50cm 34 58 5 3 92 silty clay
SE21-30cm 24 70 5 1 94 silty clay with sand
SE21-50cm 20 68 6 6 88 clayey silt
SE29-30cm 27 65 7 1 92 silty clay with sand
SE29-50cm 29 63 7 1 92 silty clay with sand
SE37-30cm 33 57 5 5 90 silty clay
SE37-50cm 31 61 5 3 92 silty clay
SE41-30cm 19 70 8 3 89 clayey silt
SE41-50cm 19 66 8 7 85 clayey silt
Rd1 -30cm 18 73 9 0 91 clayey silt
Red soils Rd1-100cm 20 74 6 0 94 clayey silt
Rd1-200cm 24 69 7 0 93 silty clay with sand
Rd1-400cm 20 72 8 0 92 silty clay with sand
soils are presented in the chart (Fig. 7.3b). A more summarised classification of all tested
samples based on this chart is given in Table (7.7).
In this study therefore, the grading curves provided a means by which soils could be classified
(Table 7.7) and their engineering properties broadly assessed. Engineering implications of
grain size distribution and grading characteristics of black clays and red soils on construction
practice are discussed in the next chapter.

102
Particle size distribution curves
100
90
80
70
% passing
60
50
40
30
20
10
0
0,001 0,010 0,100 1,000 10,000
Grain size d (mm)
Rd1-100cm (red soils) Rd1-400cm (red soils) SC17-50cm (black clays)
SD1-80cm (black clays) SE1-30cm (black clays) SB18-30cm (black clays)
SA2-105cm (black clays) SA41-50cm (black clays)
Figure 7.3a. Particle size distribution curves for black clays and red soils in this study.

103

104
7.3 Direct shear tests
7.3.1 Scope
Direct shear tests are usually carried out to measure and determine the shear strength of soils,
by sliding one portion of a soil relative to another.
Buildings and other constructed structures impose loads on the soils in and/ or on which they
are founded. The stresses set up in the soil have the effect of deforming it through
consolidation by expelling the fluid from the voids between solid particles. The deformation
of soil could also occur through shear failure, whereby slippage of particles, one on another,
causes the sliding of one body of soil relative to the surrounding mass. Shear failure is
therefore a result of shear stresses set up in the soil exceeding the maximum shear resistance
the soil can offer, i.e. its shear strength.
Shear strength determinations of soils therefore serve to facilitate stability analyses under
various external loading conditions with the purpose of guarding against failures which could
cause destruction of constructed structures. In practice, economically feasible and safe
constructed structures require that the stresses set up in the soil under loading are everywhere
less than its shear strength by a suitable margin (Colombo, 1983).
According to Head ( 1984), shear strength is usually related to conditions prevailing in situ
and can also vary with time. The value determined in the laboratory is, however, dependent
upon the conditions imposed during the test and, in some instances, upon the duration of the
test. In this study, the spatial distribution of shear strength parameters (c, φ) of soils found is
investigated and presented.
7.3.2 Procedure
A shearbox apparatus was used in this study to investigate the shear strength characteristics of
soils found. The apparatus is relatively quick and simple to use; and consists of a rigid metal
box, square in plan with internal dimensions of 100 mm length and 30 mm depth, consisting
of two halves and in which the soil was placed. The shear test involved sliding the upper
portion of the soil contained in upper half of the box along and relative to the lower portion in
the lower half, by the action of a steadily increasing horizontal shearing displacement (Plate
7.7). A constant load normal to the plane of relative movement was applied at the same time.
During this process, the lower half of the box was restrained from moving by a loadmeasuring
device. The horizontal shearing force was applied by a motorised drive unit which
pulled on the upper half of the box; while the normal pressure was provided by a yoke
supporting a load hanger.
The area of contact between the soil in the two halves of the shearbox usually decreases as the
test proceeds, so that a correction to allow for this may be necessary. However, according to
Petley (1966), this affects the shear stress and normal stress in equal proportions so that the
net effect on the failure envelope is usually negligible.

105
Plate 7.7a. Shear testing showing preconsolidation
and shearing of specimens.
Plate 7.7b. Shear testing procedure
showing automatic data acquisition.
In this study, slow consolidated-drained (CD) direct shear tests (according to ASTM 3080;
DIN 18 137) were carried out on undisturbed specimens of soil samples previously collected
from the field using 100 mm diameter U-tubes. The consolidated-drained conditions were
provided for by allowing soil specimens to completely consolidate under selected normal
pressure before the shearing process to allow for complete dissipation of excess pore
pressures. Further drainage was provided for during shear by adjusting the motor speed to
give a suitably slow rate of displacement and thereby dissipate any additional pore water
pressure which could have further developed. In practice, the drained shear strength
parameters (Cd, φd) differ only slightly from effective shear strength parameters (c´, φ´ )
obtained from undrained tests in which pore water pressures are measured (Head, 1984). For
many purposes therefore, the two sets of parameters are considered to be equal so that this
study adopts the symbols c´, φ´; for the drained shear strength parameters of soils involved.
The shear tests served to provide the angle of internal friction, φ (degrees), as well as amount
of cohesion, c (kN/m²), of the soils investigated. The relative displacement (mm) of the two
portions of soil specimen as well as the applied shearing force (kN) were measured with time
(t ); and the results used to plot a shear stress/ displacement curve. The vertical movement
(mm) of the top surface of specimen, which indicates changes in volume, was also recorded
and enabled changes in density and voids ratio during shear to be evaluated.
Up to three tests at different normal loading conditions were carried out on specimens of the
same soil sample and the appropriate shear stress/ displacement curves drawn. The maximum
drained shear resistance and/ or shear strength offered by the soil at a given normal loading
condition was given by the peak value of the respective shear stress/ displacement curve. The
peak or maximum shear stress from each curve was read off and plotted against the
corresponding normal stress to give the failure envelope, i.e. an approximately straight linegraph
whose inclination to the horizontal axis represents the angle of shearing resistance, φ.
The cohesion, C, was given by the intercept of the line on the vertical stress axis. The straight
line relationship of the Coulomb or failure envelope is defined by Coulomb´s law (Coulomb,
1773), i.e.
Tf = C + σntanφ (7.16)
where Tf = maximum shearing resistance (kN/m²)
σn = normal stress (kN/m²)
C = cohesion (kN/m²)
tanφ = frictional component

106
According to Terzaghi and Peck (1967), the cohesion, C, is the component of shearing
resistance due to internal forces holding the particles together in a solid mass; while tanφ is
the component of shearing resistance due to interlocking of soil particles and friction between
them when subjected to normal stress. The tanφ component usually increases with increased
normal stress and disappears under unconfined conditions of zero normal loading. On the
other hand, C is independent of normal stress and remains constant with increased normal
loading.
7.3.3 Results
The drained shear strength parameters (c´, φ´ ) obtained for the soils in this study are
presented in Table (7.9); with c´ reported to two significant figures and φ´ to the nearest
degree. The black clays exhibit angles of shear resistance (φ´ ) of between 11° and 30°, giving
a mean value of 18°. The red soils are characterised by shear resistance angles of 28° to 29°.
These are all typical values of clay soils which are usually less than 28° (Lambe and
Whitman, 1979; Johnson and DeGraff, 1988). The characteristic shear stress/ displacement
curves obtained for the soils are included in Fig. (7.4); with the generally cohesive and high
plasticity black clays giving rise to sharp peaks while the loose and low plasticity red soils are
characterised by flattened-out peaks. As a result, the difference between peak strength and
residual strength is large in black clays and quite insignificant in red soils. A diagrammatic
representation of the strength characteristics of these soils is illustrated in the form of Mohr-
Coulomb failure envelopes in Fig. (7.5).
The very cohesive nature of the black clays is evidenced by the relatively large values of
cohesion (c´ ) obtained, i.e.12 – 48 kN/m², giving a mean value of 35 kN/m² (Table 7.9; Fig.
7.5). On the contrary, cohesion is insignificant in red soils due to their generally loose and
friable nature.
Typical values of the angle of shear resistance for other geological materials (quartz grains
and noncohesive soils) are provided in Table (7.8); for comparison purposes.
Table 7.8. Typical values of φ for dry noncohesive soils and clay (after Lambe and Whitman,
1979).
Type of soil and grading Angle of shear resistance (φ) in degrees
Loose
Dense
Rounded Angular Rounded Angular
Uniform sand- fine to 30 35 37 43
medium
Well-graded sand 34 39 40 45
Sand and gravel 36 42 40 48
Gravel 35 40 45 50
Silt 28-32 30-35
Clay < 28 < 30

107
Shear stress / Displacement Curves:
black clays and red soils
200,0
180,0
160,0
140,0
Shear stress [kN/m²]
120,0
100,0
80,0
60,0
40,0
20,0
0,0
0,00 5,00 10,00 15,00 20,00 25,00
Horizontal displacement [mm]
SA2-70cm (black clays) SB1-30cm (black clays) SC1-30cm (black clays)
SD7-50cm (black clays) RD1-100cm (red soils) RD1-30cm (red soils)
Figure 7.4. Shear stress/ displacement curves of black clays and red soils in this study.
The variation of strength characteristics of black clays across the study area would be
reflected by the distribution of shear strength parameter, (φ`), as shown in Table (7.10). Soil
depths of less than 0,50m are characterised by relatively higher maximum (30°), minimum
(16°) and mean (21°) values; than those of 0,50m depth and greater which have maximum,
minimum and mean values of 23°, 11° and 17°, respectively. According to Johnson and
DeGraff (1988), the shear strength of soils is usually inversely proportional to their plasticity.
As a result, the observed slight decrease of strength characteristics of black clays with depth
could be attributed to a corresponding slight increase of their plasticity (PI) with depth (Table
7.10). However, the two depth intervals exhibit more or less similar variation trends of soil
strength across the study area as characterised by standard deviation values which are of the
same order of magnitude (5,50 and 3,76); and a very strong correlation (R = 0,89) for the two
sets of data (Table 7.10). As a result, the observed slight increase in strength parameters (φ`)
with depth could be safely taken as insignificant and the black clays therefore classified as
generally homogeneous in their strength characteristics with depth across the study area.

108
Table 7.9. Shear strength parameters of black clays and red soils. Results of natural moisture
content (Wn), plasticity index (PI) and relative consistency (Cr) also included for
comparison purposes.
Profile Sample No. Wn (%) PI (%) Cr
Cohesion
c´(kN/m²)
Shear angle
φ´ (degrees)
A SA2-30cm 34 54 1,094 23,10 22
SA2-70cm 34 54 1,04 35,60 19
(black clays) SA41-30cm 23 47 1,387 29,80 29
SA41-50cm 25 49 1,334 41,00 19
SB1-30cm 23 47 1,237 46,70 17
B SB1-50cm 27 48 1,197 39,50 12
SB1-70cm 30 51 1,187 17,40 19
(black clays) SB5-30cm 24 47 1,212 47,40 17
SB5-50cm 27 48 1,205 36,60 13
SB5-70cm 31 51 1,19 16,00 19
SB7-30cm 23 48 1,235 44,90 17
SB7-50cm 26 49 1,222 36,20 13
SB7-70cm 30 49 1,179 37,60 14
SB41-30cm 23 44 1,491 30,20 30
SB41-50cm 24 47 1,415 28,60 22
SC1-30cm 24 48 1,218 47,20 18
C SC1-50cm 26 48 1,224 31,80 14
SC17-30cm 21 41 1,329 11,50 22
(black clays) SC17-50cm 22 48 1,219 33,70 14
SC33-30cm 20 49 1,248 25,10 25
SC33-50cm 21 50 1,233 34,90 22
SD1-30cm 27 46 1,203 47,90 17
D SD1-50cm 27 47 1,19 38,40 16
SD1-80cm 30 49 1,248 33,80 15
(black clays) SD5-30cm 26 47 1,217 44,00 16
SD5-50cm 27 48 1,188 36,10 13
SD5-80cm 30 50 1,243 35,10 13
SD7-30cm 27 48 1,198 45,40 16
SD7-50cm 27 49 1,205 36,80 14
SD7-70cm 30 52 1,177 35,50 13
SD33-30cm 20 47 1,37 32,00 30
SD33-50cm 21 50 1,287 28,00 21
SE1-30cm 24 48 1,229 46,40 16
E SE1-50cm 27 48 1,214 39,00 12
SE1-70cm 30 52 1,181 42,50 11
(black clays) SE13-30cm 21 47 1,391 42,60 16
SE13-50cm 24 49 1,338 35,10 12
SE29-30cm 22 46 1,301 25,80 26
SE29-50cm 23 47 1,279 36,20 23
SE41-30cm 22 47 1,381 26,00 28
SE41-50cm 21 48 1,403 29,00 22
Rd1 -30cm 39 18 0,512 0,00 29
Red soils Rd1-100cm 24 21 1,316 0,00 29
Rd1-200cm 24 20 1,27 0,00 28
Rd1-400cm 25 18 1,278 0,00 28

109
Table 7.10. Distribution and/ or variation of shear strength parameter (φ´) across study area.
Profile
Depth (m) Sample No.
Sta. position
(m) PI (%)
Shear angle
(φ´) in
degrees Depth (m) Sample No. PI (%)
SA1-30cm 0 41 SA1-50cm 39
Shear angle
(φ´) in
degrees
A < 0,50 SA2-30cm 250 54 22 ≥ 0,50 SA2-70cm 55 19
SA28-30cm 6750 45 SA28-50cm 47
SA37-30cm 9000 43 SA37-50/80cm 39
SA41-30cm 10000 47 29 SA41-50cm 49 19
SB1-30cm 0 47 17 SB1-50/70cm 50 15
B < 0,50 SB5-30cm 1000 47 17 ≥ 0,50 SB5-50/70cm 50 16
SB7-30cm 1500 48 17 SB7-50/70cm 49 14
SB18-30cm 4250 43 SB18-50cm 39
SB27-30cm 6500 47 SB27-50cm 49
SB31-30cm 7500 49 SB31-50cm 48
SB41-30cm 10000 44 30 SB41-50cm 47 22
SB42-30cm 10250 44 SB42-50cm 47
SC1-30cm 0 48 18 SC1-50cm 48 14
C < 0,50 SC5-30cm 1000 48 ≥ 0,50 SC5-50cm 47
SC9-30cm 2000 48 SC9-50cm 48
SC17-30cm 4000 41 22 SC17-50cm 48 14
SC25-30cm 6000 49 SC25-50cm 48
SC29-30cm 7000 49 SC29-50cm 43
SC33-30cm 8000 49 25 SC33-50cm 50 22
SC41-30cm 10000 47 SC41-50cm 47
SD1-30cm 0 46 17 SD1-50/80cm 48 15
D < 0,50 SD5-30cm 1000 47 16 ≥ 0,50 SD5-50/70cm 49 13
SD7-30cm 1500 48 16 SD7-50/70cm 51 14
SD17-30cm 4000 44 SD17-50cm 49
SD25-30cm 6000 39 SD25-50cm 45
SD29-30cm 7000 47 SD29-50cm 51
SD33-30cm 8000 47 30 SD33-50cm 50 21
SD41-30cm 10000 46 SD41-50cm 49
SE1-30cm 0 48 16 SE1-50/70cm 50 11
E < 0,50 SE5-30cm 1000 48 ≥ 0,50 SE5-50/70cm 49
SE13-30cm 3000 47 16 SE13-50cm 49 12
SE21-30cm 5000 49 SE21-50cm 49
SE29-30cm 7000 46 26 SE29-50cm 47 23
SE37-30cm 9000 48 SE37-50cm 48
SE41-30cm 10000 47 28 SE41-50cm 48 22
max 30 max 22
Statistical < 0,50 min 16 ≥ 0,50 min 11
analysis range 14 range 11
median 17 median 15
mode 17 mode 22
mean 21 mean 16
standard dev. 5,50 standard dev. 3,76
variance 30,26 variance 14,10
covariance covariance -0,08
correlation correlation 0,89

110
Maximum shear stress / Normal stress:
black clays & red soils
300,0
250,0
200,0
Shear stress at failure [kN/m²]
150,0
100,0
50,0
0,0
0,0 100,0 200,0 300,0 400,0 500,0 600,0
Normal stress [kN/m²]
SA2-70cm (black clays) SB1-30cm (black clays) SC1-30cm (black clays)
SD7-50cm (black clays) RD1-100cm (red soils) RD1-30cm (red soils)
RD1-400cm (red soils)
Figure 7.5. Coulomb/ shear failure envelopes for black clays and red soils in this study.
Plotting of drained peak values of shear stress against normal stress (failure envelopes) was
done on the same diagram, both for black clays and red soils (Fig. 7.5).
Possible engineering implications of shear strength and/ or shear strength parameters of soils
on construction practice are discussed under a relevant section in the next chapter.
7.3.4 Evaluation and analysis of results
The possible forms of relationship between shear parameters (cohesion, angle of shear
resistance) measured on undisturbed samples and index properties of black clays are

111
illustrated in Figures 7.6, 7.7, 7.8 and 7.9. The index parameters were obtained by performing
tests on disturbed and/ or fractioned samples of the clays.
Fig. (7.6a) shows the correlation between angle of shear resistance (φ`) and moisture content
in relation to Atterberg limits, i.e. the relative consistency (Cr), to be fair and of moderate
strength (R=0,58). A general increase in the angle of shear resistance with increased
consistency is observed. However, a broad spread of results on the plot diagram occurs, and
serves to be indicative of the influence of other important factors such as soil fabric and/ or
structure and clay mineralogy on the magnitude of shear angles and, therefore, the strength
characteristics of the soils.
Black clays
Angle of shear resistance
φ´ (°)
35
n = 41
30
25
20
R 2 = 0,34
15
10
1,015 1,215 1,415 1,615
Relative consistency, Cr
Shear angle vs Cr
Figure 7.6a. Correlation between angle of shear resistance (φ´ ) and relative
consistency, Cr, of black clays.
Black clays
Cohesion c´(kN/m²)
60,00
n = 41
50,00
40,00
30,00
20,00
10,00
0,00
15 20 25 30 35
Moisture content Wn (%)
Cohesion vs Wn
Figure 7.6b. Correlation between cohesion (c´ ) and moisture content, Wn,
of black clays.

112
Black clays
Cohesion c´ (kN/m²)
60,00
50,00
40,00
30,00
20,00
10,00
0,00
n = 41
Cohesion vs Cr
1 1,1 1,2 1,3 1,4 1,5 1,6
Rel. Consistency, Cr
Figure 7.6c. Correlation between cohesion (c´ ) and relative consistency, Cr,
of black clays.
A poor correlation (R = 0,30) is obtained by relating shear angles and plasticity indices of the
clays (Fig. 7.7a). A slight improvement in correlation is obtained by plotting the shear angles
against the reciprocal of plasticity indices (Fig. 7.8a); but once again, the strength of
correlation is still low (R = 0,32).
Black clays
Angle of shear resistance
φ´ (°)
35
n = 41 R² = 0,09
30
25
20
15
10
35,015 40,015 45,015 50,015 55,015 60,015
Plasticity index PI (%)
Shear angle vs PI
Figure 7.7a. Correlation between angle of shear resistance (φ´ ) and plasticity
index (PI) of black clays.

113
Black clays
Cohesion c´ (kN/m²)
60,00
n = 41
50,00
40,00
30,00
20,00
10,00
0,00
40 45 50 55
Plasticity index PI (%)
Cohesion vs PI
Fig. 7.7b. Correlation between cohesion (c´ ) & plasticity index (PI) of black clays.
Black clays
Angle of shear resistance
φ´ (°)
35
n = 41
30
25
20
R² = 0,102
15
10
0,015 0,020 0,025 0,030
Reciprocal of plasticity index 1/P
Shear angle vs 1/PI
Figure 7.8a. Correlation between angle of shear resistance (φ´ ) and reciprocal
of plasticity index (1/PI) of black clays.
Black clays
Cohesion c´ (kN/m²)
60,00
50,00
n = 41
40,00
30,00
20,00
10,00
0,00
0,018 0,020 0,022 0,024 0,026
1/PI
Cohesion vs 1/PI
Figure 7.8b. Correlation between cohesion (c´ ) and reciprocal of plasticity
index (1/PI) of black clays.

114
The combined influence of relative consistency and plasticity indices on the shear angles and
shear strength of clays is illustrated in Fig. (7.9a). A fair or moderate correlation (R = 0,54)
showing a general increase in shear angles and / or shear strength with an increased ratio of
Cr/PI is observed. There is also an improvement in correlation over that based on plasticity
index (Figs. 7.7a & 7.8a) alone.
Black clays
Angle of shear resistance
φ´ (°)
35
n = 41
30
25
20
R 2 = 0,29
15
10
0,015 0,025 0,035 0,045
Ratio Cr/PI
Shear angle vs Cr/PI
Figure 7.9a. Correlation between angle of shear resistance (φ´ ) and ratio Cr/PI
for black clays.
Black clays
Cohesion c´ (kN/m²)
60,00
50,00
n = 41
40,00
30,00
20,00
10,00
0,00
0,016 0,021 0,026 0,031 0,036
Cr/PI
Cohesion vs Cr/PI
Figure 7.9b. Correlation between cohesion (c´ ) and ratio Cr/PI
for black clays.
On the other hand, correlations between values of cohesion and those of index parameters
(natural moisture content, relative consistency, Atterberg limits) are generally weak, with the
resulting plots giving more spread out data points (Figures 7.6b & c, 7.7b, 7.8b, 7.9b).
All in all, however, the strengths of correlations between cohesion and shear angles on one
hand and relative consistency and plasticity indices on the other are generally poor and, at
their best, only moderate. This serves to point to the uncertainities accompanying assessment
and characterisation of shear strengths of clays based on results of index tests performed on
disturbed and fractioned / sieved soil samples.

115
7.4 Oedometer consolidation tests
7.4.1 Scope
Oedometer consolidation tests were carried out in this study to investigate and establish
consolidation-settlement characteristics of normally consolidated as well as overconsolidated
clay soils found, in terms of estimating the amount and rate of settlement when externally
loaded. The tests were carried out on saturated clays, and were also aimed at determining and
establishing percentage swelling and swelling pressures of the clay soils.
Determined parameters of coefficient of volume compressibility and coefficient of
consolidation were used to describe the consolidation-settlement characteristics of soils. The
coefficient of volume compressibility (i.e. modulus of volume change) was used to indicate
the compressibility of soils in terms of the amount by which they would compress when
loaded and allowed to consolidate. The coefficient of consolidation is a time related parameter
(Head, 1984), and served to indicate the rate of compression and hence the time period over
which consolidation -settlement would take place.
Results of the tests and parameters derived thereof would be usefully applied in foundation
design in limiting settlements to within tolerable limits when structural foundation loads are
imposed on the soils. In addition, the rate of settlements and the time within which such
settlements could be virtually complete would be estimated.
The theory of consolidation (Terzaghi, 1925; Terzaghi and Peck, 1948; Taylor, 1948) is based
on the fact that, consolidation process of soils subjected to external loading mainly entails
escape of water from the voids between the skeleton of solid grains. The loss of pore water is
generally rapid in free-draining materials, but is limited in soils of impeded drainage. As a
result, settlements in high permeability materials of sands and gravels generally take place in
a short time, usually as construction work proceeds, thereby causing no major problems. On
the other hand, low permeability clays could undergo settlements over much longer periods of
time after completion of construction, thereby subjecting constructed structures to a prolonged
state of instability (Nelson and Miller, 1992; Johnson and DeGraff, 1988; Head, 1984).
In this study, the amounts and rates of consolidation settlement of soils found under various
loading conditions have been studied, analysed and established. The spatial distribution and
variation of consolidation parameters across the study area have also been investigated and
established.
7.4.2 Theory of consolidation
According to Terzaghi (1926), saturated clays which are externally loaded have the total
applied stress, σ, initially supported by the excess pore water pressure, µ, which is induced.
As water drains out of the soil, the pore pressure correspondingly falls so that an increasing
proportion of the applied load is transferred to the grains forming the soil skeleton. The
consolidation process therefore involves gradual transfer of stress from pore water to soil
skeleton. The degree of consolidation, U, is a measure of the extent of this stress transfer at
any instant during consolidation; and is given by the equation
U (%) = ((µo – µ)/ µo) * 100 (7.21)
where µo = initial excess pore pressure

116
µ = excess pore pressure at time t from start of consolidation
U = degree of consolidation or percentage pore pressure dissipation at time t
The effective stress, σ`, is the numerical difference between total applied stress and pore
water pressure at any instant during consolidation; and is approximately equal to the stress
carried by the soil skeleton (Simons and Menzies, 1977; Terzaghi, 1926), i.e.
where σ = total applied stress
σ` = effective stress
µ = pore water pressure
σ` = σ – µ (7.22)
However, no measurements of pore water pressure are made in the oedometer test so that the
degree of consolidation has to be related to the change in height of the specimen. By taking
percentage U to be related to the average pore pressure at time t, it can be assumed that the
degree of consolidation is proportional to the amount of settlement which has taken place by
time t. Thus
U (%) = (∆H/∆Hf) * 100 (7.23)
Where ∆H = settlement up to time t
∆Hf = settlement which will ultimately take place when U = 100%
In practice, the flow of water and displacements which take place during consolidation are
nearly always three – dimensional. However, the analysis of three-dimensional effects is
extremely complex and rarely practicable (Davis and Poulos, 1965). In this study, Terzaghi´s
one –dimensional analysis was applied in estimating the magnitude of settlements and the rate
at which they develop. In applying Terzaghi´s analysis, it would be usually assumed that soils
subjected to consolidation are horizontal, laterally confined, fully saturated, homogeneous and
uniformly thick materials; in which Darcy´s law for the flow of water is valid and the
coefficient of permeability and other soil properties remain constant during any one increment
of applied stress. The analysis also holds for soils in which drainage and compression is onedimensional
in a vertical direction, and soil particles and water are practically incompressible.
In addition, initial excess pore pressure due to application of a load should be uniform
throughout the clay layer while one or both of adjacent strata to the clay layer are to be
perfectly free-draining in comparison to the clay. The effect of the weight of the clay layer
under consolidation should be also negligible.
Based on the above assumptions, the simple one-dimensional case of consolidation of a clay
layer subjected to uniform loading could be summarised in the form of a differential equation
(Terzaghi, 1943; Scott, 1974), i.e.
δµ/δt = (K/ρwgmv ) δ²u/dz² (7.24)
where µ = excess pore water pressure at time t, at a given point
z = vertical height of point in consideration
K = coefficient of permeability of clay
mv = coefficient of volume compressibility
ρw = mass density of water
g = acceleration due to gravity

117
The compound coefficient, K/ρwgmv, is defined as the coefficient of consolidation, cv, i.e. a
parameter which relates the change in excess pore pressure with respect to time, to the amount
of water draining out of the voids of a clay layer during the same time, as a result of
consolidation, or
cv = K/ρwgmv (7.25)
so that Equation (7.24) above becomes
δµ/δt = cv .δ²u/dz² (7.26)
The solution of Equation (7.26) yields the percentage consolidation, U, as a function of cv, h,
and time t, i.e.
U/100 = f(cvt/h²) (7.27)
where h = length of longest drainage path
cvt/h² = a dimensionless number, replaceable by a time factor Tv.
Thus Tv = cvt/h² (7.28)
Equation (7.27) then becomes
U/100 = f(Tv) (7.29)
Computed values of U for various values of Tv and √Tv are given in Table (7.11) after
Leonards (1962).
Table 7.11. Time factors for one-dimensional
consolidation (Leonards, 1962).
Time factor
U (%)
Tv √Tv
0 0 0
10 0,0077 0,0877
20 0,031 0,176
30 0,071 0,266
40 0,126 0,355
50 0,197 0,443
60 0,286 0,535
70 0,403 0,635
80 0,567 0,753
90 0,848 0,921
95 1,129 1,063
100 ∞ ∞
Fig. 7.10. Time factor Tv related to degree
of consolidation U% (Terzaghi, 1943).

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A graphical relationship between U and Tv is given in Figs. (7.10, 7.11, 7.12) where U is
plotted against Tv, log10(Tv) and √Tv, respectively (Terzaghi, 1943).
Fig. 7.11. Time factor Tv (log scale) related to Fig. 7.12. Square-root of time factor, √Tv,
degree of consolidation U% related to degree of consolidation U%
(Terzaghi, 1943). (Terzaghi, 1943).
The compression of soils under a given load is usually divided into three phases (Head, 1988),
and include initial compression, primary consolidation and secondary compression (Fig.
7.13). The initial compression takes place almost simultaneously with the application of a
load increment in a laboratory test and before commencement of drainage. It is a result of
compression of small pockets of gas within the pore spaces and partly to bedding down of
contact surfaces in the cell and load frame. A small proportion may be due to elastic
compression which is recoverable when the load is removed. The initial compression is
usually responsible for deviation of the laboratory curve from the theoretical curve near the
beginning of a loading increment.
Fig. 7.13. Log-time/settlement curve analysis Fig. 7.14. Analysis of square-root time/
method, showing phases of consolidation settlement curve (Taylor, 1942).
(Casagrande, 1936).

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Primary consolidation is a time-dependent compression due to dissipation of excess pore
pressure under loading. This phase is accounted for by the Terzaghi consolidation theory
(Terzaghi, 1925), and relates closely to the theoretical curve for most clays. It is therefore
used in many applications to make estimation of settlements.
Secondary compression continues after the excess pore pressure of the primary consolidation
phase has virtually dissipated, and is most probably a result of continued movement of
particles as the soil structure adjusts itself to increasing effective stress.
The primary consolidation phase is usually used to determine the coefficient of consolidation,
cv, for each increment of loading through a curve fitting process, which entails comparing a
laboratory consolidation curve with the theoretical curve. In this study, two curve fitting
procedures were adopted and include the log-time method (after Casagrande, 1936), in which
a graphical analysis of log-time/ settlement curves was done (Fig. 7.13); and the square-roottime
method (after Taylor, 1942; Barden, 1965) in which a graphical analysis of square-roottime/
settlement curves was undertaken (Fig. 7.14).
In this study, the log –time plot was used in the analysis of black clays whose settlement
curves were generally of the conventional shape (Fig. 7.16). However, the square-root plot
was used in the analysis of the rather silty red soils, in which otherwise evaluation of initial
consolidation conditions, i.e. the do point of deformation corresponding to U = 0% primary
consolidation using the log-time method was difficult due to the unconventional shape of
settlement curves produced (Fig. 7.17).
7.4.3 Consolidation coefficients and other parameters
7.4.3.1 Coefficient of consolidation (cv)
This parameter relates the change in excess pore pressure with respect to time, to the amount
of water draining out of the voids of a clay layer during the same time, as a result of
consolidation.
The coefficient of consolidation for a given load increment in a consolidation test can be
determined by re-writing Equation (7.28), i.e.
cv = (Tv/t) * h² (mm²/min) (7.30)
where t (min) = time for a given percentage of primary consolidation
h (mm) = length of maximum drainage path
A log-time graphical analysis using time t50 corresponding to 50% primary consolidation (i.e.
U = 50%) and theoretical time factor T50 = 0,197 (Table 7.11) transforms Equation (7.30) into
cv = (T50/t50) * h²
= 0,197 * (h²/t50)
or cv = (0,197 * (h/1000)²)/(t50/(60*24*365,25)
i.e., cv = 0,1036h²/t50 (m²/year) (7.31)
In the standard oedometer consolidation test as used in this study, with double drainage to
both the lower and upper surfaces, the height of the specimen H' (mm) is equal to 2h.
Therefore Equation (7.31) becomes

120
cv = (0,1036/t50)* (H'/2)²
or cv = 0,026(H')²/t50 (m²/year) (7.32)
where H' (mm) = mean specimen height during the load increment
t50 (min) = time for 50% primary consolidation
A square-root time analysis using t90 corresponding to U = 90% primary consolidation and T90
= 0,848 (Table 7.11) transforms Equation (7.30) into
cv = (T90/t90)* h²
= 0,848 * (h²/t90) mm²/min, or
cv = (0,848 * (h/1000)²)/(t90/(60*24*365,25))
= 0,446* (h²/t90) m²/year (7.33)
In terms of mean height of specimen, H',
cv = (0,446*(H'/2)²)/t90, or
cv = 0,112 * H'²/t90 (m²/year) (7.34)
where t90 (min) = time for 90% primary consolidation for a given load increment
H' (mm) = mean specimen height during this load increment
In practice, however, cv is usually calculated from t50 rather than t90 because the middle
section of the laboratory settlement curve is the portion which agrees most closely with the
theoretical curve (Head, 1988).
Typical values of coefficient of consolidation (Lambe and Whitman, 1979) obtained from
laboratory oedometer tests on specimens of uniform soil are related to their approximate
plasticity ranges in Table (7.14). Values of cv obtained for black clays and red soils in this
study have also been related to their plasticity, and included alongside for comparison
purposes.
7.4.3.2 Voids ratio
Voids ratio, e, is the ratio of the volume of voids occupied by water and/ or air; to the volume
of solid particles in a mass of soil.
Voids ratio in a consolidation test is defined by
Voids ratio e = (Gs/ρo) –1 (7.35)
where Gs = specific gravity of solid particles
ρ0 = dry density of soil (Mg/m³)
Voids ratio, e, is related to degree of saturation, S, through the relation
S (%) = (w * Gs/e) * 100 (7.36)

121
where w (%) = moisture content of soil
S (%) = degree of saturation, i.e. volume of water contained in the void space between
soil particles expressed as a percentage of total voids
The volume change which occurs during consolidation takes place only in the voids so that a
change in height ∆H from an initial height Ho in the specimen corresponds to a change in
voids ratio ∆e from an initial value eo. By proportion therefore,
∆H/Ho = ∆e/(1+eo), so that
∆e = ((1+eo)/Ho) * ∆H, or
∆e = F∆H (7.37)
Where F (mm¯¹) = change in voids ratio per unit height, with respect to initial conditions of
test specimen
The change in height of specimen is used to give the voids ratio,e, at any stage of a
consolidation test through
e = eo – ∆e (7.38)
where eo = initial voids ratio of test specimen
7.4.3.3 Coefficient of compressibility (av), coefficient of volume compressibility (mv)
and compression index (cc)
These coefficients are derived from consolidation tests and are indicative of the
compressibility of soils. They serve to give estimates of the amount of settlement due to
primary consolidation.
The coefficient of compressibility, av, and the coefficient of volume compressibility, mv, are
usually calculated for each load increment while the compression index, cc, is derived from an
e/ log p curve obtained by plotting voids ratio, e, against applied pressure, p, (log scale), i.e.
(Fig. 7.15a). In practice, mv is applied to overconsolidated clays and cc to normally
consolidated ones. The swell index, cs, gives an indication of expansion of a soil on
unloading.
The coefficient of compressibility, av, is the change in voids ratio, δe, per unit pressure
change, during a given load increment, due to consolidation caused by that incremental
pressure δp, i.e.
av = (e2 – e1)/ δp
= - δe/ δp (m²/kN) (7.39)
where e1 and e2 = voids ratios at start and end of consolidation under the load increment
δp in kN/m²
The negative sign indicates decrease of e as p increases.

122
Fig. 7.15a. Log-pressure/ voids ratio (e/ log p)
curve .
Fig. 7.15b. Determination of coefficient of
secondary compression cα.
The coefficient of volume compressibility, mv, is the change in volume of soil test specimen
per unit volume, per unit pressure change, as a result of consolidation due to that pressure
change. It is a modulus of volume change and serves to indicate the compressibility per unit
thickness of soil under test, i.e.
mv = av/(1+e1) = 1/(1+e1) * (- δe/ δp) m²/kN, or
mv = 1000/(1+e1) * (- δe/ δp) m²/MN (7.40)
where e1 = voids ratio at start of load increment δp (kN/m²)
Typical values of mv for a number of types of clay are given in Table (7.13), alongside values
of mv obtained for black clays and red soils of this study, for comparison purposes.
The compression index, cc, is a dimensionless number. It refers to the slope of the linear
section of a consolidation curve of voids ratio, e, plotted against loading pressure, p (log
scale). It is therefore the change in voids ratio for one log cycle of pressure change on the
curve whose linear section is represented by the equation
e = eo – cclog10 (po+ δp)/po (7.41)
where eo and po refer to initial conditions of voids ratio and pressure.
Skempton (1944), found cc to be related to liquid limit, LL, of clays to a reasonable degree of
approximation through
cc = 0,009 (LL-10%) (7.42)
for undisturbed clay, and
cc = 0,007(LL-10%) (7.43)
for remoulded clay.

123
Relationships between values of cc and liquid limit for the soils in this study are discussed in a
later section and represented in Table (7.15) and Figs. (7.23 & 7.24). Measured values of cc
obtained from consolidation tests in this study are also compared and correlated with
calculated values, both from the new relationship established in this study as well as
Skempton´s relationship (Table 7.16).
7.4.3.4 Swell index (cs)
This is the slope of the swelling or unloading curve of e plotted against log p (Fig. 7.18). It is
the change in voids ratio, e, over one log cycle of pressure change in the linear section of the
swelling/ unloading curve of e/ log p. The straight-line section of the curve is given
approximately by the equation
e = eo ± cslog10 (po+δp)/po (7.44)
where eo and po refer to initial conditions of voids ratio and loading pressure.
Correlations between swell index and index properties are discussed in a later chapter.
7.4.3.5 Coefficient of permeability (K)
The coefficient of permeability, K, is usually computed from Equation (7.25), i.e.
cv = K/(ρwgmv ), or by re-writing
K = cvmvρwg (mm/s), or in practical units
K = cv/(365,25*24*3600) * (mv*10^¯6) * (1*10³) * 9,81 (m/s), or
K = cvmv * 0,31*10^¯9 (m/s) (7.45)
where cv (m²/year) and mv (m²/MN) are consolidation parameters whose values are known,
g (m/s²) = acceleration due to gravity
ρw (kg/m³) = mass density of water
Values of K calculated for black clays and red soils of this study using the above equation are
given in Table (7.12).
7.4.3.6 Coefficient of secondary compression (cα)
This is the compression which continues after primary consolidation has virtually finished,
and is time dependent. According to Head (Head, 1988), it is a significant factor in the
settlement of soft soils, especially organic clays and peats where it usually increases with
increased load application.
Secondary compression is equal to the slope of the linear portion of the secondary
compression part of log-time/ settlement plot, in terms of strain per log cycle of time (Fig.
7.15b), i.e.
cα = (δHs/Ho)/ δlogt, or for one log cycle of time change

124
cα = δHs/Ho (7.46)
where δHs (mm) = change in specimen height over one log-cycle of time
Ho (mm) = initial height of specimen
cα = a dimensionless number
Ranges of values of cα determined for black clays and red soils in this study are presented and
compared with typical values for other clays in Table (7.17).
7.4.4 Procedure
Oedometer consolidation tests were carried out in this study according to British Standard (BS
1377: 1975, Test (17)). The tests were carried out on undisturbed specimens of the soils
previously collected from the project area. Use was made of a fixed-ring type of consolidation
cell consisting of a mould and/ or steel cutting ring (for rigidly supporting the soil specimen),
upper and lower drainage surfaces, a loading cap and a provision for adding and/ or
containing distilled water to keep and maintain the soil specimen saturated. The ring was
capable of accommodating specimens of 14 mm thickness and 71,40 mm diameter. According
to British Standard (BS 1377: 1975), the height to diameter ratio of specimens is usually
about 1: 4 to 1:5. The small thickness of test specimens used served to ensure that testing
times were not excessively long; and the tests could also be easily extended to long-term tests
to provide secondary compression characteristics of soils. The set-up of apparatus as used in
this study is presented in Plate (7.8).
A standard sequence of vertical loads ranging from 6 to 2400 kPa was applied to the laterally
confined specimen contained in the consolidation ring; with 6-12 kPa as extended range for
very soft soils; 25-800 kPa as normal range; and 1600-2400 kPa as extended range for stiff to
hard and/ or overconsolidated soils. The actual initial incremental load used for the black
clays depended on the amount of swelling pressure exhibited by the soils. The resulting
vertical compression (mm) under each load was observed and recorded over a 24 hour period
of time. The laterally confining ring had the purpose of permitting vertical deformation of the
soil specimen, without lateral deformation, so that one-dimensional consolidation
characteristics of soils could be investigated.
At the end of the last incremental loading stage, the specimen was unloaded in a sequence of
decrements and allowed to swell in about half the number of stages as were applied during
consolidation. Compression readings were plotted cumulatively against time, during both
loading (consolidation) and unloading (swelling) stages (Figs. 7.16 & 7.17). Compression/
time curves obtained for loading stages were used for derivation of one-dimensional
consolidation parameters.
Subsequent calculations involving consolidation test data made use of an average value of
specific gravity of soil particles of 2,72 for red soils and 2,70 for black clays; assuming
inorganic clays for the two types of soil.

125
Plate 7.8. Oedometer consolidation testing in this study.
The British Standard consolidation testing procedure adopted in this study is very similar in
principle to the one-dimensional consolidation test specified by the ASTM Designation
D2435. However, the ASTM standard incorporates either a fixed-ring type or floating-ring
type of consolidation cell, with minimum specimen dimensions of 50 mm diameter and 12,5
mm thickness; and a minimum height to diameter ratio of 1:2,5. In addition, a copper disc is
also used for the deformation calibration of the apparatus, while an initial seating load of 2,5-
5,0 kPa is usually applied to the soil specimen depending on the softness of the soil. In
practice, a standard loading sequence of pressures consist of 5, 12,5; 25, 50, 100, 200, 400,
800 kPa, even though smaller increments could be used on very soft soils (BS 1377: 1975;
ASTM D2435-70).
7.4.5 Analysis of consolidation test data
A graphical analysis of consolidation test data obtained for black clays made use of
conventional log-time method. However, both log-time and square-root-time methods were
employed in the graphical analysis of test data for the more silty red clay soils, whose early
portion of log- time/ settlement curves depart from the conventional shape as derived from
theoretical relationships (Fig. 7.17). Determination of the do point corresponding to 0%
primary consolidation was therefore not possible. The departure from conventional shape
could be attributed to the relatively much higher permeability (k = 7,57E-11 to 7,33E-9) of
the red soils which caused rapid settlement immediately after initial loading, so that the initial

126
convex-upwards portion of the primary curve was passed before any readings could be taken.
In this case, the do point was estimated from the square-root-time/ settlement curve and then
transferred to the log-time curve for the conventional analysis to be carried out. According to
Head (1988), cv values in the range of 10-100 m²/year are indicative of quite rapid settlements
which would not be expected to cause long-term instability problems.
Some of the red soils classified as clayey silts to silt types gave rise to log-time/ settlement
curves which were concave upwards from the start implying relatively rapid draining soils.
Both the initial convex portion and point of inflexion (Figs. 7.11 & 7.13) were passed before
the first recorded reading was made so that determination of do and d100 points corresponding
to 0% and 100% primary consolidation, respectively, was difficult. The square-root-time
curve could also not be used for determination of the do point due to lack of the necessary
linear portion of the curve (Figs. 7.12 & 7.14). The value of cv was therefore estimated by
assuming the occurrence of the d50 point somewhere mid-way between points do and df on the
log-time curve (Fig. 7.13). A value of t50 was then estimated and used to calculate cv. A range
of t50 less than 0,10 minutes, implied
cv > (0,0256 * H'²)/t50 = 0,0256 * 14²/0,1 or
cv > 50 m²/year, implying very rapid drainage and settlement of the soil,
where H' (mm) = mean specimen height
d50 (mm) = compression reading for 50% primary consolidation
dc (mm) = corrected initial compression reading
df (mm) = final compression reading
t5o (min) = time for 50% primary consolidation
Use of a Rowe consolidation cell (Rowe,1966), capable of accommodating larger soil
specimens would be recommended for oedometer tests on the clayey silt and silty varieties of
red soils, so as to obtain a more definite value of cv for estimation of rates of settlement under
various loads.
7.4.6 Results of consolidation tests
Results of log-time settlement curves obtained for black clays and red soils in this study are
illustrated in Figures (7.16) and (7.17), respectively. The red soil specimen is shown to
undergo settlements of more than 6mm over a loading range of up to 2400 kPa. Settlements
undergone by black clay specimen are less than 3mm over the same loading range. Results in
this study therefore show the red soils to be relatively more compressible than the black clays.
In addition, unloading curves for the black clays are more inclined than those of the red soils
which are more or less flat. This is indicative of the black clays having relatively more swelling
capabilities and/ or expansion tendencies on unloading than the red soils.

127
Cumulative log-time/settlement
curves: black clays (SA2-70cm)
Cumulative log-time/settlement
curves: Red soils (RD1-100cm)
cumulative settlement (mm)
time - minutes (log scale)
0,1 10,0 1000,0 100000,0
0
0,2
0,4
0,6
0,8
1
1,2
1,4
1,6
1,8
2
2,2
cumulative settlement (mm)
0,100 10,000 1000,000
0,000
1,000
2,000
3,000
4,000
5,000
time - minutes (log scale)
100000,0
00
2,4
2,6
6,000
2,8
3
Fig. 7.16. Conventional log-time/settlement
curves for black clays during loading
and unloading stages.
7,000
Fig. 7.17.Unconventional log-time/settlement
curves for red soils during loading
and unloading stages.
Results of voids ratio change of soils during loading and unloading stages are summarised in
the form of voids ratio/ log pressure (e/log p) curves as shown in Figure (7.18). The curves
for the red soils are generally placed higher up the diagram, implying higher voids ratios and
porosities. On the other hand, e/log p curves for black clays occupy lower positions on the
diagram due to their relatively lower voids ratios and porosities. As a result, red soils would
be expected to be more compressible than the black clays over the same loading range. In
addition, unloading portions of the curves are more or less flat and/ or horizontal for the red
soils, while those for black clays are relatively steeper. This serves to show once again the
higher potential swelling capabilities of black clays relative to those of red soils.

128
Consolidation curves of e/log p
Voids ratio (e)
1,501
1,401
1,301
1,201
1,101
1,001
0,901
0,801
0,701
0,601
0,501
0,401
0,301
0,201
0,101
0,001
10 100 1000 10000
Pressure on log scale (log P (kPa))
RD1-30cm: red soils RD1-100cm: red soils RD1-400cm: red soils
SA2-70cm: black clays SA37-50cm: black clays SB1-70cm: black clays
SB42-50cm: black clays SC17-50cm: black clays SC29-50cm: black clays
Figure 7.18. Voids ratio/ log Pressure (e/ log p) curves for black clays and red soils.
Results of oedometer consolidation tests in terms of voids ratio, compression coefficients and/
or indices as well as permeability over the loading range of 25-800 kPa are summarised in
Table (7.12). Complete results for the loading and unloading stages of black clays and red soils
are tabulated in Appendix A.
Table 7.12. Derived parameters from results of oedometer consolidation tests for the loading
range of 25-800 kPa in this study.
Sample
t50 Cv
No. E (%) e av (m²/kN) (min) (m²/year) K (m/s) Cc Cs Cα
mv
(m²/MN)
SA2-70cm 0,31-11,11 0,52-0,70 0,000052-0,00045 0,03-0,26 1,00 - 60 0,07-5,08 2,6E-12 - 4,8E-11 0,3 0,07 0,0002-0,009
SA37-50cm 1,84-27,68 0,52-1,06 0,0003-0,0033 0,18-1,62 2,90-55 0,05-1,73 2,94E-12 - 3,94E-10 0,4 0,075 0,003-0,017
SB1-70cm 1,85-19,46 0,65-1,01 0,000148-0,0015 0,09-0,74 0,48-18 0,19-2,36 5,11E-12 - 1,2E-9 0,29 0,043 0,0002-0,006
SB42-50cm 8,02-29,11 0,23-0,59 0,0003-0,0014 0,22-0,80 9,5-50 0,06-0,494 3,92E-12 - 1,23E-10 0,34 0,03 0,005-0,024
SC17-50cm 1,74-20,64 0,37-0,7 0,00028-0,0010 0,17-0,60 9,0- 50 0,07-0,56 1,67E-12 - 3,0E-11 0,44 0,04 0,006-0,016
SC29-50cm 3,33-18,24 0,56-0,84 0,00026-0,0008 0,16-0,44 2,5 -35 0,104-1,97 5,03E-12 - 2,03E-10 0,38 0,065 0,003-0,13
RD1-30cm 10,98-32,07 0,74-1,28 0,00035-0,0047 0,19-2,0 0,35-1,85 1,38-11,85 7,99E-11 - 7,33E-9 0,42 0,03 0,003-0,004
RD1-100cm 7,35-39,01 0,59-1,42 0,00036-0,0074 0,21-2,94 0,98-1,75 1,18-4,67 7,57E-11 - 4,26E-9 0,4 0,03 0,005-0,008
RD1-400cm 3,09-19,60 0,74-1,1 0,00029-0,0024 0,16-1,11 0,40-1,30 2,71-12,14 1,32E-10 - 4,19E-9 0,39 0,02 0,002-0,007

129
Ranges of values of coefficient of volume compressibility, mv, obtained for black clays and
red soils are given in Table (7.13), where they have been used to classify the soils based on
compressibility. Ranges of values of mv for other types of soil are also included for
comparison purposes. Results in Table (7.13) show that black clays would generally exhibit
medium to high compressibility over the normal loading range of 25-800kPa; however, cases
of low to medium compressibility are also to be expected. The red soils would be
characterised by medium to very high compressibility over the same loading range.
Comparatively, therefore, the red soils tend to be more compressible than the black clays.
This difference could be attributed to the generally loose and friable nature of the red soils
giving rise to higher porosities, as evidenced by voids ratios (e) of 0,59-1,42 (Table 7.12).
Lower compressibility of the black clays could be attributed to lower porosities (e = 0,23-
1,06), a most probable result of the cohesive nature and dense compact structure of these
clays.
Table 7.13. Compressibility classification of black clays and red soils in this study.
Coeff. of volume
Category
Clay type/ Sample No.
compressibility mv
(m²/MN)
Compressibility
classification
Typical Very organic alluvial clays > 1,5 very high
examples (after and peats
Lambe & Whitman Normally consolidated 0,3-1,5 high
-1979) alluvial/ estuarine clays
Fluvio-glacial clays and 0,1-0,3 medium
lake clays
Boulder clays 0,05-0,1 low
Heavily overconsolidated < 0,05 very low
boulder clays
SA2-70cm 0,03-0,26 very low to medium
This study: SA37-50cm 0,18-1,62 medium to very high
Black clays SB1-70cm 0,09-0,74 low to high
SB42-50cm 0,22-0,80 medium to high
SC17-50cm 0,17-0,60 medium to high
SC29-50cm 0,16-0,44 medium to high
RD1-30cm 0,19-2,0 medium to very high
This study: RD1-100cm 0,21-2,94 medium to very high
Red soils RD1-400cm 0,16-1,11 medium to high
Ranges of values of coefficient of consolidation, cv, obtained for the black clays and red soils
are given in Table (7.14). Typical ranges for other inorganic soils are included for comparison
purposes. From the results, the red soils would undergo medium to high rates of consolidation
–settlement when externally loaded in the range of 25-800 kPa. The rates of consolidationsettlement
for the black clays would be generally low over the same loading range. The high
rates of consolidation by the red soils could be attributed to their relatively high porosity and
high permeability (K = 7,57*E-11 to 7,33*E-9 m/s ) which facilitate faster drainage and rapid
dissipation of pore water pressures. On the other hand, the black clays are characterised by
relatively lower porosity and permeability (K = 1,67*E-12 to 1,2*E-9 m/s) so that drainage
and dissipation of pore pressures on external loading would be less rapid, causing slower
consolidation –settlements.

130
Table 7.14. Rates of consolidation-settlement of black clays and red soils.
Category
Clay type/ Sample No.
Coeff. of consolidation cv
(m²/year)
Rate of consolidationsettlement
Typical examples High plasticity 0,1-1,0 low
for inorganic clays Medium plasticity 1,0- 10 Medium
(after Lambe & Low plasticity 10-100 High
Whitman –1979) Silts > 100 very high
SA2-70cm 0,07-5,08 low - medium
This study: SA37-50cm 0,05-1,73 low -medium
Black clays SB1-70cm 0,19-2,36 low -medium
SB42-50cm 0,06-0,494 low
SC17-50cm 0,07-0,56 low
SC29-50cm 0,104-1,97 low -medium
RD1-30cm 1,38-11,85 medium to high
This study: RD1-100cm 1,18-4,67 medium
Red soils RD1-400cm 2,71-12,14 medium to high
Variations of values of mv, cv and K with increased loading during oedometer consolidation
tests have also been investigated by plotting them mid-way between applied pressure (log
scale) values (Figs. 7.19, 7.20, 7.21 and 7.22), since they relate to increase from one loading
stage to the next.
7,000E-09
K/ log p curves: black clays & red soils
6,000E-09
Permeability K (m/s)
5,000E-09
4,000E-09
3,000E-09
2,000E-09
1,000E-09
0,000E+00
1 10 100 1000 10000
Log pressure (kPa)
SA2-70cm (black clays) SA37-50cm (black clays) SB1-70cm (black clays)
SB42-50cm (black clays) SC17-50cm (black clays) SC29-50cm (black clays)
RD1-400cm (red soils)
Figure 7.19. Permeability/ log pressure (K/log p) curves for black clays and red soils.
In Fig. (7.19), the K/ log p curves of the red soils generally plot higher up the diagram than
those of black clays, and this especially at the initial stages of loading. This serves to indicate
the generally higher permeability of red soils relative to the black clays. However, the
permeability of red soils is also observed to decrease with increased depth (Fig. 7.20), and this

131
is most probably due to the tendency of deeper soil horizons to be relatively more
preconsolidated by increased overburden material.
K/ log p curves: red soils
Permeability K (m/s)
4,000E-08
3,000E-08
2,000E-08
1,000E-08
0,000E+00
1 10 100 1000 10000
Log pressure (kPa)
RD1-30cm
RD1-100cm
RD1-400cm
Figure 7.20. Permeability/ log pressure (K/log p) curves for red soils.
In Fig. (7.21), mv/ log p curves for red soils also generally plot higher than those for black
clays. This once again signifies a generally higher compressibility per unit thickness of the red
soils relative to black clays. However, the compressibility of the red soils alone tends to
decrease with increased depth; and this may once again be a result of preconsolidation and/ or
overconsolidation tendencies of deeper soil horizons.
mv/ log p curves: black clays & red soils
3,500
3,000
2,500
mv (m²/MN)
2,000
1,500
1,000
0,500
0,000
1 10 100 1000 10000
Log pressure (kPa)
SA2-70cm (black clays) SA37-50cm (black clays) SB1-70cm (black clays)
SB42-50cm (black clays) SC17-50cm (black clays) SC29-50cm (black clays)
RD1-100cm (red soils)
RD1-400cm (red soils)
Figure 7.21. Coefficient of volume compressibility/ log pressure (mv/log p) curves for black
clays and red soils.

132
In Fig. (7.22), cv/ log p curves for red soils once again plot above those of the black clays.
This implies that red soils exhibit higher rates of consolidation-settlement than black clays.
The difference, once again, could be due to the higher voids ratios and/ or porosity (Fig. 7.18)
and higher permeability (Fig. 7.19) of the red soils relative to the black clays.
18,00
cv/ log p curves: black clays & red soils
16,00
14,00
12,00
cv (m²/year)
10,00
8,00
6,00
4,00
2,00
0,00
1 10 100 1000 10000
Log pressure (kPa)
SA2-70cm (black clays) SA37-50cm (black clays) SB1-70cm (black clays)
SB42-50cm (black clays) SC17-50cm (black clays) SC29-50cm (black clays)
RD1-30cm (red soils)
RD1-400cm (red soils)
Figure 7.22. Coefficient of consolidation/ log pressure (cv/log p) curves for black
clays and red soils.
Results of compression index, cc, obtained for the soils in this study are presented in Table
(7.15b) together with corresponding values of liquid limit and plasticity index. Based on
compression index (Lambe and Whitman, 1979), clay soils could be approximately grouped
into those of low compressibility, medium to high compressibility, high compressibility and
very high compressibility; as shown in Table (7.15a).
Table 7.15a. Compression indices, Cc, and compressibility classification
of clay soils.
Compression index Cc
Compressibility class
2,6 Very high

133
The results of correlation show that liquid limit, LL, would be a better estimator of the
compression index and compressibility of soils than the plasticity index, PI, (Table 7.15b).
The strength of correlation between cc and LL is very high (R = -0,85) for the black clays; but
is reduced to moderate (R = -0,58) for combined black clays and red soils (Figs. 7.23 and
7.24). The relationships between compression index and liquid limit take the form of
cc = 0,0099(122-LL) (7.47)
for black clays, and
cc = 0,0016(308-LL) (7.48)
for both black clays and red soils.
Table 7.15b. Results of compression indices correlated with Atterberg limits.
Sample No. Load
(kPa) Wn (%) LL (%) PI (%) Cc Approx. Compressibility
SA2-70cm 100-2400 34 90 54 0,3 medium-high
SA37-50cm 25-1600 22 83 44 0,4 medium-high
SB1-70cm 50-2400 30 90 51 0,29 medium-high
SB42-50cm 100-2400 21 91 47 0,34 medium-high
SC17-50cm 100-2400 22 81 48 0,44 medium-high
SC29-50cm 100-2400 20 80 43 0,38 medium-high
RD1-30cm 12,0-2400 39 49 18 0,42 medium-high
RD1-100cm 12,0-2400 24 51 21 0,4 medium-high
RD1-400cm 12,0-2400 25 48 18 0,39 medium-high
Correlations (R) Black & red soils -0,58 -0,55
Correlations (R ) Black clays only -0,85 -0,66
Results of correlations and analysis show that cc values generally decrease with increasing
liquid limit and plasticity index of soils ( Table 7.15b; Figs. 7.23 & 7.24). This serves to
suggest that compressibility of soils generally decreases with increased plasticity, and vice
versa.

134
Black clays
0,45
0,43
n = 6
Compression index Cc
0,41
0,39
0,37
0,35
0,33
0,31
0,29
Cc = 0,0099(122-LL)
R 2 = 0,73
Cc vs LL
0,27
0,25
75 80 85 90 95
Liquid limit LL (%)
Figure 7.23. Correlation between laboratory determined liquid limits and
compression indices for black clays.
Black clays and red soils
Compression index Cc
0,5
0,4
0,3
0,2
0,1
0
n = 9
Cc = 0,0016(308-LL)
R 2 = 0,34
Cc vs LL
0 20 40 60 80 100
Liquid limit, LL (%)
Figure 7.24. Correlation between laboratory determined liquid limits and
compression indices for black clays and red soils.
Calculated values of compression index for black clays, cc(b), as well as for combined black
clays and red soils, cc(br), as derived from laboratory measured liquid limits using the
relationships of Equations (7.47 & 7.48) are presented in Table (7.16); alongside laboratory
determined compression indices, cc. The strength of correlation between so calculated and
laboratory measured compression indices are strong (R = 0,85) for the black clays alone; and
only moderate (R = 0,58) for combined black clays and red soils. However, values of
compression indices calculated from laboratory liquid limits using Skempton´s relationship,
i.e. ( Cc(sk) = 0,009(LL-10) are generally overestimated with respect to laboratory determined
compression indices for black clays (Table 7.16). The overestimation could be accounted for

135
by differences in lithology, mineralogy and/ or organic matter content between tropical soils
as encountered in this study, and those of temperate climates on which Skempton´s analysis
and derivation were based. This leaves the new relationships developed in this study as better
and preferred estimators of compressibility characteristics of soils using laboratory measured
liquid limits.
Table 7.16. Correlation: calculated and laboratory derived compression indices.
Sample No. Load kPa Wn (%) LL (%) PI (%) Cc Cc(b) Cc(br) Cc(Sk)
SA2-70cm 100-2400 34 90 54 0,30 0,32 0,35 0,72
SA37-50cm 25-1600 22 83 44 0,40 0,39 0,36 0,66
SB1-70cm 50-2400 30 90 51 0,29 0,32 0,35 0,72
SB42-50cm 100-2400 21 91 47 0,34 0,31 0,35 0,73
SC17-50cm 100-2400 22 81 48 0,44 0,41 0,36 0,64
SC29-50cm 100-2400 20 80 43 0,38 0,42 0,36 0,63
RD1-30cm 12,0-2400 39 49 18 0,42 0,41 0,35
RD1-100cm 12,0-2400 24 51 21 0,40 0,41 0,37
RD1-400cm 12,0-2400 25 48 18 0,39 0,42 0,34
Correlations (R ) Black clays only 0,85
Correlations (R ) Black clays & red soils 0,58
Future works could attempt to perform correlations between laboratory determined
compression indices and index properties, based on a larger amount of data than is currently
available in this study. This would serve to indicate possible improvements in the strength and
reliability of correlations, as well as assessment of compressibility characteristics of the soils
based on laboratory measured index properties.
So far, it can be safely stated that laboratory determined liquid limits could be used to
approximately estimate the compression indices and , therefore, assess compressibility
characteristics of the black clays and red soils found.
Values of secondary compression ,cα, obtained for black clays and red soils are given in Table
(7.17), and have been used to classify the soils into various groups. Typical values of cα for
other types of soil (after Lambe and Whitman, 1979) are also included for comparison
purposes. The black clays fall in the class of normally consolidated clays while the red soils
are classified as normally consolidated to slightly overconsolidated.

136
Table 7.17. Classification of black clays and red soils based on coefficient of secondary
compression.
Coeff. of
secondary
compression Average
Category Sample No. cα value cα Clay type
Typical examples < 0,001 Overconsolidated clays
of clays 0,005-0,02 Normally consolidated clays
(after Lambe & 0,03 and over Very plastic clays and/ or
Whitman –1979)
organic clays
SA2-70cm 0,0002-0,011 0,006 Normally consolidated clays
This study: SA37-50cm 0,003-0,017 0,009 Normally consolidated clays
Black clays SB1-70cm 0,0002-0,012 0,005 Normally consolidated clays
SB42-50cm 0,005-0,024 0,014 Normally consolidated clays
SC17-50cm 0,006-0,02 0,012 Normally consolidated clays
SC29-50cm 0,003-0,014 0,008 Normally consolidated clays
RD1-30cm 0,002-0,004 0,003 Slightly overconsolidated clays
This study: RD1-100cm 0,003-0,008 0,006 Normally consolidated clays
Red soils RD1-400cm 0,002-0,007 0,004 Slightly overconsolidated clays
Results of swell index, Cs, obtained for black clays and red soils during unloading phase of
oedometer tests are summarised in Table (7.18). The black clays generally exhibit higher
swelling capabilities and/ or expansion tendencies (Cs = 0,03-0,075; mean value = 0,054) on
unloading than the red soils (Cs = 0,02-0,03; mean value = 0,027). Correlations between swell
indices and index properties (liquid limit, plasticity index, free swell) are generally poor
(Figures 7.25, 7.26 & 7.27). This implies that uncertainties would be encountered in trying to
assess and/ or predict soils swelling and/ or expansion tendencies based on results of index
tests performed on disturbed and fractioned samples.
Table 7.18. Results of swell indices for black clays and red soils correlated with Atterberg
limits and free swell.
Sample No. Load kPa Wn (%) LL (%) PI (%) FS (%) Cs
SA2-70cm 100-2400 34 90 54 170 0,07
SA37-50cm 25-1600 22 83 44 115 0,075
SB1-70cm 50-2400 30 90 51 145 0,043
SB42-50cm 100-2400 21 91 47 145 0,03
SC17-50cm 100-2400 22 81 48 110 0,04
SC29-50cm 100-2400 20 80 43 110 0,065
RD1-30cm 12,0-2400 39 49 18 20 0,03
RD1-100cm 12,0-2400 24 51 21 15 0,03
RD1-400cm 12,0-2400 25 48 18 15 0,02
Correlations (R ) Black & red soils 0,61 0,64 0,63
Correlations (R ) Black clays only -0,34 -0,14 -0,06
Diagramatically, the generally weak correlations are reflected in terms of plot points which
tend to be more spread out (Figures 7.25, 7.26 & 7.27).

137
Black clays and red soils
Swell index Cs
0,08
0,07
0,06
0,05
0,04
0,03
0,02
0,01
0
n = 9
40 50 60 70 80 90 100
Liquid limit LL (%)
LL vs Cs
Figure 7.25. Correlation between swell index and liquid limit for black clays and red soils.
Black clays and red soils
Swell index Cs
0,08
0,07
0,06
0,05
0,04
0,03
0,02
0,01
0
n = 9
10 20 30 40 50 60
PI vs Cs
Plasticity index PI (%)
Figure 7.26. Correlation between swell index and plasticity index for black
clays and red soils.
Black clays and red soils
Swell index Cs
0,08
0,07
0,06
0,05
0,04
0,03
0,02
0,01
0
n = 9
0 50 100 150 200
Free swell FS (%)
FS vs Cs
Figure 7.27. Correlation between swell index and free swell for black clays
and red soils.

138
7.4.7 Swelling pressure and percentage swelling
Black clays have been shown to exhibit considerable swelling capability, i.e. free swell of
between 100 and 200% when allowed free access to water from a dry state. On the other hand,
red soils exhibit a limited swelling capability when saturated with water (free swell= 15-
20%); and as a result, no swelling pressure determination was performed on these soils.
7.4.7.1 Swelling pressure test
Swelling pressure, SP, of clays is the pressure required to prevent swelling by constraining the
clays to maintain constant volume when saturated with water (Nelson and Miller, 1992).
Swelling pressures of over 1 MN/m² have been registered for some heavily overconsolidated
clays (Head, 1988). In this study, swelling pressures of 49-104 kPa have been determined for
the black clays.
Swelling pressure determination was carried out according to a method devised by Head
(1988), and employed the British Standard oedometer consolidation cell as described in a
previous section (Plate 7.9).
Plate 7.9. Swelling pressure determination of clay soils.

139
Testing procedure involved cutting an undisturbed soil specimen into the consolidation ring,
assembling it in the oedometer cell and mounting the cell in the load frame of a consolidation
press. Distilled water was added into the cell to saturate the soil specimen and a stop-clock
started at the same time. Changes in compression gauge reading due to swelling of specimen
were noted and recorded with time; and small weights added to the weight hanger of the
apparatus to bring the compression gauge reading back to zero or initial reading and,
therefore, prevent swelling. The compression gauge reading was observed over a period of 24
hours, during which time more weights were added as necessary to maintain the reading as
close as possible to zero or initial reading.
120
Swelling Pressure curves for black clays
100
SA41-30cm (104 kPa)
Swelling pressure (kN/m²)
80
60
40
SA2-70cm (95 kPa)
SB1-70cm (49 kPa)
SB42-50cm (53 kPa)
SC17-50cm (70 kPa)
20
SC29-50cm (50 kPa)
0
0 10 20 30 40 50 60
Square-root-time (minutes)
Figure 7.28. Swelling pressure curves obtained for black clays in this study.
A record of the amount of each load increment and time from start when added was tabulated
(Table 7.19), and a graph of total pressure on specimen against square root of time plotted.
Swelling pressure test curves obtained for the black clays in this study are presented in Fig
(7.28). Flattening of the curves serves to indicate equilibrium having been virtually reached.
Table 7.19. Results of swelling pressures for black clays, alongside corresponding index
properties and clay mineralogy (smectites).
Sample No. LL (%) PL (%) PI (%) FS (%) Wn (%) Cr LI
% smectites
(min.)
SP (kPa)
SA2-70cm 90 36 54 170 34 1,04 -0,04 90,00 95
SA41-30cm 89 41 47 135 23 1,39 -0,39 95,00 104
SB1-70cm 90 39 51 145 30 1,19 -0,19 95,00 49
SB42-50cm 91 45 47 145 21 1,50 -0,50 95,00 53
SC17-50cm 81 33 48 110 22 1,22 -0,22 95 70
SC29-50cm 80 37 43 110 20 1,38 -0,38 95 50
Correlations 0,22 -0,11 0,41 0,35 0,34 -0,31 0,31 -0,51

140
The black clays exhibit significant swelling pressures of 49-104 kPa (Table 7.19). However,
no well defined relationship was found to occur between swelling pressure determined on
undisturbed specimens on one hand; and index properties as well as smectite content of the
clays, on the other. Attempted correlations are generally poor and of low strength (R< 0,6) as
shown in Table (7.19); and evidenced by more spread out plot points on scatter diagrams
(Figures 7.29, 7.30, 7.31, 7.32 & 7.33). This shows the unreliability with which swelling
characteristics of the clays could be estimated and / or predicted from results of index tests.
Black clays
Black clays
Swelling pressure SP (kPa)
120
100
80
60
40
20
n=7
LL vs SP
Swelling pressure SP (kPa)
120
100
80
60
40
20
n=7
PI vs SP
0
0
75 80 85 90 95
30 40 50 60
Liquid limit LL (%)
Plasticity index PI (%)
Figure 7.29. Correlation between LL and SP.
Figure 7.30. Correlation between PI and SP.
Once again therefore, serious uncertainties would still be encountered in attempting to assess
the possible magnitude of swelling pressures of the clay soils in situ based on results of index
tests performed on disturbed and fractioned soil samples.
Black clays
Black clays
120
120
Swelling pressure SP (kPa)
100
80
60
40
20
n = 7
Cr vs SP
Swelling pressure SP (kPa)
100
80
60
40
20
n = 7
FS vs SP
0
0,80 1,00 1,20 1,40 1,60
Relative consistency Cr
0
80 130 180
Free swell FS (%)
Figure 7.31. Correlation between Cr and SP.
Figure 7.32. Correlation between FS and SP.

141
Black clays
Swelling pressure SP (kPa)
120
n = 7
100
80
60
40
20
0
10 15 20 25 30 35 40
Natural moisture Wn (%)
Wn vs SP
Figure 7.33. Correlation between Wn and SP.
7.4.7.2 Swelling test
Swelling is the increase in volume of a soil due to absorption of water within the voids when
the applied stress is reduced. It is therefore the reverse and opposite process of consolidation,
and occurs when overconsolidated clays are allowed free access to water (Nelson and Miller,
1992). In consolidation tests, swelling is represented by the unloading portion of the e/ log p
curve (Fig. 7.18 ). The mechanism of swelling is explained by an overconsolidated soil
possessing high suction tensions within its skeleton on unloading, thereby drawing water into
the voids. The resulting increase in volume of voids causes the soil to swell and eventually
disintegrate.
Testing procedure involved use of same apparatus as in the standard oedometer consolidation
test. However, the height of soil specimen was 3 mm less than the height of consolidation
ring; so that the specimen was laterally confined at all times during swelling. The space in the
ring above test specimen was taken up by a flat metal disc of 3 mm thickness and diameter
about 1 mm less than the internal diameter of ring. The specimen in the ring was weighed,
assembled in oedometer cell and mounted in the load frame of consolidation press. Initial
height of specimen was given by height of ring less the average disc thickness.
Swelling pressure, SP, was determined as described above. After establishment of
equilibrium, the swelling test was performed by unloading the specimen in stages from the
swelling pressure, SP, as the starting point; and by following an unloading sequence of
halving the loads at each successive decrement, i.e. 1/2SP, 1/4SP, 1/8SP, and so on down to
the smallest load required. Each unloading stage was maintained until equilibrium had been
reached and no further change in compression gauge reading was observed. During
unloading, cumulative amount of swell (mm) undergone by the specimen for successive
decrements of load was recorded by noting the change in compression gauge reading from its
initial position.
The cumulative swell (S mm) under each load decrement was recorded and expressed as a
percentage of the ultimate amount of swell (Smax mm) which was registered with specimen
under zero load. Results of cumulative percentage swelling (S %) recorded against

142
corresponding load decrements (P kPa) and as obtained for black clays in this study are
presented in Appendix (B). A summary of results of percentage swelling under various
loading for tested samples is given in Table 7.20. The variation of percentage swelling (S)
with pressure loads (P) is presented in Fig. (7.34). The variation is generally similar and close
for all tested samples, and this once again reflects on the general homogeneity of black clays.
Table 7.20. Percentage swelling of black clays under various load decrements (S% in relation
to ultimate amount of swelling Smax).
Sample No. S/Smax S (%) P (kPa) P/SP P (%)
0,01 1 82,46 1,00 100
0,1 10 41,23 0,50 50
SA2/70cm 0,24 24 19,99 0,24 24
0,31 31 10,00 0,12 12
0,52 52 5,00 0,06 6
0,7 70 2,50 0,03 3
1 100 0,01 0,00 0
0,01 1 37,48 1,00 100
0,13 13 18,74 0,50 50
SB1/70cm 0,3 30 10,00 0,27 27
0,46 46 5,00 0,13 13
0,66 66 2,50 0,07 7
1 100 0,01 0,00 0
0,01 1 36,23 1,00 100
0,14 14 17,49 0,48 48
SC25/50cm 0,34 34 8,75 0,24 24
0,55 55 5,00 0,14 14
0,73 73 2,50 0,07 7
1 100 0,01 0,00 0
0,01 1 44,98 1,00 100
0,12 12 22,49 0,50 50
SC29/50cm 0,3 30 11,24 0,25 25
0,54 54 5,00 0,11 11
0,72 72 2,50 0,06 6
1 100 0,01 0,00 0

143
Black clays: P (kPa) vs S (%)
Pressure load P (kPa)
90,00
80,00
70,00
60,00
50,00
40,00
30,00
20,00
10,00
0,00
0 20 40 60 80 100 120
Swelling S (%)
SA2/70cm
SB1/70cm
SC25/50cm
SC29/50cm
Figure 7.34. Variation of percentage swelling of black clays with loading pressures.
A classification of the relative degree of swelling achieved under various loading pressure
decrements for each of the soil samples tested is given in Table (7.21). As a result, a general
classification for the black clays as a whole, has been derived and presented in Table (7.22). It
is observed that black clays would exhibit high percentage swelling when externally loaded to
below 5 kPa; and very low percentage swelling when loaded to over 80 kPa. This implies that
external engineering structures imposing loads of up to 5 kPa and less would still give way to
maximum swelling of the clays on wetting, with a corresponding maximum destabilisation of
the same structures. However, the structures would be relatively more stable when designed to
impose loads of over 80 kPa.
Table 7.21. Relative degree of swelling of black clays under various load
decrements.
Swelling degree
Sample No. S (%) P (kPa) (after author, 2003)
SA2/70cm 100-52 0-4,96 high
52-31 4,96-10 moderate
31-0 10,0-82,46 low
SB1/70cm 100-85 0-1,18
high
85-30 1,18-10 moderate
30-0 10,0-37,48 low
SC25/50cm 100-75 0-1,70 high
75-34 1,70-8,75
moderate
34-0 8,75-36,23 low
SC29/50cm 100-85 0-1,58 high
85-30 1,58-11,24 moderate
30-0 11,24-44,98 low

144
Table 7.22. Derived classification of relative degree of swelling of black
clays based on percentage swelling (after author, 2003).
P (kPa) S (%) Swelling degree
0-2,5 100-75 high
2,5-10 75-30,0 moderate
10-50,0 30-10,0 low
> 50,0 < 10 very low
A correlation between percentage swelling and corresponding load decrements (expressed as
percentage of respective swelling pressures) for the individual soil samples is presented
diagrammatically in Fig. (7.35). The variation is generally logarithmic with a very strong
correlation (R = 0,99 – 1,0).
Individual samples: S (%) vs P (%)
120
Loading P (%)
100
80
60
40
R 2 = 0,98
R 2 = 0,99
R 2 = 1,0
SA2-70cm
SB1-70cm
SC25-50cm
SC29-50cm
Logarithmic SA2-70cm
Logarithmic SB1-70cm
20
0
R 2 = 1,0
0 50 100 150
Logarithmic SC25-50cm
Logarithmic SC29-50cm
Swelling S (%)
Figure 7.35. Correlation between percentage swelling and percent loading pressure of
individual black clay samples.
Results of an overall correlation combining results of all samples tested are presented Fig.
(7.36). The relationship between percentage swelling and extent of loading is once again
logarithmic (Fig. 7.36a) with a very strong correlation (R = 0,995), and could be summarised
by an overall equation, i.e.
P = -22,3Ln(S) + 100,8 (7.49)
where P = extent of loading (%)
S = cumulative swelling (%)

145
The same relationship is represented by a straight-line graph by plotting S% on a logarithmic
scale (Fig. 7.36b).
The above relationship could be used as a guide to estimate the amount of external loading (P
kPa) necessary to produce an allowed amount of swelling (S mm) or percentage swelling
(S%) of the clays, as may be required for engineering design purposes, if the swelling
pressure (SP), as well as ultimate swelling (Smax) under zero loading, are already known. For
instance, limited swelling of the clays ( i.e. S/ Smax < 0,01or S% < 1) would only be realised
in practice with external loads (P ) close to the swelling pressure (SP), i.e. P% (or P/SP *100)
is close or equal to 100. This could also be confirmed by the above established relationship
by substituting for S/Smax = 0,01 (or S% = 1), i.e.
P% = -22,3Ln(1) + 100,8 = 0 + 100,8, or
P% = (P/SP)*100 = 100,8
On the other hand, zero external loading conditions would be expected to allow for maximum
swelling of the clays, i.e. S% = 100, in normal practice. However, substituting P% (i.e. P = 0
kPa) in the above relationship gives an underestimated value of S% = 92, i.e.
0 = -22,3Ln(S) + 100,8 or Ln(S) = 100,8/22,3
so that S = e^4,52 = 92%
The underestimation of percentage swelling from known loading pressures could be explained
by the fact that, test specimens of clays underwent some amount of plastic deformation (or
consolidation) when initially loaded to their swelling pressure values, and could not therefore
recover their original sizes on complete unloading. In this case, the extent of plastic
deformation is equivalent to S% = 8 (i.e., 100-92), on the average. A suggested remedy of this
shortcoming would be to determine the ultimate amount of swelling (Smax) under zero
loading on separate test specimens from those used in the swelling test. Cumulative swelling
values obtained during the test would then be expressed as a percentage of this separately
determined maximun swelling value of Smax.
Future works on swelling tests involving larger clay sample sizes and data could yield more
closely approximating logarithmic relationships with relatively stronger correlations.

146
Combined sample results: S (%) vs P (%)
Loading P (%)
120
100
80
60
40
n = 25
P = -22,3Ln(S) + 100,8
R 2 = 0,99
Combined sample
results
Logarithmic
20
0
0 50 100 150
Swelling S (%)
Figure 7.36a. Correlation between percentage swelling and extent of loading using
combined sample results of black clays.
Combined sample results: S (%) vs P (%)
120
n = 25
100
P= -22,3Ln(S) + 100,8
Loading P (%)
80
60
R 2 = 0,99
Combined
sample results
40
20
0
1 10 100
Swelling S (%) on Log scale
Figure 7.36b. Correlation between percentage swelling (log scale) and extent of loading using
combined sample results of black clays.

147
Alternatively, percentage swelling of clays could be derived by expressing the swelling (S
mm) which occurs under any load decrement (P kPa) as a percentage of the initial thickness
(Ho mm) of test specimen. Complete results of percentage swelling calculated for tested black
clay samples using this second method are given in Appendix C. A more summarised
representation of the results for the samples is given in Table (7.23). A derived classification
of relative degree of swelling of black clays based on this new method is also given in Table
(7.24).
Table 7.23. Percentage swelling of black clays under various load decrements (S% is in
relation to initial specimen height, Ho, i.e. 2 nd method).
Sample No. P (kPa) SP = Pmax (kPa) P (%) S (mm) Smax (mm) S (%) Smax(%)
82,46 100 0,000 0,00
41,23 50 0,106 0,96
SA2-70cm 19,99 24 0,253 2,30
10,00 82,46 12 0,321 1,036 2,92 9,42
5,00 6 0,539 4,90
2,50 3 0,725 6,59
0,00 0 1,036 9,42
37,48 100 0,000 0,00
18,74 50 0,054 0,49
SB1-70cm 10,00 37,48 27 0,126 0,425 1,15 3,86
5,00 13 0,197 1,79
2,50 7 0,279 2,54
0,00 0 0,425 3,86
36,23 100 0 0,00
17,49 48 0,066 0,60
SC25/50cm 8,75 36,23 24 0,16 0,475 1,45 4,32
5,00 14 0,26 2,36
2,50 7 0,347 3,15
0,00 0 0,475 4,32
44,98 100 0 0,00
22,49 50 0,061 0,55
SC29-50cm 11,24 44,98 25 0,149 0,493 1,35 4,48
5,00 11 0,266 2,42
2,50 6 0,356 3,24
0,00 0 0,493 4,48
Average 50,29 0,607 5,52
Table 7.24. Classification of relative degree of swelling of black clays based
on percentage swelling derived using the new/ 2 nd method (author, 2003).
P (kPa) P (%) S (%) Swelling degree
< 2,5 < 5 > 5 high
2,5-7,5 5-15 5 – 2,5 moderate
7,5-25,0 15-50 2,5- 0,5 low
> 25,0 > 50 < 0,5 very low

148
The variation of so calculated percentage swelling (S%) with extent of external loading P%
(i.e., load decrements, P kPa, expressed as a percentage of respective swelling pressure, SP
kPa) is presented diagrammatically in Fig. (7.37a). The variation is also closely logarithmic
with a very strong correlation (R = 0,98), i.e.
P = -15,86Ln(S) + 29,84 (7.50)
where P = extent of loading (%)
S = cumulative swelling (%), derived w.r.t initial specimen height Ho
This relationship is also represented as a straight line graph in Fig. (7.37b) where percentage
swelling (S%) has been plotted on a logarithmic scale.
The second relationship could also be possibly used to estimate and/ or anticipate percentage
swelling (S%) and/ or swelling (S mm) that may arise from certain or chosen projected
external loading conditions. For instance, choosing zero loading conditions implies P (kPa)
and P% [i.e. (P/SP)*100] are also zero, so that from Equation (7.50),
0 = -15,86Ln(S) + 29,84 or Ln(S) = 29,84/15,86 = 1,88, so that
S = e^ 1,88 = 6,55%
Or for initial specimen thickness Ho of 11mm,
S = (6,55/100) * 11 = 0,72mm
On the average, therefore, the black clays would undergo a maximum percentage swelling of
6,55% (or swelling of 0,72 mm relative to the initial specimen thickness Ho of 11 mm), under
zero external loading conditions. Other values of percentage swelling and/ or amount of
swelling that may arise from selected external loading could be similarly calculated or just
read off from the curves of Figs. (7.37a &b).
The second relationship could also be possibly used for estimating the extent of external
loading (P%) and/ or external load (P kPa) necessary to give chosen or allowable percentage
swelling and/ or amount of swelling, as may be required by foundation designers. For
instance, a selected average percentage swelling of S% = 2 (or S = 0,22 mm for Ho = 11 mm)
would be exhibited by the clays when the extent of external loading P% is such that
P% = -15,86 * Ln(2) + 29,84 = -(15,86 * 0,693) + 29,84, or
P% = 18,85,
and this is the same value given by the curve of Fig. (7.37) for S% = 2.
For an average swelling pressure, SP = 50,29 kPa as calculated for tested samples of black
clays in Table (7.23), the external loading pressure is given by
P (kPa) = (P% * SP)/100 = (18,85 * 50,29)/100 or
P (kPa) = 9,48

149
Combined sample results: S % vs P% (2nd method)
120
100
n = 25
Loading P (%)
80
60
40
P = -15,86Ln(S) + 29,84
R 2 = 0,96
Combined
sample results
20
0
0,00 2,00 4,00 6,00 8,00 10,00
Swelling S (%)
Figure 7.37a. Correlation between percentage swelling (derived w.r.t initial specimen height
Ho) and extent of external loading for combined sample results of black clays.
Combined sample results: S % vs P% (2nd method)
120
n = 25
Loading P (%)
100
80
60
40
P = -15,86Ln(S) + 29,84
R 2 = 0,96
Combined
sample results
20
0
0,01 0,10 1,00 10,00
Swelling S (%) on log scale
Figure 7.37b. Correlation between percentage swelling (derived w.r.t initial specimen height
Ho) on log scale, and extent of external loading for combined sample results of black clays.

150
In comparison, black clays from field locations representing sample No. SA2-70cm alone
have a swelling pressure of SP = 82,46 kPa, and would therefore require an external load of
15,54 kPa to permit the same amount of percentage swelling, S% = 2, i.e.
P (kPa) = (18,85 * 82,46)/100 = 15,54
Other extents of external loading necessary to give certain permissible percentage swelling
could be similarly calculated if the swelling pressure (SP) of the clays is already known.
Tsiambaos and Tsaligopoulos (1995) investigated swelling characteristics of expansive clays
based on examples from Greece. As a way of estimating the swelling characteristics, they
suggested plotting the ratio of percentage swelling (w.r.t original specimen height, Ho) to
corresponding load decrement, S (%)/P (kPa) on log scale, against the ratio of load decrement
to swelling pressure, [P (kPa)/SP (kPa)] * 100%. Results of such a relationship as applied to
data of black clays obtained in this study are presented in Table (7.25) and Fig.(7.38).
Table 7.25. Computed values of ratio S(%)/P and P/SP (%) (according to Greek method)
for black clays in this study.
Sample No. P (kPa) SP = Pmax (kPa) Smax (mm) S (%) S (%) / P P (%) = (P/SP)*100
82,46 0,00 0,01 100
41,23 0,96 0,02 50
SA2/70cm 19,99 82,46 1,036 2,30 0,12 24
10,00 2,92 0,29 12
5,00 4,90 0,98 6
2,50 6,59 2,64 3
37,48 0,00 0,01 100
18,74 0,49 0,03 50
SB1/70cm 10,00 37,48 0,425 1,15 0,12 27
5,00 1,79 0,36 13
2,50 2,54 1,02 7
36,23 0,00 0,01 100
17,49 0,60 0,03 48
SC25/50cm 8,75 36,23 0,475 1,45 0,17 24
5,00 2,36 0,47 14
2,50 3,15 1,26 7
44,98 0,00 0,01 100
22,49 0,55 0,02 50
SC29/50cm 11,24 44,98 0,493 1,35 0,12 25
5,00 2,42 0,48 11
2,50 3,24 1,30 6
Average value: 50,29 0,607(5,52%)

151
Black clays: Greek method
P% = (P/SP) * 100
120
100
80
60
40
20
n = 21
P/SP = -17,32Ln(S/P) + 1,18
R 2 = 0,85
S/P vs P/SP (%)
0
0,00 0,50 1,00 1,50 2,00 2,50 3,00
S (%)/P (kPa)
Fig. 7.38a. Correlation between ratios S(%)/P and P/SP (%), i.e. Greek method, for combined
sample results of black clays ( S% derived w.r.t initial specimen height Ho).
Black clays: Greek method
P% = (P/SP) * 100
120
100
80
60
40
20
n = 21
P/SP = -17,32Ln(S/P) + 1,18
R 2 = 0,85
S/P vs P/SP (%)
0
0,01 0,10 1,00 10,00
S (%)/ P (kPa)
Figure 7.38b. Correlation between S(%)/P on log scale and P/SP (%) for combined sample
results of black clays, based on the Greek method (S% w.r.t initial specimen height, Ho).
Correlations using the Greek method resulted in the following relationship, i.e.
P (%) = -17.32Ln(S/SP) + 1,18 (7.51)
Where S (%) = percentage swelling in relation to initial specimen height, Ho mm.
P (kPa) = load decrement
SP (kPa) = swelling pressure
P% = [P(kPa)/SP (kPa)] * 100

152
For loading conditions equal to swelling pressure, SP, (i.e. P% or P/SP * 100 = 100%), the
percentage swelling is usually expected to be zero, i.e. S% = 0. However, percentage swelling
value given by the Greek method (Eq. 7.51) for these loading conditions is overestimated, i.e.
by substitution,
100 = -17.32Ln(S/SP) + 1,18, or
S/SP = e^-5,706 = 0,0033
For an average swelling pressure of SP = 50,29 kPa for black clays,
S% = 0,0033* 50,29 or
S% = 0,17
In comparison, the second method (Eq. 7.50) developed in this study gives a more reliable
estimation of S% for the same loading conditions above, i.e. S% = 0,01.
On the other hand, estimation of loading pressures necessary to realise permitted percentage
swelling values using the Greek method will require solution of rather cumbersome
mathematical equations of the form
aP + bLn(P) + c = 0,
where P = the loading pressure (kPa)
a, b, c = constants
In addition, the strength of estimating swelling characteristics by the Greek method (Eq. 7.51)
is inferior (R = 0,92) relative to that of the second method (Eq. 7.50) developed in this study
(R = 0,98).
In conclusion, the second method (Eq. 7.50) developed in this study would be preferred and
more reliable than the Greek one in estimating swelling characteristics of black clays.
Abebe Solomon (2002), undertook a laboratory investigation of the swelling characteristics of
partially saturated and remoulded highly expansive clay materials in Germany. He adopted
an improved testing procedure and apparatus in the form of a modified triaxial compression
machine and/ or cell to determine swelling pressure and associated volume change (swelling
strain) simultaneously. The obtained results could be presented in the form of curves of both
swelling pressure and moisture/ water up take variations with time, the interpretation of which
serves to provide more accurate insights regarding swelling behaviour and capability of clays
soils. However, his engineering computations of swelling strain rate (in terms of variations of
swelling stress and moisture) are beyond the scope of the present work.
7.4.8 Evaluation of results of oedometer consolidation, swelling pressure and
percentage swelling
Results of consolidation parameters cv, mv, cc and secondary compression, cα, show the red
soils to be slightly overconsolidated, especially the deeper horizons (Table 7.17), so that they
have in the past been subjected to a pressure greater than the present overburden pressure.
This is most probably due to the clays having been covered by deposits of soil or rock
(volcanic lava), and which were subsequently eroded away in the course of geological time;
leading to a reduction in effective pressure. Preconsolidation effects could have also been

153
partly produced by effects of weathering characterised by removal of soluble bases and
minerals, and partly due to partial drying; thereby causing a reduction in effective pressure in
the soil mass. As a result, the soils tend to exhibit a stiff to hard consistency with depth.
However, upper sections of soils up to 1,5 - 2m depth are generally soft to firm, and this is
most likely due to reduced preconsolidation effects (most probably accounted for by chemical
weathering and partial drying only).
In spite of overconsolidation, the red soils generally exhibit limited percentage swelling and
swelling pressure effects when allowed free access to water. This could be explained by the
relatively high content of iron oxide in the soils; and this has the effect of cementing the clay
particles together, thereby counteracting, inhibiting and/ or suppressing possible swelling and
swelling pressure effects that could arise from overconsolidation character on saturation with
water (Fig. 7.18).
Results of swelling tests performed on black clays show variation of percentage swelling
(S%) with progressive load decrements (P kPa) to be logarithmic in relationship with a very
strong correlation (R = 0,99), when the external loading is expressed as a percentage of the
swelling pressure (SP kPa) and amount of swelling (S mm) as a percentage of ultimate
swelling (Smax mm) which occurs under zero external loading. The above relationship could
be used as a guide to estimate the amount of external loading (P kPa) necessary to produce an
allowed amount of swelling (S mm) or percentage swelling (S%) of the clays, as may be
required for engineering design purposes, if the swelling pressure (SP), as well as ultimate
swelling (Smax) under zero loading, are already known. In general, the clays would exhibit
high percentage swelling (S > 75%) when imposed loads from light engineering structures
located on and/ or within, fall below 5 kPa. On the other hand, external imposed loads greater
than 80 kPa would mean minimal swelling of the clays (S < 10%).
The swelling characteristics of black clays could also be expressed in terms of the percentage
swelling which occurs with respect to initial test specimen thickness, Ho mm [i.e., S% =
(S/Ho) * 100] and extent of loading (external loading, P kPa, expressed as a percentage of
swelling pressure, SP kPa). This second variation and/ or relationship is also logarithmic with
a similarly very strong correlation (R = 0,98). For a known swelling pressure, SP, of the clays,
this relationship may be used for the estimation of percentage swelling which could occur for
a selected external loading, as well as external loading conditions necessary to give rise to
certain allowable percentage swelling; with the purpose of meeting specific design
requirements.
In practice, preconsolidation pressure, pc, and overconsolidation ratio, OCR (i.e OCR = pc/po;
po = overburden pressure), are derived through engineering analysis methods by constructing
field consolidation curves representing soils loaded in situ from laboratory consolidation
curves (Casagrande, 1936; Leonards, 1962; Schmertmann, 1953/54). The analysis usually
relies on there being sufficient load increments in the laboratory test to obtain three points on
a straight line, as well as data on specific gravity of soil particles and Atterberg limits for
cross –checking the compression index, cc. However, no such construction of field
consolidation curves was done in this study due to limitations imposed by the capacity of
consolidation press used.
On the other hand, results of consolidation parameters and secondary compression values
(Table 7.17) show the black clays as being normally consolidated, so that the soils have never
been subjected to an effective stress greater than the present effective overburden pressure.
They are generally soft to firm and stiff with depth. The large percentage swelling and

154
swelling pressures exhibited by these clays when allowed access to water are therefore a
result of the high content of expansive clay minerals (smectites) found within, rather than
overconsolidation character.
The black clays studied and sampled during the dry season were observed to exhibit strong
dessication cracks and a firm to stiff, sometimes hard consistency in the field. Later laboratory
studies have shown the clays to posses high swell and expansion potential by exhibiting large
percentage swelling and high swelling pressures when allowed free access to water (Table
7.19; Fig. 7.28). The appreciable amount of swelling and swelling pressures which develop on
saturation of the soils with water could be due partly to some form of slight seasonal
overconsolidation of black clays, previously preconsolidated by effects of lowering of the
ground water table and partial drying during the hot and dry season; and partly to the high
smectite content, as already stated.

155
Chapter 8
Distribution of index and strength properties in black clays
8.1 Natural moisture content
The distribution of natural moisture content of soils at depths of less than 0,50m as well as
those at 0,50m and greater, is summarised diagrammatically in Figures (8.1a & b),
respectively.
Natural moisture content varation; < 0,50m depth
natural moisture content
(%)
40
35
30
25
20
15
10
5
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Figure 8.1a. Natural moisture content variation across the study area for
Depths less than 0,50m (north-south distance: 4000 m).
Field profiles A, B, C, D and E show moisture contents of 19-34%, 20-24%, 18-26%, 18-27%
and 21-25% for depths of less than 0,50m; and 23-32%, 20-29%, 17-26%, 21-29% and 21-
29% for depths of 0,50m and greater, respectively.
Natural moisture content variation; 0,50m and greater
Natural moisture content
(%)
35
30
25
20
15
10
5
0
0 2000 4000 6000 8000 10000 12000
Profile A
Profile B
Profile C
Profile D
Profile E
Distance along profile (m)
Figure 8.1b. Natural moisture content variation across the study area for the
depth of 0,50m and greater (north-south distance: 4000 m).

156
Table 8.1. Values of index properties of soils along field profiles in this study.
Profile
Depth
(m)
Sample No.
Sta.
position (m)
Wn
(%) LL (%) PI (%)
LS
(%)
FS
(%)
Depth
(m)
Sample No.
Wn
(%) LL (%) PI (%)
SA1-30cm 0 32 78 41 25 135 SA1-50cm 30 75 39 23 100
SA2-
A < 0,50 SA2-30cm 250 34 93 54 27 135 ≥ 0,50 70/105cm 32 89 55 26 145
SA28-30cm 6750 26 85 45 25 125 SA28-50cm 27 89 47 25 135
SA37-
SA37-30cm 9000 19 81 43 24 115
50/80cm 23 84 39 26 115
SA41-30cm 10000 23 89 47 26 135 SA41-50cm 25 90 49 24 120
SB1-30cm 0 23 82 47 23 125 SB1-50/70cm 29 87 50 25 140
B < 0,50 SB5-30cm 1000 24 81 47 23 120 ≥ 0,50 SB5-50/70cm 29 88 50 25 140
SB7-30cm 1500 23 83 48 24 130 SB7-50/70cm 28 87 49 25 138
SB18-30cm 4250 20 82 43 23 120 SB18-50cm 20 72 39 21 120
SB27-30cm 6500 22 85 47 24 130 SB27-50cm 22 88 49 25 130
SB31-30cm 7500 23 87 49 24 130 SB31-50cm 23 86 48 25 130
SB41-30cm 10000 23 89 44 26 145 SB41-50cm 24 91 47 25 145
SB42-30cm 10250 22 88 44 25 145 SB42-50cm 21 91 47 26 145
SC1-30cm 0 24 82 48 23 125 SC1-50cm 26 85 48 24 135
C < 0,50 SC5-30cm 1000 23 83 48 23 125 ≥ 0,50 SC5-50cm 25 84 47 24 135
SC9-30cm 2000 25 84 48 24 130 SC9-50cm 26 85 48 24 135
SC17-30cm 4000 21 76 41 25 105 SC17-50cm 22 81 48 24 110
SC25-30cm 6000 26 88 49 25 125 SC25-50cm 26 89 48 25 145
SC29-30cm 7000 21 82 49 25 110 SC29-50cm 20 80 43 26 110
SC33-30cm 8000 20 82 49 25 120 SC33-50cm 21 83 50 26 120
SC41-30cm 10000 18 85 47 29 125 SC41-50cm 17 84 47 29 125
SD1-30cm 0 27 82 46 26 125 SD1-50/80cm 29 87 48 27 135
D < 0,50 SD5-30cm 1000 26 84 47 26 125 ≥ 0,50 SD5-50/70cm 29 88 49 27 135
SD7-30cm 1500 27 84 48 27 110 SD7-50/70cm 29 89 51 26 138
SD17-30cm 4000 20 84 44 24 120 SD17-50cm 22 84 49 25 130
SD25-30cm 6000 22 80 39 25 125 SD25-50cm 21 86 45 25 135
SD29-30cm 7000 18 85 47 27 125 SD29-50cm 22 86 51 26 145
SD33-30cm 8000 20 85 47 27 125 SD33-50cm 21 86 50 25 145
SD41-30cm 10000 23 85 46 26 130 SD41-50cm 25 90 49 26 135
SE1-30cm 0 24 82 48 23 125 SE1-50/70cm 29 88 50 25 140
E < 0,50 SE5-30cm 1000 25 85 48 24 125 ≥ 0,50 SE5-50/70cm 28 89 49 26 138
SE13-30cm 3000 21 86 47 24 125 SE13-50cm 24 89 49 25 130
SE21-30cm 5000 25 88 49 25 125 SE21-50cm 26 90 49 26 145
SE29-30cm 7000 22 81 46 25 120 SE29-50cm 23 83 47 26 120
SE37-30cm 9000 21 85 48 24 125 SE37-50cm 22 85 48 25 130
SE41-30cm 10000 22 87 47 25 135 SE41-50cm 21 88 48 26 135
max 34 93 54 29 145 max 32 91 55 29 145
Statistical < 0,50 min 18 76 39 23 105 ≥ 0,50 min 17 72 39 21 100
analysis range 16 17 15 6 40 range 15 19 16 8 45
median 23 84 47 25 125 median 25 87 48 25 135
mode 23 85 47 25 125 mode 29 89 49 25 135
mean 23 84 47 25 126 mean 25 86 48 25 132
std 3,37 3,30 2,78 1,38 8,26 std 3,56 4,11 3,28 1,29 11,33
128,2
var 11,36 10,90 7,74 1,89 68,25 var 12,69 16,86 10,76 1,67 8
cov 9,71 8,28 5,60 1,05 35,52 cov 9,71 8,28 5,60 1,05 35,52
correlation 0,83 0,63 0,63 0,61 0,39 correlation 0,83 0,63 0,63 0,61 0,39
There is a general decrease of moisture content of soils west-eastwards across the study area
at depths of less than 0,50m, as well as those at 0,50m and more. This could be attributed to a
similar variation in the total thickness of soils which also decreases west-eastwards across the
study area, from depths of 1,50-2,0m in the western parts to 0,50-0,70m in the eastern parts of
LS
(%)
FS
(%)

157
the study area, respectively. Representation of this moisture variation in contour form is given
in Fig. (8.2).
Natural moisture (Wn) variation (< 0,5 m)
3234 26
1°19´S 4000
19 23
Profile distance (m)
27 26 27
3000
20 22 18 20 23
23 24 23 20 22 23 23
2000
West
24 25 21 25 22 21 22
1000
24 23 25 21 26
1°21.5´S 0
21 20 18
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
Natural moisture (Wn) variation (> 0,5 m)
36°49´E
3032 27 23 25
1°19´S 4000
Wn%
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
Profile distance (m)
29 29 29 22 21 22 21 25
3000
29 29 28 20 22 23 24
2000
West 29 28 24 26 23 22 21
1000
26 25 26 22 26 20 21 17
0
1°21.5´S
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South Profile distance (m)
36°55´E
17
18
19
20
21
22
23
24
25
26
27
28
29
Wn%
30
Fig. 8.2. Contoured representation of distribution and variation of natural moisture content
in black clays.
The area is dotted with patches of relatively low moisture content (Fig. 8.2) corresponding to
increasingly lateritic black clays; as well as those of relatively high moisture content in
swampy parts and vicinity of stream and/ or river valleys.

158
The distribution and variation of natural moisture content at depths of less than 0,50m across
the study area are characterised by a maximum and minimum of 34% and 18%, giving a range
of 16% (Table 8.1). Also characteristic are median, mode and mean values of 23% as well as
a standard deviation of 3,37 and variance of 11,36. The distribution is similar at depths of
0,50m and greater, with maximum and minimum values of 32% and 17% giving a range of
15% (Table 8.1). However, a slight increase in moisture content with depth of soils occurs
(due to reduced drying effects), and is reflected by a median of 25%, mode of 29% and mean
of 25%. Also calculated for this depth are a standard deviation of 3,56 and a variance of
12,69.
The variation of natural moisture content of soils at the two depth intervals across the study
area is generally similar, so that the two sets of data give a covariance of 9,71 and a strong
correlation of 0,83 (Table 8.1).
The upper 50cm of red soils were found to exhibit a relatively elevated moisture content of
38-39%, most probably a result of recent rainfall. However, depths greater than 0,50m are
generally characterised by a moisture content of 24-26%.
8.2 Liquid limit
The distribution of liquid limit values across the study area is presented in Figures (8.3a & b),
which show a concentration about a mean of 84% and 86%, respectively.
Liquid limit variation; < 0,50m depth
Liquid limit (%)
100
90
80
70
60
50
40
30
20
10
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Figure 8.3a. Liquid limit variation across the study area for the depth of less than
0,50m (north-south distance: 4000 m).

159
Liquid limit variation; 0,50m and greater
Liquid limit (%)
100
90
80
70
60
50
40
30
20
10
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Figure 8.3b. Liquid limit variation across the study area for the depth of
0,50m and greater (north-south distance: 4000 m).
The distribution is generally similar along the five field profiles, both at depths of less than
0,50m and those of 0,50m and deeper. The profiles A, B, C, D and E exhibit liquid limit
values of 78-93%, 81-89%, 76-88%, 80-85% and 81-88% for depths of less than 0,50m; and
75-90%, 72-91%, 80-89%, 84-90% and 83-90% for depths of 0,50m and greater.
The distribution and variation of liquid limit at depths of 0,50m and those at 0,50m and
greater across the study area are generally similar, being characterised by maximum and
minimum values of 93% and 76% with a range of 17%; and 91% and 72% with a range of
19%, respectively (Table 8.1).

160
Liquid limit (LL) variation (< 0,5 m)
36°55´E
LL (%)
1°19´S
Profile distance (m)
West
1°21.5´S
36°49´E
LL%
88
87.5
87
86.5
86
85.5
85
84.5
84
83.5
83
82.5
82
81.5
81
80.5
80
79.5
79
78.5
South
Profile distance (m)
Fig. 8.4a. Block diagram showing distribution and variation of liquid limit values at depths of
less than 0,5m in black clays.
A contoured representation of the distribution is shown in Figures (8.4a & b) in which
isolated patches of relatively low liquid limits correspond to zones of more lateritic clays
and/or swampy environments. On the whole, however, there is a slight tendency of liquid
limit values to increase with depth. This could be attributed to the relatively higher organic
matter content of black clays for depths of less than 0,50m, and which has the effect of
inhibiting the plasticity of soils. As a result, soil depths of less than 0,50m show median,
mode and mean liquid limit values of 84%, 85%, and 84%, respectively, as well as standard
deviation of 3,30 and variance of 10,90; while soil depths of 0,50m and greater, exhibit
relatively larger median, mode and mean values of 87%, 89% and 86%, respectively. The
latter soil depths are also characterised by standard deviation of 4,11 and variance of 16,86.
On the whole, however, the two sets of data at the two different depth intervals are generally
comparable and in good agreement, this being indicated by a covariance of 8,28 and a fairly
strong correlation of 0,63 (Table 8.1).

161
Liquid limit (LL) variation (> 0,5 m)
36°55´E
89
88
87
Profile distance (m)
LL%
90
LL (%)
South
84
85
86
83
82
1°19´S
81
80
Profile distance (m)
West
1°21.5´S
36°49´E
Fig. 8.4b. Block diagram showing distribution and variation of liquid limit values at depths of
more than 0,5m in black clays.
76
77
78
79
Liquid limit values obtained for the red soils in this study are comparatively lower, and range
from 48% to 57%.
8.3 Plasticity index
The variation of plasticity index of soils along the five field profiles of this study is
represented diagrammatically in Figures (8.5a & b), for depths of less than 0,50m and those of
0,50m and greater, respectively.

162
Plasticity index variation; < 0,50m depth
Plasticity index (%)
60
50
40
30
20
10
Profile A
Profile B
Profile C
Profile D
Profile E
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Figure 8.5a. Plasticity index variation across the study area for the depth
of less than 0,50m (north-south distance: 4000 m).
Profiles A, B, C, D and E show plasticity indices of 41-54%, 43-49%, 41-49%, 39-48% and
46-49% for depths of less than 0,50m; and 39-55%, 39-50%, 43-50%, 45-51% and 47-50%
for depths of 0,5m and greater, respectively.
Plasticity index variation; 0,50m and greater
60
Plasticity index (%)
50
40
30
20
10
Profile A
Profile B
Profile C
Profile D
Profile E
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Figure 8.5b. Plasticity index variation across the study area for the depth of
0,50m and greater (north-south distance: 4000 m).
The plasticity index of soils is similarly distributed across the study area for both depths of
less than 0,50m and those of 0,50m and deeper, exhibiting maximum and minimum values of
54% and 39% with a range of 15%; and 55% and 39% with a range of 16%, respectively
(Table 8.1). Soil depths of less than 0,50m are also characterised by a median, mode and
mean value of 47%, as well as a standard deviation of 2,78 and variance of 7,74. Similar and
closely comparable parameters were calculated for depths of 0,50m and greater, in terms of
median, mode and mean values of 48%, 49% and 48%, respectively. A standard deviation of
3,28 and variance of 10,76 were also computed for the latter depth interval. A summary of
the distribution in the form of contour maps for the two depth intervals is presented in Fig.

163
(8.6). The few isolated patches of relatively low plasticity values imply areas of increasingly
lateritic clays and/ or swampy environments .
PI%
Plasticitx index (PI) variation (< 0,5m)
1°19´S 41 54 45 43 47
4000
(m)
Profile distance (m)
46
3000
47 48 44 39 47 47 46
47 47 48 43 47 49 44
2000
West
48 48 47 49 46 48 47
1000
1°21.5´S48 48 48 41 49 49 49
0
47
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 (m)
36°49´E
South
Profile distance (m) 36°55´E
49
48.5
48
47.5
47
46.5
%
46
45.5
45
44.5
44
43.5
43
42.5
42
41.5
41
40.5
Plasticity index (PI) variation (> 0,5m)
1°19´S 39 55 47 39 49
4000
(m)
Profile distance (m)
48 49 51 49 45 51 50 49
3000
50 50 49 39 49
2000
48 47
West
50 49 49 49 47 48 48
1000
1°21.5´S48 47 48 48 48 43 50
0
47
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 (m)
36°49´E
South Profile distance (m)
36°55´E
41.5
42
42.5
43
43.5
44
44.5
45
45.5
46
46.5
47
47.5
48
48.5
49
49.5
50
PI%
50.5
Figure 8.6. A contoured representation of the distribution and variation of plasticity index
values in black clays.
The two depth intervals show more or less similar plasticity values and parameters, implying
a more or less uniform and/ or homogeneous character of the black clays, both across the
study area and with depth. This is further evidenced by a covariance of 5,60 and a fairly
strong correlation of 0,63 between the two sets of data obtained for the two depth intervals
(Table 8.1).
The plasticity indices obtained for the red soils in this study are relatively low, ranging from
18-22%.

164
8.4 Linear shrinkage
The spatial variation and distribution of values of linear shrinkage determined in this study are
presented in Table (8.1) and Figures (8.7a & b).
Linear shrinkage variation; < 0,5m depth
30
Linear shrinkage (%)
25
20
15
10
5
Profile A
Profile B
Profile C
Profile D
Profile E
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Fig. 8.7a. Linear shrinkage variation across the study area for the depth of less than 0,50m (north-south distance: 4000 m).
Black clays of less than 0,50m depth generally exhibit little variation in linear shrinkage
values along each of the five field profiles across the study area (Fig. 8.7a); with percentage
values of 24-27, 23-26, 23-29, 24-27 and 23-25 determined for profiles A, B, C, D and E,
respectively. On the whole, soil thicknesses in this depth interval across the study area exhibit
linear shrinkage values of between 23-29%, with a range of 6% as well as mode, median and
mean values of 25%. The distribution is also characterised by a standard deviation of 1,38
and variance of 1,89.
Linear shrinkage variation; 0,50m and more
30
25
Linear shrinkage (%)
20
15
10
5
Profile A
Profile B
Profile C
Profile D
Profile E
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Figure 8.7b. Linear shrinkage variation across the study area at depths of 0,5m and greater (north-south distance: 4000 m).

165
Similarly, soil thicknesses of black clays with depths equal to and/ or greater than 0,50m show
linear shrinkage values generally concentrated about 25% (Figure 8.7b); with percentage
values of 23-26, 21-26, 24-29, 25-27 and 25-26 recorded for profiles A, B, C, D and E,
respectively. On the whole across the study area, values of 21-29% exhibiting a range of 8%
as well as a mode, median and mean of 25% have been recorded for the depth. A standard
deviation of 1,29 and variance of 1,67, have also been calculated. Fig. (8.8) shows a
contoured distribution pattern of linear shrinkage at the two depth intervals.
Linear shrinkage (LS) variation (< 0,5 m)
2527 25 24 26
1°19´S 4000
Profile distance (m)
26 26 27 24 25 27 27 26
3000
23 23 24 23 24 24 26
2000
West
23 24 24 25 25 24 25
1000
23 23 24 25 25 25 25 29
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South
Profile distance (m) 36°55´E
LS (%)
27.5
27
26.5
26
25.5
25
24.5
24
23.5
23
Linear shrinkage (LS) variation (> 0,5 m)
2326 25 26 24
1°19´S 4000
22.5
Profile distance (m)
27 27 26 25 25 26 25 26
3000
25 25 25 21 25 25 25
2000
West 25 26 25 26 26 25 26
1000
1°21.5´S
24 24 24 24 25 26 26 29
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m) 36°55´E
27
26.5
26
25.5
25
24.5
24
23.5
23
22.5
22
Figure 8.8. A contoured distribution pattern of linear shrinkage in black clays.
27.5
LS (%)
A comparison of linear shrinkage values of black clays for depths of less than 0,50m with
those obtained for depths equal to/ and greater than 0,50m yield a moderately strong
correlation of 0,61 and a covariance of 1,05. In addition, there is a strong agreement between
values of linear shrinkage obtained for the two depth intervals in terms of comparable
respective parameters of mode, median, mean, standard deviation and variance. This serves to

166
strongly suggest that the whole soil thickness of black clays of the current study area is more
or less homogenous and uniform in terms of linear shrinkage and, possibly, other index and/or
engineering properties.
Linear shrinkage values obtained for the red soils are comparatively low and range from 10-
11%.
8.5 Free swell
The distribution of free swell values along the five field profiles in this study is presented in
Figure (8.9). Profiles A, B, C, D and E show free swell values of 115-135%, 120-145%, 105-
130%, 110-130 and 120-135% for depths of less than 0,50m concentrated about a mean value
of 126%; and 100-145%, 120-145%, 110-145%, 130-145% and 120-145% for depths 0f 0,5m
and over, concentrated about a mean value of 132%, respectively. The complete set of data is
given in Table (8.1).
Distribution of free swell values at a depth of less than 0,50m across the study area is
characterised by minimum and maximun values of 105% and 145% respectively, giving a
range of 40%, median and mode of 125%, mean of 126% as well as a standard deviation of
8,26 and variance of 68,25.
Free swell variation; < 0,50m depth
Free swell (%)
160
140
120
100
80
60
40
20
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Figure 8.9a. Free swell variation across the study area at depths of less than
0,5m (north-south distance: 4000 m).
On the other hand, black clays at depths of 0,50m and over exhibit free swell values of
between 100% and 145% with a range of 45%, across the study area. A median and mode of
135%, mean of 132%, standard deviation of 11,33 and variance of 128,28 were also
calculated for the depth.

167
Free swell variation; 0,50m depth and greater
Free swell (%)
160
140
120
100
80
60
40
20
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Figure 8.9b. Free swell variation across the study area at depths of 0,50m
and greater (north-south distance: 4000 m).
The two sets of data obtained for depths of less than 0,50m and those of 0,50m and over, are
generally comparable and similar in their distribution in terms of maximum and minimum
values as well as their range. A slight variation is, however, exhibited in terms of the median,
mode and mean values as well as the standard deviation and variance. As a result, a
covariance of 35,52 and relatively weak correlation of 0,39 has also been recorded for the two
sets of data. Relatively lower values of free swell and/ or swelling capability in upper soil
depths could be attributed to increased organic matter content.
Fig. (8.10) shows a contoured representation of the above described distribution, in which
patches of low swelling capabilities are associated with lateritic clays; as well as clays of
swampy environments and/ or those adjacent to river valleys and stream channels with
impeded drainage and relatively higher organic matter content.
Free swell values obtained for red soils in this study are comparatively low and range from
15-20%.

168
Free swell (FS) variation (< 0,5 m)
1°19´S 135135 125 115 135
4000
Profile distance (m)
125 125 110 120 125 125 125 130
3000
125 120 130 120 130 130 145
2000
West 125 125 125 125 120 125 135
1000
125 125 130 105 125 110 120 125
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
Free swell (FS) variation (> 0,5 m)
1°19´S 100 145 135 115 120
4000
FS%
140
138
136
134
132
130
128
126
124
122
120
118
116
114
112
110
108
Profile distance (m)
135 135 138 130 135 145 145 135
3000
140 140 138 120 130 130 145
2000
West 140 138 130 145 120 130 135
1000
135 135 135 110
145
110 120 125
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South
Profile distance (m) 36°55´E
114
116
118
120
122
124
126
128
Figure 8.10. Contour maps showing distribution of free swell values and variation of swelling
capability in black clays.
130
132
134
136
138
FS%
140
8.6 Clay and fine fraction
The distribution and variation of percentage clay and fines (clay + silt) content of black clays
across the study area is summarised in Table (8.2) in the order of profiles. The variation of
clay fraction at depths of less than 0,50m is characterised by maximum and minimum values
of 37% and 13% giving a range of 24%; as well as median, mode and mean values 29%, 31%
and 27%, respectively. A similar variation of clay fraction occurs at depths of 0,50m and
greater, where closely comparable maximum and minimum values of 44% and 14% giving a
range of 30%; as well as median/ mode of 27% and mean of about 26% have been registered.
Closely agreeable standard deviation values of 6,09 and 6,54; as well a strong correlation of
0,82 for the two sets of data serve as further evidence for similarity of variation of clay

169
fraction at the two depth intervals across the study area. The distribution and variation of
percent clay in black clays across the study area is also summarised in contour form in Fig.
(8.11).
Table 8.2. Values of clay and fines fractions of soils along field profiles in this study.
Profile Depth (m) Sample No. Sta. position (m) PI (%) % clay % fines Depth (m) Sample No. PI (%) % clay % fines
SA1-30cm 0 41 18 90 SA1-50cm 39 16 72
A < 0,50 SA2-30cm 250 54 26 90 ≥ 0,50 SA2-70/105cm 55 25 87
SA28-30cm 6750 45 23 94 SA28-50cm 47 18 88
SA37-30cm 9000 43 32 89 SA37-50/80cm 39 33 91
SA41-30cm 10000 47 37 96 SA41-50cm 49 44 89
SB1-30cm 0 47 30 90 SB1-50/70cm 50 26 91
B < 0,50 SB5-30cm 1000 47 29 89 ≥ 0,50 SB5-50/70cm 50 26 91
SB7-30cm 1500 48 31 89 SB7-50/70cm 49 27 91
SB18-30cm 4250 43 13 84 SB18-50cm 39 14 88
SB27-30cm 6500 47 32 94 SB27-50cm 49 33 92
SB31-30cm 7500 49 35 90 SB31-50cm 48 30 93
SB41-30cm 10000 44 20 88 SB41-50cm 47 22 86
SB42-30cm 10250 44 23 88 SB42-50cm 47 21 86
SC1-30cm 0 48 30 87 SC1-50cm 48 27 90
C < 0,50 SC5-30cm 1000 48 31 88 ≥ 0,50 SC5-50cm 47 27 91
SC9-30cm 2000 48 31 89 SC9-50cm 48 27 89
SC17-30cm 4000 41 29 88 SC17-50cm 48 27 86
SC25-30cm 6000 49 23 94 SC25-50cm 48 18 89
SC29-30cm 7000 49 28 91 SC29-50cm 43 26 92
SC33-30cm 8000 49 27 91 SC33-50cm 50 28 91
SC41-30cm 10000 47 16 74 SC41-50cm 47 19 85
SD1-30cm 0 46 32 91 SD1-50/80cm 48 27 90
D < 0,50 SD5-30cm 1000 47 31 91 ≥ 0,50 SD5-50/70cm 49 27 90
SD7-30cm 1500 48 32 90 SD7-50/70cm 51 26 90
SD17-30cm 4000 44 13 84 SD17-50cm 49 21 84
SD25-30cm 6000 39 24 84 SD25-50cm 45 14 74
SD29-30cm 7000 47 27 90 SD29-50cm 51 28 92
SD33-30cm 8000 47 29 90 SD33-50cm 50 28 91
SD41-30cm 10000 46 35 92 SD41-50cm 49 41 89
SE1-30cm 0 48 31 89 SE1-50/70cm 50 27 91
E < 0,50 SE5-30cm 1000 48 31 89 ≥ 0,50 SE5-50/70cm 49 27 91
SE13-30cm 3000 47 33 93 SE13-50cm 49 34 92
SE21-30cm 5000 49 24 94 SE21-50cm 49 20 88
SE29-30cm 7000 46 27 92 SE29-50cm 47 29 92
SE37-30cm 9000 48 33 90 SE37-50cm 48 31 92
SE41-30cm 10000 47 19 89 SE41-50cm 48 19 85
max 54 37 96 max 55 44 93
Statistical < 0,50 min 39 13 74 ≥ 0,50 min 39 14 72
analysis range 15 24 22 range 16 30 21
median 47 29 90 median 48 27 90
mode 47 31 90 mode 49 27 91
mean 47 27 89 mean 48 26 89
std 2,78 6,09 3,81 std 3,28 6,54 4,50
var 7,74 37,04 14,48 var 10,76 42,82 20,25
cov 31,86 5,92 cov 31,86 5,92
correlation 0,82 0,36 correlation 0,82 0,36

170
1°19´S 4000
Profile distance (m) Profile distance (m)
3000
2000
West
1000
1°21.5´S
1°19´S 4000
3000
2000
West
1000
% clay variation (< 0,5 m)
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
% clay variation (> 0,5 m)
%clay
35
34
33
32
31
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
1°21.5´S
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South
Profile distance (m) 36°55´E
38
36
34
32
30
28
26
24
22
20
18
16
14
Figure 8.11. Contour maps showing distribution and variation of %clay in black clays.
40
% clay
The variation of percentage fines is also identical at the two depth intervals, in terms of
maximum and minimum values of 96% and 74% (range: 22%); and 93% and 72% (range:
21%), respectively; as well as median, mode and mean values of about 90%. Standard
deviations registered for the data sets at the two depth intervals are also of the same order of
magnitude, i.e. 3,81 and 4,50, respectively. The variation along field profiles at the two depth
intervals is also illustrated in Figure (8.12) and a contoured representation in Fig.(8.13).

171
Variation of fines (%), < 0,50m depth
% fines
100
90
80
70
60
50
40
30
20
10
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Fig. 8.12a. Variation of fines fraction across the study area at depths of less than 0,5m (north-south distance: 4000 m).
Variation of fines (%), 0,50m depth and greater
% fines
100
90
80
70
60
50
40
30
20
10
0
0 2000 4000 6000 8000 10000 12000
Distance along profile (m)
Profile A
Profile B
Profile C
Profile D
Profile E
Fig. 8.12b. Variation of fines fraction across the study area at depths of 0,50m and greater (north-south distance: 4000 m).

172
% fines variation (< 0,5 m)
9090 94 89 96
1°19´S 4000
Profile distance (m)
91 91 90 84 84 90 90 92
3000
90 89 89 84 94 90 88
2000
West 89 89 93 94 92 90 89
1000
87 88 89 88 94 91 91
74
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
%fines
94
93
92
91
90
89
88
87
86
85
84
83
82
81
80
79
78
77
1°19´S
Profile distance (m)
% fines variation (> 0,5 m)
7287
88 91 89
4000
90 90 90 84 74
3000
92 91 89
91 91 91 88 92 93 86
2000
West 91 91 92 88 92 92 85
1000
1°21.5´S
90 91 89 86 89 92 91 85
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
78
79
80
81
82
83
84
85
86
87
88
89
90
91
%fines
92
Figure 8.13. A contoured representation of the distribution and variation of fines fraction in
black clays.
Once again, areas of relatively low percent clay and fines fractions are associated with zones
of increasingly lateritic clays (Figs. 8.11 & 8.13). Patches of lateritic clays found are therefore
characterised by relatively higher contents of the coarse fraction (Fig. 8.14).

173
%coarse
% coarse fraction variation (< 0,5 m)
1010 6 11 4
1°19´S 4000
Profile distance (m)
9 9 10 16 16 10 10 8
3000
10 11 11 16 6 10 12
2000
West 11 11 7 6 8 10 11
1000
13 12 11 12 6 9 9
26
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
% coarse fraction variation (> 0,5 m)
28 13 12 9 11
1°19´S 4000
Profile distance (m)
10 10 10 16 26
8 9 11
3000
9 9 9 12 8 7 14
2000
West
9 9 8 12 8 8 15
1000
1°21.5´S
10 9 11 14 11 8 9 15
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
20
19
18
17
16
15
14
13
12
11
10
9
8
7
%coarse
21
Fig. 8.14. Distribution and variation of coarse fraction in black clays.
The red soils exhibit average clay and fines contents of 21% and 93%, respectively. Despite
the similar proportions of fines in black clays and red soils, the former are found to exhibit
higher plasticity and potential expansiveness relative to the latter. The difference could be
attributed to the predominating type of clay mineral composition, the high content of
smectites (over 90%) rendering the black clays higher swelling capabilities on wetting. The
red soils are mainly kaolinite (over 80%), a clay mineral characterised by limited swelling
capabilities on wetting.
8.7 Shear strength
Variation of strength characteristics in black clays is presented in Figures (8.15a & b) in
which distribution of shear strength parameters of apparent cohesion (c´ ) and angle of
frictional resistance (phi or φ´ ) are given in contour form.

174
1°19´S 4000
Profile distance (m)
Cohesion (c`) variation (< 0,5 m)
23.1 29.8
47.9 44.0 45.4 32.0
3000
46.7 47.4 44.9 30.2
2000
West 46.4 42.6 25.8 26.0
1000
c` (kN/m²)
46
44
42
40
38
36
34
32
30
1°21.5´S
47.2
11.5 25.1
28
0
26
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m) 36°55´E 24
22
20
18
16
1°19´S 4000
Cohesion (c`) variation (> 0,5 m)
35.6 41.0
Profile distance (m)
36.1 35.6 36.1 28.0
3000
28.4
2000
26.3 36.9 28.6
West 40.8 35.1 36.2 29.0
1000
1°21.5´S
31.8 33.7 34.9
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m) 36°55´E
38.5
38
37.5
37
36.5
36
35.5
35
34.5
34
33.5
33
32.5
32
31.5
31
30.5
30
29.5
29
Fig. 8.15a. Distribution and variation of cohesion (c´ ) in black clays.
c` (kN/m²)
There is a slight increase in values of cohesion with increased depths (Fig. 8.15a), and this
could be attributed to a corresponding decrease of organic matter content with depth of soils.
In practice, organic matter would have the effect of counteracting cohesive effects in clay
soils. Similarly, the general decrease in the thickness of black clays eastwards across the study
area would also mean progressively increased counteractive effects of organic matter content
on cohesion of the clays. As a result, values of cohesion are also observed to generally
decrease eastwards at any chosen depth interval (Fig. 8.15a).
The angle of shear resistance is usually a more reliable parameter in characterising strength
characteristics of soils (Head, 1988).

175
1°19´S 22
Shear angle variation (< 0,5 m)
29
4000
Profile distance (m)
17 16 16
30
3000
17 17 17 30
2000
West 16 16 26 28
1000
18 22 25
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South Profile distance (m)
36°55´E
phi (°)
29
28
27
26
25
24
23
22
21
20
19
18
17
16
1°19´S
4000
Profile distance (m)
Shear angle variation (> 0,5 m)
19 19
15 13 14 21
3000
15 16 14 22
2000
West
11 12
1000
23 22
14 14 22
1°21.5´S 0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
36°49´E
South
Profile distance (m) 36°55´E
22.5
22
21.5
21
20.5
20
19.5
19
18.5
18
17.5
17
16.5
16
15.5
15
14.5
14
13.5
13
12.5
Fig. 8.15b. Distribution and variation of angle of shear resistance (φ´ ) in black clays.
phi (°)

176
Chapter 9 Implications of index/ engineering properties of black clays and red soils
on construction practice
9.1 Atterberg limits and other index parameters
The spatial distribution of values of Atterberg limits for black clays across the study area at
soil depths of less than 0,50m and above 0,50m is presented in the last chapter (Figs. 8.3-
8.6). The distribution also serves to be indicative of the possible variation of other important
engineering soil properties across the study area. This is evidenced by plotting on the same
axes results of Atterberg limits against those of free swell and linear shrinkage (see next
chapter), all of which demonstrate generally similar variation trends in their spatial
distribution across the area. This points to the possibility that the Atterberg limits and other
index properties are interrelated, so that parameters of one property could be readily estimated
from measured values of the other, and vice versa. In this study, a correlation of results of
Atterberg limits with those of other index properties has been attempted, and the forms and
strengths of interrelationship determined and discussed in the next chapter.
The results of Atterberg limits could also be usefully applied in the selection of soils for use
as compacted fill in various types of earthworks construction. This is because clays of high
plasticity are also usually associated with properties of relatively low permeability and a
tendency to consolidate over longer periods of time under load than clays of low plasticity
(Head, 1984). As a result, the high plasticity black clays involved in this study would be more
difficult to compact, and are therefore unsuitable for use as fill material; while the relatively
low plasticity red soils could be preferred for the purpose.
The liquid and plastic limits also serve to indicate and describe the consistency state of clay
soil involved, i.e. the condition of the clay soils in their natural state would be dependent upon
and described by their natural moisture content in relation to these limits, as expressed by the
liquidity index, LI, and relative consistency, Cr, (Table 7.2). The black clays could be
described as having a generally firm consistency; while the red soils have firm to stiff
consistency. (However, swampy areas of black clays and forested areas of red soils were
found to show generally soft upper thicknesses of soils). As a result, red soils are expected to
be comparatively more stable with respect to supporting light engineering structures than
black clays. This is because the natural moisture content of soils as related to the plastic and
liquid limits through the liquidity index (LI) and relative consistency (Cr) is also usually
related to, and serves to be indicative of the shear strength and compressibility of soils in situ
(Nelson and Miller, 1992). A correlation of LI and Cr values with shear strength and
consolidation parameters of soils involved in this study is presented and discussed in
respective sections of laboratory shear strength and consolidation tests.
Black clays have been found to harbour potentially high swelling (over 100% free swell) and
shrinkage (21-29% linear shrinkage) capabilities than red soils (less than 50% free swell;
about 10% linear shrinkage). As a result, volumetric changes and/ or expansive movements in
soils that usually accompany alternating seasons of wet and dry months are likely to have a
comparatively more destabilising effect on light engineering structures constructed on black
clays than those on red soils.

177
9.2 Grain size distribution
The results of particle size analysis obtained in this study have been used to complement those
of Atterberg limits to give a complete description, and thereby reflect the probable physical
and/ or engineering behaviour of the clay soils. This is illustrated in Tables (7.1) and (7.4), as
well as Figure (7.2) where the colloidal activity and probable expansive behaviour of the soils
have been calculated and/ or inferred from measured values of Atterberg limits (plasticity
index), free swell and percentage clay fraction. The black clays and red soils both show high
contents of the fine fraction (silt and clay sizes) of over 70%. The higher plasticity and
potentially expansive behaviour exhibited by the black clays could be accounted for in terms
of clay mineral composition, i.e. a higher content of smectites (over 90%) which usually
exhibit high swelling capabilities on wetting. On the contrary, the red soils are predominated
by the rather less expansive kaolinite (over 80%) which usually exhibits limited activity and
swelling capabilities on wetting.
In addition, the results could be used in conjuction with those of Atterberg limits and free
swell (swelling capability) when considering the soils for use as fill material in various civil
and construction works. This is because specific civil engineering works such as
embankments, earth dams and/ or sub-base materials for roads and airfield runways; all
require the soils to meet certain grading specifications to provide a mechanically stable
foundation (Nelson and Miller, 1992). The results of particle size analysis would also serve to
assess and indicate the probable feasibility and/ or effectiveness of dynamic compaction
technique when used to improve poor ground conditions in projected sites. In the present
study for instance, the black clays and red soils have been classified into soils of high and low
activity and/ or swelling capability/ expansiveness, respectively; on the basis of grain sizes,
Atterberg limits and free swell (Tables 7.1 and 7.4). As a result, the black clays with probable
high swelling capability/ expansive behaviour would generally be difficult to compact and
thus unsuitable for use as fill material; while the relatively low swelling capability/ expansive
red soils would be preferred.
Results of particle size analysis, and especially the clay fraction and/ or fine fraction, have
been correlated with other soil parameters and index properties (see next chapter). This had
the purpose of exploring and establishing possible forms of interrelationships that could aid
in approximately estimating and/ or inferring for important soils` physical/ mechanical and/ or
engineering behaviour relevant to projection/ planning and construction of light structures.
The results of the analysis have also aided in classification of the black soils as being mainly
silty clays and/ or silty clays with sand, while varieties classified as clay and clayey silt also
occur but to a lesser extent. The red soils fall in the classes of clayey silts for depths of up to
1,0m; and silty clays with sand, for depths greater than 1,0m (Table 7.7). The classification
implies that the fine fraction (silt and clay sizes) predominates both types of clays, and would
most likely be the significant component in influencing and/ or controlling the engineering
properties/ behaviour of these soils.
9.3 Shear strength parameters
The red soils are characterised by larger shear angles (mean φ´: 28,5°) than the black clays
(mean φ´: 18°); and this implies that the former are relatively more stable and have a larger
capacity in supporting light engineering structures than the latter.

178
The variation of shear strength parameter φ´ for depths of less than 0,5m (mean 21°, std 5,5)
and those greater than 0,5m (mean 17°, std 3,8) in black clays was shown to be generally
similar across the study area. This implies that geo-engineers and planners involved in
planning, design and construction of projected structures could safely treat the black clays as a
single homogeneous layer with respect to their strength characteristics.
Results of correlations between index (Atterberg limits, relative consistency) and shear
strength (c´, φ´ ) parameters have been found to be only weak to moderate in strength. This
implies that index parameters cannot be reliably used to characterise shear strength of soils in
situ. The uncertainy accompanying the estimation and assessment of strength characteristics
could be a result of index tests having been performed on disturbed and/ or fractioned samples
having destroyed/ broken down soil fabric and structure not representative of conditions in
situ.
The results of shear strength parameters (c´, φ´ ) obtained for black clays and red soils in this
study would serve to be useful in computations involving stability analyses of soils when
subjected to imposed loads from projected foundation structures. The parameters would
especially be useful in determining the limiting value of external imposed loads capable of
just mobilising the shear strength of the soils, and thereby cause their failure. In order to limit
deformations to within tolerable limits, foundation design practices and/ or procedures would
require that a suitable factor of safety be adopted and applied so that shear stresses induced in
the soil due to weights of constructed structures are everywhere less than a certain proportion
of its maximum shear strength (Terzaghi and Peck, 1967; Art. 26-32). The drained shear
strength parameters would also serve to aid in the analysis of long-term stability problems that
may arise in a number of projects; whereupon the angle of shear resistance would be useful in
deriving earth pressure coefficients and/ or bearing capacity coefficients for use in relevant
computations.
The analysis of long-term stability problems for the conditions prevailing after construction of
structures may include a number of determinations such as bearing capacity of footings and
foundations for structures on clay soils, earth pressure on a retaining wall, earth pressure
against bracing in temporary excavations, safeguards against heave of the bottom of
temporary and/ or deep open excavations in clays, as well as long-term stability of earth dams
and embankments (Terzaghi & Peck, 1967; Art. 29). In addition, residual shear strength
parameters (cr´, φr´ ) when available, could be useful in long-term stability analyses of slopes
and cuttings, especially in overconsolidated clays (Skempton, 1964; Skempton & La
Rochelle, 1965; Symons, 1968). These parameters are usually obtained by allowing for large
additional horizontal displacements during shearbox testing after the peak strength had been
mobilised. However, their exact and/ or accurate determination in this study was limited by
the capacity of the apparatus. In practice, accurate determination of residual shear strength
parameters is achieved by employing a ring shear apparatus which was, however, unavailable
for the present study.
9.4 Consolidation-settlement characteristics
Results of laboratory consolidation tests (Tables 7.12 & 7.14 ) show that red soils exhibit
relatively larger voids ratios and higher porosity (initial voids ratio, eo: 1,56 at 0,30m depth &
1,16 at 4m depth); with voids ratio, e, varying from 0,59-1,42 for the normal loading range of
25-800 kPa. As a result of the above, together with their generally loose and friable nature,
these soils tend to be more or less free-draining, more permeable (K: 7,57E-11 to 7,33E-9
m/s) and more compressible (mv =0,16-2.94 m²/MN ). They would therefore exhibit faster and

179
rapid dissipation of pore water pressures and faster consolidation settlements (cv = 1,18-12,14
m²/year) when externally loaded. Constructed structures located on red soils are therefore
expected to undergo faster consolidation –settlements, much of them during the construction
stage, with minimum long-term instability implications after construction.
On the other hand, the black clays have been found to be characterised by relatively smaller
voids ratios and lower porosity (eo: 0,709-1,098; e = 0,23-1,06 over 25-800 kPa loading
range). These clays are generally more compact, dense and cohesive, and therefore less
permeable (K = 1,67E-12 to 1,20E-9 m/s), less compressible (mv = 0,03-1,62 m²/MN) and
would exhibit slower dissipation of pore water pressures and, therefore, slower consolidation
settlements (cv = 0,05-5,08 m²/year) on external loading. The low rates of consolidationsettlement
exhibited by the black clays would also mean structural settlements occurring
beyond the construction stage. Long-term instability problems would therefore be expected
for structures constructed on black clays.
Results of oedometer consolidation tests obtained in this study could be used to solve
problems related to foundations for light structures in the current area. The consolidation data
and parameters could be applied in conjuction with classification data, as well as knowledge
of the loading history of the soils to make estimates and predictions as regards probable
behaviour of foundations under various loading conditions. The results would specifically
assist in calculating the amount of settlement which would ultimately take place for the
structure as a whole, while variations in long-term settlements and/ or differential settlements
between individual footings could also be estimated; by applying methods described by
Terzaghi (1939), MacDonald and Skempton (1955) and Skempton and Bjerum (1957).
According to Skempton (1956), Terzaghi (1934), Mitchell, Vivatrat & Lambe (1977),
differential settlements are usually more critical than overall settlement, and can cause tilting
of a structure as a whole and/or distortions within the structure; and should therefore be kept
within acceptable limits to avoid possible structural deterioration and damage. In addition,
possible settlement of piled foundations as a result of the presence of deep-seated weak
compressible clay layers, especially in red soils, could also be estimated.
On the other hand, estimates of the rate of consolidation could be further used to give the
duration of time within which structural settlements would be completed, either during or
after construction. In situations of long-term settlements as may occur in black clays,
settlement/ time graphs could serve to show the duration of the most significant part of
settlements, and this could be compared with the economic life of the structure. The
settlement/ time relationship would also assist in ascertaining possible development of
unacceptable differential settlements in the long-term, after construction.
Areas of soft ground and clays in the form of alluvial and / or swampy environments are
common in the present study area. The shear strength and capacity of these grounds to support
increased foundation loads could be increased and improved by consolidation through preloading
of the ground with a surcharge of temporary fill. In this case, results of laboratory
consolidation tests would be useful in estimating the extent and rate of the resulting
settlement; and thereby indicate whether a provision for accelerating the consolidation, such
as installation of sand drains, is justifiable.
In situations where piles could be driven through the pre-consolidated soft strata to transmit
structural loads to a deeper firm stratum, the tendency of continued settlement of the fill and
soft strata would have the effect of throwing additional loads onto the piles. Here also,

180
knowledge of the consolidation characteristics would serve as a basis for safeguarding against
overloading of the piles due to this effect.
Results of consolidation tests could also be used to monitor changes in effective strength in
structures such as earth-dams and embankments (which usually consolidate under their own
weight); as well as in the underlying soil strata. This would ultimately serve to aid in the
analysis of the long-term stability of the structures.
The use of results of oedometer consolidation tests is, however, usually limited when it comes
to the estimation of rates of settlement of foundation soils. According to Rowe (1972), the
underestimation is attributed to the small size of laboratory test specimens used which make it
impracticable to represent many of the natural features of soil fabric (fissures, pores,
laminations, other discontinuities), and which have a profound effect and control on drainage
and therefore the rate of consolidation-settlement. As a result, a given proportion of the
ultimate consolidation-settlement is reached in a shorter time in real field situations, than that
predicted from laboratory test data. In addition, extent of consolidation is based only on
measurements of change in height of the specimen; while means for measuring excess pore
pressure, the dissipation of which actually controls the consolidation process is, however, not
provided for in the oedometer consolidation cell.
9.5 Percentage swelling and swelling pressure effects
Results of swelling tests show that, in general, black clays would exhibit high percentage
swelling (S > 75%) when imposed loads from light engineering structures located on and/ or
within, fall below 5 kPa. On the other hand, external imposed loads greater than 80 kPa would
mean minimal swelling of the clays, i.e. S < 10% [S% in this case is the amount of swelling
(S mm) under a given load decrement, expressed as a percentage of ultimate swelling (Smax
mm) which occurs under zero external loading]. In addition, swelling pressure tests have
shown the black clays to exhibit swelling pressures of 49-104 kPa. It would therefore be
appropriate that in practice, light engineering structures are initially designed to impose loads
of well over 100 kPa in order to minimise potential destabilisation effects that may arise from
possible swelling of the clays on wetting.

Chapter 10
181
Correlation of index properties
10.1 Linear shrinkage and plasticity index
Laboratory measured values of linear shrinkage, LS, and plasticity index, PI meas.,
Table 10.1. Calculated and laboratory measured values of plasticity index of black clays.
Profile
Depth
(m)
Sample No.
LS
(%)
PI meas.
(%) PI BS (%) PILS (%)
Depth
(m) Sample No. LS (%)
PI meas.
(%) PI BS (%)
SA1-30cm 25 41 52 46 SA1-50cm 23 39 49 44
A < 0,50 SA2-30cm 27 54 57 50 ≥ 0,50 SA2-70/105cm 26 55 55 49
SA28-30cm 25 45 54 48 SA28-50cm 25 47 54 48
SA37-30cm 24 43 51 45 SA37-50/80cm 26 39 55 49
SA41-30cm 26 47 55 48 SA41-50cm 24 49 52 46
SB1-30cm 23 47 49 44 SB1-50/70cm 25 50 53 47
B < 0,50 SB5-30cm 23 47 49 44 ≥ 0,50 SB5-50/70cm 25 50 53 47
SB7-30cm 24 48 51 45 SB7-50/70cm 25 49 53 47
SB18-30cm 23 43 49 43 SB18-50cm 21 39 44 39
SB27-30cm 24 47 50 44 SB27-50cm 25 49 52 46
SB31-30cm 24 49 51 45 SB31-50cm 25 48 53 47
SB41-30cm 26 44 55 49 SB41-50cm 25 47 54 48
SB42-30cm 25 44 54 48 SB42-50cm 26 47 55 48
SC1-30cm 23 48 49 43 SC1-50cm 24 48 51 45
C < 0,50 SC5-30cm 23 48 49 44 ≥ 0,50 SC5-50cm 24 47 50 44
SC9-30cm 24 48 50 44 SC9-50cm 24 48 51 45
SC17-30cm 25 41 53 47 SC17-50cm 24 48 51 45
SC25-30cm 25 49 52 46 SC25-50cm 25 48 54 48
SC29-30cm 25 49 53 47 SC29-50cm 26 43 56 50
SC33-30cm 25 49 53 47 SC33-50cm 26 50 55 48
SC41-30cm 29 47 61 54 SC41-50cm 29 47 61 54
SD1-30cm 26 46 56 49 SD1-50/80cm 27 48 57 50
D < 0,50 SD5-30cm 26 47 55 48 ≥ 0,50 SD5-50/70cm 27 49 57 50
SD7-30cm 27 48 57 50 SD7-50/70cm 26 51 56 50
SD17-30cm 24 44 50 44 SD17-50cm 25 49 52 46
SD25-30cm 25 39 53 47 SD25-50cm 25 45 52 46
SD29-30cm 27 47 57 50 SD29-50cm 26 51 55 48
SD33-30cm 27 47 58 51 SD33-50cm 25 50 53 47
SD41-30cm 26 46 55 48 SD41-50cm 26 49 56 49
SE1-30cm 23 48 49 44 SE1-50/70cm 25 50 53 47
E < 0,50 SE5-30cm 24 48 50 44 ≥ 0,50 SE5-50/70cm 26 49 55 49
SE13-30cm 24 47 51 45 SE13-50cm 25 49 53 47
SE21-30cm 25 49 53 47 SE21-50cm 26 49 55 48
SE29-30cm 25 46 52 46 SE29-50cm 26 47 56 50
SE37-30cm 24 48 50 44 SE37-50cm 25 48 54 48
SE41-30cm 25 47 53 47 SE41-50cm 26 48 55 48
max 29 54 61 54 max 29 55 61 54
Statistical < 0,50 min 23 39 49 43 ≥ 0,50 min 21 39 44 39
analysis range 6 15 12 11 range 8 16 17 15
median 25 47 53 47 median 25 48 54 48
mode 25 47 53 47 mode 25 49 53 47
mean 25 47 53 47 mean 25 48 54 47
std 1,38 2,78 2,93 2,59 std 1,29 3,28 2,76 2,43
var 1,89 7,74 8,58 6,69 var 1,67 10,76 7,60 5,92
cov 0,57 0,51 cov 3,23 2,85
correlation 0,70 0,70 correlation 0,37 0,37
PILS
(%)

182
Table 10.2. Calculated and laboratory measured values of plasticity index of red soils.
Soil type Depth (m) Sample No. LS (%) PI meas. (%) PIBS (%) PILS (%)
0,30 Rd1-30cm 10 18 21 18
Red soils 1,00 Rd1-100cm 11 21 23 20
2,00 Rd1-200cm 11 20 23 20
4,00 Rd1-400cm 11 18 23 20
0,30 Rd2-30cm 10 19 21 18
1,00 Rd2-100cm 11 22 23 20
2,00 Rd2-200cm 11 21 23 20
4,00 Rd2-400cm 11 19 23 20
max 11 22 23 20
Statistical min 10 18 21 18
analysis range 1 5 2 2
median 11 20 23 20
mode 11 #NV 23 20
mean 11 20 23 20
std 0,46 1,62 0,99 0,85
var 0,21 2,63 0,97 0,72
cov 0,76 0,65
correlation 0,54 0,54
as obtained for the black clays in this study at depths of less than 0,50m as well as those at
0,50m and greater, are given in Table (10.1).
Calculated values of plasticity index, PI(BS), as estimated from laboratory determined linear
shrinkage values using the British Standard relationship (BS 1377: 1967), i.e.
PI = 2,13 * LS ( derivation based on British soils) (10.1)
are also included in the table for the two depth intervals. The calculated plasticity index
values are observed to be overestimated with respect to the laboratory determined values for
both the two depth intervals. A comparison of the respective maximum and minimum values
as well as the median, mode and mean values of calculated and measured plasticity indices
further indicates and confirms an overestimation of the former with respect to the latter (Table
10.1). The overestimation could be attributed to the generally higher organic content of
tropical black clays which tends to inhibit and/ or suppress their plasticity to some degree.
The coefficient of variation (covariance) with respect to the calculated and measured values
are 0,57 and 3,23 for the two depth intervals, respectively. The standard deviation and
variance for the calculated and measured plasticity values are in close agreement and more or
less of the same order of magnitude in each case, implying a similar form and/ or mode of
distribution and variation across the study area.
A correlation of laboratory measured values of plasticity index and linear shrinkage is
presented diagrammatically in Fig. (10.1).

183
Black clays; plasticity index/ linear shrinkage
60
n = 36
Plasticity index PI (%)
55
50
45
40
35
PI = 1,89LS
PI = 1,87LS
less than 0,50m depth
0,50m depth and
greater
Linear (less than
0,50m depth)
Linear (0,50m depth
and greater)
30
20 25 30
Linear shrinkage LS (%)
Figure 10.1. Correlation of laboratory measured values of plasticity index and linear
shrinkage of black clays.
From the diagram, the plasticity index of black clays could be approximately estimated by the
relationship
and
PI = 1,87*LS (10.2)
PI = 1,89*LS (10.3)
for soil depths of less than 0,50m and those of depths of 0,50m and greater, respectively. The
two relationships derived for the two depth intervals are practically identical, and this serves
to support and confirm an earlier suggestion that the black clays of the study area are
homogeneous in their engineering character. On the average therefore, the plasticity index of
the whole soil thickness of black clays across the study area could be estimated from the
average relationship of
PI = 1,88*LS (10.4)
Calculated values of plasticity index, PILS, as estimated from laboratory measured values of
linear shrinkage, LS, using the above new relationship in Equation (10.4) are presented in
Table (10.1), both for depths of less than 0,50m and those of 0,50m and greater. A
comparison of the so calculated values of plasticity index (PILS) with plasticity index values
measured in the laboratory (PI meas.) reveals a generally close agreement between the two sets
of data. The maximum and minimum values as well as the median, mode and mean values
obtained for the two sets of data are identical at each one of the two depth intervals. This
implies that Equation (10.4) above is a better and preferred estimator for the plasticity index
of black clays than the British standard relationship of Equation (10.1). In addition, values of

184
standard deviation and variance derived for the calculated and measured plasticity values
(Table 10.1) are comparable and of the same order of magnitude at each of the two depth
intervals; and this further pointing to the possible general uniform character and/ or
homogeneity of the soils across the study area. A covariance of 0,51 and 2,85 is also
registered for the two sets of data at depths of less than 0,50m and those of 0,50m and greater,
respectively.
A correlation of calculated and laboratory measured plasticity indices is presented in Fig.
(10.2).
Black clays; measured/ calculated plasticity indices
Measured plasticity index PImeas. (%)
60
55
50
45
40
35
n = 36
PImeas = 1,01PILS
(> 0,50m depth)
PImeas. = 0,89PIBS.
(> 0,50m depth)
PImeas. = 1,00PILS
(< 0,50m depth)
PImeas. = 0,88PIBS
(< 0,50m depth)
PI (BS) vs PI (meas.), <
0,50m depth
PI (BS) vs PI (meas.),
0,50m depth and greater
PI (LS) vs PI (meas.), <
0,50m depth
PI (LS) vs PI (meas.),
0,50m depth and greater
30
40 45 50 55 60 65
Calculated plasticity index PILS (%)
Figure 10.2. A correlation of calculated and laboratory measured plasticity indices of black
clays.
It is observed that actual laboratory determined/ measured values of plasticity index (PImeas.)
would usually be underestimated with respect to calculated plasticity values (PIBS) obtained
using the British Standard relationship (PI = 2,13*LS). However, the calculated and measured
plasticity indices are practically related in the same way for depths of less than 0,50m (PImeas.
= 0,88PIBS) and those of 0,50m and greater (PImeas. = 0,89PIBS), implying homogeneous black
clays across the study area. An average relationship of
PImeas. = 0,885PIBS (10.5)
could therefore be safely adopted for the whole thickness of black clays across the study area.

185
On the other hand, laboratory measured plasticity values (PImeas.) compare exactly and are
practically the same as calculated values (PILS) derived using the new relationship (PI =
1,88*LS) developed and established in this study for the black clays. The relationship
between the calculated and measured plasticity indices is practically identical at both depths
of less than 0,50m i.e.
PImeas. = 1,00PILS (10.6)
and those of 0,50m and greater, i.e.
PImeas. = 1,01PILS (10.7)
so that the black clays must be uniform and homogeneous in their engineering character
across the study area. An average relationship of
PImeas. = 1,005PILS (10.8)
could be adopted for the two sets of plasticity indices.
The red soils involved in this study were investigated and analysed in a similar way.
Laboratory determined values of plasticity index are presented alongside those derived by
calculation using the British Standard relationship in Table (10.2). A comparison of the
minimum and maximum values as well as the median, mode and mean values of the two sets
of data reveals a slight overestimation of the calculated plasticity indices with respect to the
laboratory measured values, and this most probably due to reasons already stated above as for
black clays. A covariance of 0,76 and moderately strong correlation of 0,54 was also recorded
for the two sets of data.
A correlation of laboratory measured linear shrinkage (LS) and plasticity index (PI) yielded
new relationships for estimating the latter parameter from the former for the red soils (Fig.
10.3), i.e.
PI = 1,77LS (10.9)
for the sampling area Rd1 and
PI = 1,90LS (10.10)
for the sampling area Rd2, respectively. An average relationship of
PI = 1,83LS (10.11)
was derived and established for the red soils. Plasticity index values (PILS) derived by
calculation from laboratory linear shrinkage (LS) values using this new relationship are also
included in Table (10.2) for comparison purposes. The so calculated and laboratory measured
(PImeas.) plasticity index values are perfectly in close agreement and/ or practically identical in
terms of their maximum and minimum values as well as median and mean values. A
covariance of 0,65 and moderately strong correlation of 0,54 were registered for the two sets
of data.

186
Red soils; plasticity index/ linear shrinkage
23
22
n = 8
Plasticity index PI (%)
21
20
19
18
17
PI = 1,90LS
PI = 1,77LS
PI = 1,83LS
Rd1+Rd2
Rd1
Rd2
Linear (Rd1+Rd2)
Linear (Rd1)
Linear (Rd2)
16
15
9,8 10 10,2 10,4 10,6 10,8 11 11,2
Linear shrinkage LS (%)
Figure 10.3. Correlation between laboratory measured values of plasticity index and linear
shrinkage of red soils.
A correlation of laboratory measured plasticity indices (PImeas.) with plasticity indices derived
by calculation (PIBS &PILS) using the British Standard relationship as well as the new
relationship in Equation (10.11) developed in this study for the red soils, is presented
diagrammatically in Fig.( 10.4). Once again, laboratory measured plasticity indices would be
slightly underestimated when derived from the British Standard relationship, i.e.
PImeas. = 0,86PIBS. (10.12)
On the other hand, the newly developed relationship (PI = 1,83LS) would perfectly reproduce
and/ or better estimate actual measured plasticity indices, with the measured and calculated
values being related in the form of
PImeas. = 1,00PILS (10.13)

187
Red clays; measured/ calculated plasticity index
23
22
n = 8
Measured plasticitx index (%)
21
20
19
18
17
16
PImeas. = 1,00PILS
PImeas. = 0,86PIBS
PI(BS) vs PI meas.
PI (LS) vs PI meas.
15
15 17 19 21 23 25
Calculated plasticity index (%)
Figure 10.4. A correlation of calculated and laboratory measured
plasticity indices of red soils.
10.2 Liquid limit and plasticity index
The plasticity index of black clays and red soils involved in this study could be estimated by
calculation from laboratory determined results of liquid limit using the British Standard
relationship
PI =0,73(LL-20) (10.14)
through which they are very strongly interrelated, i.e.R = 1 (Fig. 10.5). A correlation of
laboratory determined values of plasticity index and liquid limit have also given rise to a new
and strong (R = 0,96) relationship, i.e.
PI = 0,79(LL-25) (10.15)
Diagram: plasticity index/ liquid limit
Plasticity index PI (%)
60
50
40
30
20
10
0
n = 90
PIBS = 0,73(LL - 20)
R 2 = 1
PI = 0,79(LL - 25)
R 2 = 0,92
40 60 80 100
Liquid limit LL (%)
PI British Std
Black+red soils
Fig. 10.5. Correlation: laboratory measured values of plasticity index
and liquid limit of black clays and red soils.

188
Table 10.3. Calculated and laboratory measured plasticity
indices of black clays.
Profile Sample No. Wn (%) LL (%) PL (%)
PI
meas. PI BS
(%) (%) PILL (%)
SA1-30cm 32 78 37 41 42 42
A SA1-50cm 30 75 36 39 40 39
SA2-30cm 34 93 39 54 53 53
(black clays) SA2-70cm 34 90 36 54 51 51
SA2-105cm 30 88 33 55 50 50
SA28-30cm 26 85 40 45 47 47
SA28-50cm 27 89 41 47 50 50
SA37-30cm 19 81 38 43 44 44
SA37-50cm 22 83 39 44 46 46
SA37-80cm 23 85 39 46 48 47
SA41-30cm 23 89 41 47 50 50
SA41-50cm 25 90 41 49 51 51
SB1-30cm 23 82 34 47 45 45
B SB1-50cm 27 84 36 48 47 46
SB1-70cm 30 90 39 51 51 51
(black clays) SB5-30cm 24 81 34 47 44 44
SB5-50cm 27 85 37 48 47 47
SB5-70cm 31 91 40 51 52 52
SB7-30cm 23 83 35 48 46 46
SB7-50cm 26 86 37 49 48 48
SB7-70cm 30 87 38 49 49 49
SB18-30cm 20 82 40 43 46 45
SB18-50cm 20 72 34 39 38 37
SB27-30cm 22 85 38 47 47 47
SB27-50cm 22 88 39 49 49 50
SB31-30cm 23 87 38 49 49 49
SB31-50cm 23 86 38 48 48 48
SB41-30cm 23 89 45 44 50 50
SB41-50cm 24 91 44 47 52 52
SB42-30cm 22 88 44 44 49 49
SB42-50cm 21 91 45 47 52 52
SC1-30cm 24 82 35 48 45 45
C SC1-50cm 26 85 37 48 47 47
SC5-30cm 23 83 35 48 46 46
(black clays) SC5-50cm 25 84 36 47 46 46
SC9-30cm 25 84 36 48 46 46
SC9-50cm 26 85 37 48 47 47
SC17-30cm 21 76 35 41 41 40
SC17-50cm 22 81 33 48 44 44
SC25-30cm 26 88 39 49 50 50
SC25-50cm 26 89 41 48 50 50
SC29-30cm 21 82 32 49 45 45
SC29-50cm 20 80 37 43 44 44
SC33-30cm 20 82 33 49 45 45
SC33-50cm 21 83 33 50 46 46
SC41-30cm 18 85 38 47 48 48
SC41-50cm 17 84 37 47 46 46

189
These two relationships could be especially useful in the approximate estimation of the
plasticity index from measured values of liquid limit for the more silty varieties of clay soils
in which determination of the plastic limit could otherwise be difficult.
Table 10.3, continued: Calculated and laboratory measured plasticity indices of black clays.
Profile Sample No. Wn (%) LL (%) PL (%) PI meas(%) PI BS (%) PILL (%)
SD1-30cm 27 82 36 46 45 45
D SD1-50cm 27 83 36 47 46 46
SD1-80cm 30 91 42 49 52 52
(black clays) SD5-30cm 26 84 36 47 46 46
SD5-50cm 27 84 36 48 47 47
SD5-80cm 30 92 42 50 53 53
SD7-30cm 27 84 36 48 47 47
SD7-50cm 27 86 37 49 48 48
SD7-70cm 30 91 39 52 52 52
SD17-30cm 20 84 40 44 47 46
SD17-50cm 22 84 35 49 47 47
SD25-30cm 22 80 41 39 44 44
SD25-50cm 21 84 40 45 47 47
SD29-30cm 18 85 38 47 48 48
SD29-50cm 22 86 35 51 48 48
SD33-30cm 20 85 37 47 47 47
SD33-50cm 21 86 35 50 48 48
SD41-30cm 23 85 39 46 47 47
SD41-50cm 25 90 41 49 51 51
SE1-30cm 24 82 35 48 45 45
E SE1-50cm 27 85 37 48 47 47
SE1-70cm 30 91 39 52 52 52
(black clays) SE5-30cm 25 85 37 48 47 47
SE5-50cm 27 86 38 48 48 48
SE5-70cm 29 92 42 50 53 53
SE13-30cm 21 86 39 47 48 48
SE13-50cm 24 89 40 49 50 50
SE21-30cm 25 88 39 49 50 50
SE21-50cm 26 90 41 49 51 51
SE29-30cm 22 81 36 46 44 44
SE29-50cm 23 83 36 47 46 45
SE37-30cm 21 85 37 48 47 47
SE37-50cm 22 85 38 48 48 48
SE41-30cm 22 87 40 47 49 49
SE41-50cm 21 88 40 48 49 49
max 55 53 53
Statistical min 39 38 37
analysis range 17 15 16
median 48 47 47
mode 48 47 47
mean 47 48 48
standard dev. 3,00 2,85 3,09
variance 9,00 8,13 9,53
covariance 5,89 6,37
correlation 0,70 0,70
Results of plasticity index values, PIBS and PILL, derived by calculation from laboratory
measured liquid limits using the British Standard relationship (Equation 10.14) and the new
relationship developed in this study (Equation 10.15), respectively, are presented in Table (

190
10.3) for the black clays. The calculated values are shown to be strongly in close agreement
with laboratory determined plasticity indices (PImeasured) in terms of maximum (53-55%),
minimum (37-39%) and range (15-17%) values; as well as the median, mode and mean
values (47-48%). Standard deviation (2,85-3,09) and variance (8,13-9,53) are also in strong
agreement and of the same order of magnitude, signifying a similar form of distribution across
the study area. A covariance of 5,89-6,37 and a strong correlation of 0,70 was also registered
between the calculated and laboratory determined plasticity indices.
Table 10.4. Calculated and laboratory measured plasticity indices of red soils.
Profile Sample No. Wn (%) LL (%) PL (%)
PImeasured
(%) PI BS (%) PILL (%)
Rd1 -30cm 39 49 31 18 21 19
Red soils Rd1-100cm 24 51 31 21 23 21
Rd1-200cm 24 49 30 20 21 19
Rd1-400cm 25 48 30 18 20 18
Rd2-30cm 38 53 35 19 24 22
Rd2-100cm 26 57 34 22 27 25
Rd2-200cm 25 54 33 21 25 23
Rd2-400cm 26 53 34 19 24 22
max 22 27 25
Statistical min 18 20 18
analysis range 5 7 7
median 20 24 21
mode #NV 24 22
mean 20 23 21
standard dev. 1,62 2,29 2,48
variance 2,63 5,26 6,16
covariance 2,56 2,77
correlation 0,79 0,79
Calculated plasticity indices for red soils (Table 10.4) are also closely comparable to the
laboratory determined plasticity indices, with maximum and minimum values of 22-27% and
18-20%, as well as a range of 5-7%. Median, mode, and mean values of 20-24%, 22-24% and
20-23% were also recorded. The standard deviation and variance are of the same order of
magnitude while a covariance of 2,56-2,77 and strong correlation of 0,79 was recorded
between the calculated and measured sets of data.
The calculated and laboratory measured plasticity indices for both black clays and red soils
are in a very strong and near perfect agreement (Fig. 10.6), being related in the form of
and
PImeasured = 0,99PIBS, (10.16)
PImeasured = 0,99PILL (10.17)

191
Diagram: PImeasured/ PIcalculated
60
Measured plasticity index (%)
50
40
30
20
10
n = 90
PImeasured = 0,99PILL
R 2 = 0,92
PImeasured = 0,99PIBS
R 2 = 0,92
PI( BS) vs PI (measured)
PI (LL) vs PI (measured)
0
0 20 40 60
Calculated plasticity index (%)
Figure 10.6. A correlation between calculated and laboratory determined plasticity
indices of black clays and red soils.
10.3 Liquid limit and Linear shrinkage
A correlation of laboratory determined values of linear shrinkage (LS) and liquid limit (LL)
results in a very strong combined relationship (R = 0,93) for both black clays and red soils as
illustrated in Fig. (10.7). The relationship could be summarised in the form of an equation, i.e.
LS = 0,39(LL-21) (10.18)
Diagram: linear shrinkage/ liquid limit
Linear shrinkage LS (%)
35
n = 90
30
LS = 0,39(LL - 21)
25
R 2 = 0,87
20
15
10
5
0
40 50 60 70 80 90 100
Liquid limit LL (%)
Black clays
+ red soils
Figure 10.7. Correlation between laboratory measured values of linear shrinkage
and liquid limit of black clays and red soils.

192
Calculated values of linear shrinkage computed from laboratory liquid limit values using this
new relationship are presented in Table (10.5).
Table 10.5. Calculated and laboratory measured linear shrinkage
values of black clays and red soils.
Profile Sample No. LL (%) LS measured (%) LSLL (%)
SA1-30cm 78 25 22
A SA1-50cm 75 23 21
SA2-30cm 93 27 28
(black clays) SA2-70cm 90 25 27
SA2-105cm 88 26 26
SA28-30cm 85 25 25
SA28-50cm 89 25 26
SA37-30cm 81 24 23
SA37-50cm 83 26 24
SA37-80cm 85 26 25
SA41-30cm 89 26 26
SA41-50cm 90 24 27
SB1-30cm 82 23 24
B SB1-50cm 84 24 24
SB1-70cm 90 25 27
(black clays) SB5-30cm 81 23 23
SB5-50cm 85 24 25
SB5-70cm 91 26 27
SB7-30cm 83 24 24
SB7-50cm 86 24 25
SB7-70cm 87 25 26
SB18-30cm 82 23 24
SB18-50cm 72 21 20
SB27-30cm 85 24 25
SB27-50cm 88 25 26
SB31-30cm 87 24 26
SB31-50cm 86 25 25
SB41-30cm 89 26 26
SB41-50cm 91 25 27
SB42-30cm 88 25 26
SB42-50cm 91 26 27
SC1-30cm 82 23 24
C SC1-50cm 85 24 25
SC5-30cm 83 23 24
(black clays) SC5-50cm 84 24 24
SC9-30cm 84 24 24
SC9-50cm 85 24 25
SC17-30cm 76 25 21
SC17-50cm 81 24 23
SC25-30cm 88 25 26
SC25-50cm 89 25 26
SC29-30cm 82 25 24
SC29-50cm 80 26 23
SC33-30cm 82 25 24
SC33-50cm 83 26 24
SC41-30cm 85 29 25
SC41-50cm 84 29 24

193
Table 10.5, continued. Calculated and laboratory measured linear
shrinkage values of black and red soils.
Profile Sample No. LL (%) LS measured (%) LS calculated (%)
SD1-30cm 82 26 24
D SD1-50cm 83 27 24
SD1-80cm 91 26 27
(black clays) SD5-30cm 84 26 24
SD5-50cm 84 27 25
SD5-80cm 92 27 28
SD7-30cm 84 27 25
SD7-50cm 86 26 25
SD7-70cm 91 26 27
SD17-30cm 84 24 24
SD17-50cm 84 25 25
SD25-30cm 80 25 23
SD25-50cm 84 25 25
SD29-30cm 85 27 25
SD29-50cm 86 26 25
SD33-30cm 85 27 25
SD33-50cm 86 25 25
SD41-30cm 85 26 25
SD41-50cm 90 26 27
SE1-30cm 82 23 24
E SE1-50cm 85 24 25
SE1-70cm 91 25 27
(black clays) SE5-30cm 85 24 25
SE5-50cm 86 25 25
SE5-70cm 92 26 28
SE13-30cm 86 24 25
SE13-50cm 89 25 26
SE21-30cm 88 25 26
SE21-50cm 90 26 27
SE29-30cm 81 25 23
SE29-50cm 83 26 24
SE37-30cm 85 24 25
SE37-50cm 85 25 25
SE41-30cm 87 25 26
SE41-50cm 88 26 26
Rd1 -30cm 49 10 11
Red soils Rd1-100cm 51 11 12
Rd1-200cm 49 11 11
Rd1-400cm 48 11 10
Rd2-30cm 53 10 13
Rd2-100cm 57 11 14
Rd2-200cm 54 11 13
Rd2-400cm 53 11 13
max 29 28
min 10 10
range 19 18
median 25 25
mode 25 25
mean 24 24
standard dev. 4,30 4,02
covariance 15,88
correlation 0,93

194
A comparison of laboratory determined linear shrinkage values (LS measured) with those
computed (LSLL) according to Equation (10.18) reveals a very strong closeness. The two sets
of data are practically identical in terms of maximun and minimum values, the range as well
as the median, mode and mean values (Table 10.5). The two comparable standard deviations
of 4,30 and 4,02, and very strong correlation of 0,93 are further evidence of the similarity of
the two sets of data; and therefore the reliability of the new relationship in estimating linear
shrinkage of soils from laboratory measured liquid limits. The relationship between laboratory
measured and calculated linear shrinkage values is illustrated diagrammatically in Fig. (10.8),
and takes the form of
LSmeasured = 0,997LSLL (10.19)
LS measured/ LSLL
Measured linear shrinkage
LSmeas. (%)
35
30
25
20
15
10
5
0
n = 90
R 2 = 0,86
LS meas. = 0,997LSLL
Black clays and red
soils
Linear (Black clays
and red soils)
0 10 20 30
Calculated linear shrinkage LSLL
(%)
Figure 10.8. A correlation between calculated and laboratory determined linear
shrinkage of black clays and red soils.
10.4 Free swell and other index properties
The Swelling capability of black clays and red soils could be estimated with a high degree of
approximation (R² = 0,85 and over) from laboratory results of Atterberg limits and linear
shrinkage. The form and strength of correlation between laboratory determined free swell
(FS) on one hand; and liquid limits (LL), plasticity index (PI) and linear shrinkage (LS) of the
soils on the other, are illustrated in Table (10.6) and Fig. (10.9).

195
Table 10.6. Calculated and laboratory measured free swell values of black
clays and red soils.
Profile Sample No. FS (%) LL (%) PL (%) PI (%) LS (%) FSLL (%) FSPI (%) FSLS (%)
SA1-30cm 135 78 37 41 25 105 106 131
A SA1-50cm 100 75 36 39 23 95 97 117
SA2-30cm 135 93 39 54 27 151 155 146
(black clays) SA2-70cm 170 90 36 54 25 143 154 131
SA2-105cm 120 88 33 55 26 137 160 139
SA28-30cm 125 85 40 45 25 126 122 131
SA28-50cm 135 89 41 47 25 138 130 131
SA37-30cm 115 81 38 43 24 114 113 124
SA37-50cm 115 83 39 44 26 121 118 139
SA37-80cm 115 85 39 46 26 127 125 139
SA41-30cm 135 89 41 47 26 138 130 139
SA41-50cm 120 90 41 49 24 142 135 124
SB1-30cm 125 82 34 47 23 116 130 117
B SB1-50cm 135 84 36 48 24 123 131 124
SB1-70cm 145 90 39 51 25 142 143 131
(black clays) SB5-30cm 120 81 34 47 23 114 129 117
SB5-50cm 135 85 37 48 24 126 133 124
SB5-70cm 145 91 40 51 26 146 144 139
SB7-30cm 130 83 35 48 24 120 134 124
SB7-50cm 135 86 37 49 24 129 136 124
SB7-70cm 140 87 38 49 25 133 135 131
SB18-30cm 120 82 40 43 23 119 112 117
SB18-50cm 120 72 34 39 21 87 98 102
SB27-30cm 130 85 38 47 24 127 129 124
SB27-50cm 130 88 39 49 25 135 135 131
SB31-30cm 130 87 38 49 24 133 136 124
SB31-50cm 130 86 38 48 25 130 134 131
SB41-30cm 145 89 45 44 26 139 118 139
SB41-50cm 145 91 44 47 25 145 129 131
SB42-30cm 145 88 44 44 25 135 117 131
SB42-50cm 145 91 45 47 26 146 127 139
SC1-30cm 125 82 35 48 23 117 131 117
C SC1-50cm 135 85 37 48 24 126 133 124
SC5-30cm 125 83 35 48 23 121 134 117
(black clays) SC5-50cm 135 84 36 47 24 122 130 124
SC9-30cm 130 84 36 48 24 122 133 124
SC9-50cm 135 85 37 48 24 126 131 124
SC17-30cm 105 76 35 41 25 97 106 131
SC17-50cm 110 81 33 48 24 113 133 124
SC25-30cm 125 88 39 49 25 136 136 131
SC25-50cm 145 89 41 48 25 138 131 131
SC29-30cm 110 82 32 49 25 116 138 131
SC29-50cm 110 80 37 43 26 112 115 139
SC33-30cm 120 82 33 49 25 116 137 131
SC33-50cm 120 83 33 50 26 120 140 139
SC41-30cm 125 85 38 47 29 127 130 160
SC41-50cm 125 84 37 47 29 122 129 160

196
Table 10.6, continued. Calculated and laboratory measured free swell values
of black clays and red soils.
Profile Sample No. FS (%) LL (%) PL (%) PI (%) LS (%) FSLL (%) FSPI (%) FSLS (%)
SD1-30cm 125 82 36 46 26 118 127 139
D SD1-50cm 130 83 36 47 27 120 128 146
SD1-80cm 140 91 42 49 26 146 138 139
(black clays) SD5-30cm 125 84 36 47 26 122 129 139
SD5-50cm 130 84 36 48 27 124 132 146
SD5-80cm 140 92 42 50 27 150 142 146
SD7-30cm 110 84 36 48 27 125 133 146
SD7-50cm 140 86 37 49 26 130 136 139
SD7-70cm 135 91 39 52 26 145 148 139
SD17-30cm 120 84 40 44 24 123 117 124
SD17-50cm 130 84 35 49 25 123 138 131
SD25-30cm 125 80 41 39 25 111 98 131
SD25-50cm 135 84 40 45 25 125 120 131
SD29-30cm 125 85 38 47 27 128 130 146
SD29-50cm 145 86 35 51 26 130 143 139
SD33-30cm 125 85 37 47 27 126 130 146
SD33-50cm 145 86 35 50 25 129 141 131
SD41-30cm 130 85 39 46 26 126 124 139
SD41-50cm 135 90 41 49 26 143 136 139
SE1-30cm 125 82 35 48 23 118 131 117
E SE1-50cm 135 85 37 48 24 126 131 124
SE1-70cm 145 91 39 52 25 145 146 131
(black clays) SE5-30cm 125 85 37 48 24 126 132 124
SE5-50cm 130 86 38 48 25 128 131 131
SE5-70cm 145 92 42 50 26 150 140 139
SE13-30cm 125 86 39 47 24 129 128 124
SE13-50cm 130 89 40 49 25 138 135 131
SE21-30cm 125 88 39 49 25 137 137 131
SE21-50cm 145 90 41 49 26 143 138 139
SE29-30cm 120 81 36 46 25 114 123 131
SE29-50cm 120 83 36 47 26 119 127 139
SE37-30cm 125 85 37 48 24 126 131 124
SE37-50cm 130 85 38 48 25 128 131 131
SE41-30cm 135 87 40 47 25 132 128 131
SE41-50cm 135 88 40 48 26 135 131 139
Rd1 -30cm 20 49 31 18 10 11 19 22
Red soils Rd1-100cm 15 51 31 21 11 19 28 29
Rd1-200cm 20 49 30 20 11 14 26 29
Rd1-400cm 15 48 30 18 11 9 18 29
Rd2-30cm 20 53 35 19 10 26 21 22
Rd2-100cm 15 57 34 22 11 37 36 29
Rd2-200cm 15 54 33 21 11 29 31 29
Rd2-400cm 15 53 34 19 11 26 24 29
max 170 151 160 160
min 15 9 18 22
Statistical range 155 142 142 139
analysis median 130 126 131 131
mode 125 126 131 131
mean 120 118 121 123
standard dev. 34 33 32 31
covariance 1053 1013 970
correlation 0,96 0,94 0,92

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The swelling capability in terms of free swell (FS) could be estimated from the Atterberg
limits and linear shrinkage and vice versa, using the relationships,
and
FS = 3,17(LL-45) (10.20)
FS = 3,79(PI-13) (10.21)
FS = 7,29(LS-7) (10.22)
Free swell/ other index properties
180
160
140
FS = 7,29(LS - 7)
R 2 = 0,85
n = 90
FS = 3,17(LL - 45)
R 2 = 0,92
Free swell FS (%)
120
100
80
60
FS = 3,79(PI - 13)
R 2 = 0,88
FS vs LL
FS vs PI
FS vs LS
40
20
0
0 20 40 60 80 100
Liquid limit LL, plasticity index PI and linear
shrinkage LS (%)
Figure 10.9. Correlation: laboratory measured values of free swell and laboratory
liquid limit, plasticity index and linear shrinkage of black clays and red soils.
The strength of agreement between laboratory measured free swell values (FS meas.) and those
calculated by computation from laboratory Atterberg limits(FSLL; FSPI) and linear shrinkage
(FSLS) using the above new relationships is very high (Table 10.6); as characterised by very
strong correlations of 0,92 and over. Further evidence in their closeness is revealed by the
median (126-131%), mode (125-131%), mean (118-123%) and standard deviation (31-34)
values which are found to be closely comparable and of the same order of magnitude.
The relationships between laboratory measured and calculated free swell values (Fig. 10.10)
take the form of
FSmeasured = 1,01FSLL (10.23)

198
FSmeasured = 0,99FSPI (10.24)
and
FSmeasured = 0,98FSLS (10.25)
These relationships serve to support the reliability with which laboratory free swell; and
therefore swelling capability of soils could be estimated from laboratory determined Atterberg
limits and linear shrinkage.
Measured/ calculated free swell
Measured free swell (%)
200
150
100
50
0
n = 91
FSmeas. = 0,99FSPI
FSmeas. = 1,01FSLL
FSmeas. = 0,98FSLS
0 50 100 150 200
FS (meas.) vs FS (LL)
FS (meas.) vs FS (PI)
FS (meas.) vs FS (LS)
Calculated free swell (%)
Figure 10.10. A correlation between calculated and laboratory determined free
swell values of black clays and red soils.
10.5 Clay fraction and linear shrinkage, free swell
Relatioships between clay fraction and linear shrinkage and/ or swelling capability of black
clays and red soils are generally poorly defined, being characterised by very weak correlations
of less than 0,3 (Figs. 10.11 and 10.12).
Linear shrinkage/ clay fraction
Linear shrinkage LS (%)
35
30
n = 87
25
20
15
R 2 = 0,0363
10
5
0
10 20 30 40 50
% clay
LS vs % clay
Figure 10.11. Correlation between laboratory measured values of linear
shrinkage and clay fraction of black clays and red soils.

199
Free swell/ clay fraction
Free swell FS (%)
200
n = 87
150
100
R 2 = 0,0207
50
0
10 20 30 40 50
% clay
FS vs % clay
Figure 10.12. Correlation between laboratory measured values of free swell and
clay fraction of black clays and red soils.
Figures (10.11) and (10.12) illustrate a slight general increase of linear shrinkage and free
swell values with increasing clay content. The broad spread of results, however, points to the
possible influence of other factors such as soil fabric and/ or structure and clay mineralogy on
activity and engineering behaviour of the soils.

200
Chapter 11
Conclusions
This study has highlighted the presence of extensive reactive (potentially shrinking and
swelling) and/ or expansive black cotton soils across the project area. Swelling capabilities
(free swell values) of 100-170% (mean: 130%) and swelling pressures of 49-104 kPa (mean:
77 kPa) are exhibited by the soils.
The study has also highlighted the influence of environmental factors such as rainfall, solar
radiation, vegetation cover and drainage on the nature and behaviour of the soils in situ. Some
sites in areas of black clays exhibited only a fraction of anticipated shrinkage phenomena due
to limited change in soil moisture conditions; whereas at other sites, extremely strong
shrinkage cracking up to substantial depths and in excess of that anticipated based on soil
index properties, has been observed and recorded.
A range of soil index and engineering properties as well as chemical/ mineralogical
composition consistent with the geological diversity of the project area and its surroundings,
has been obtained and recorded.
The variability of index properties and chemical/ mineralogical composition of black cotton
soils is generally uniform with depth and across the study area, implying occurrence of
generally homogeneous black clays.
Most soil index properties could be characterised and/ or estimated with a high degree of
approximation through correlation with results of Atterberg limits and other index tests
performed on disturbed and fractioned samples.
Characterisation of engineering behaviour of soils (shear strength, consolidation-settlement,
swelling pressure & percentage swelling) is better accomplished by direct measurements on
undisturbed core samples rather than through correlation with results of indirect index tests
performed on disturbed and part samples which harbour destroyed soil fabric and structure.
The field and laboratory investigations and studies of soils of the project area have provided a
vast data base on soil index and engineering properties as well as chemical/ mineralogical
composition. This data will provide the basis for on-going and future research on the
characterisation of expansive and reactive black cotton soils, especially as regards their
negative environmental, engineering geological/ civil engineering and social-economic
implications.
There has been a progressive depletion in the proportionate amounts of soluble bases (MgO,
CaO, Na2O, K2O) and a corresponding increase in the proportions of iron and aluminium
compounds (Fe2O3 and Al2O3) contained in the red soils with time. This is evidenced by
comparing results of chemical analyses for the red soils obtained in this study with previous
chemical analyses results for the same soils (Dumbleton, 1967; Sherwood, 1967). The
depletion is most probably attributed to leaching of the soluble bases from the red soils,
facilitated by the relatively good drainage conditions of the soils and high rainfall received
(760-1000 mm per year), thereby leaving the soils enriched in iron oxide and haematite, as
well as aluminium in the form of clay minerals.

201
There has also been a decrease in the proportions of clay fraction as well as values of
Atterberg limits and other index properties in the red soils with time. This has been evidenced
by comparing results of soil classification tests obtained for the soils in this study with similar
and earlier results obtained for the same soils (Sherwood, 1967). This decrease could be
explained by the increasing presence of free iron oxide in the soils, and which has the effect
of cementing the clay particles together, thereby causing a progressive decrease in both the
proportions of clay as well as values of Atterberg limits of the red soils with time.
Results of X-ray diffraction (XRD) analyses show the mineralogical composition of red soils
to be mainly kaolinite (80-81%) and haematie (15-16%). Accessories of quartz (1-3%),
titanium, i.e.ilmenite and/ or rutile (~ 2%) and manganese (~ 1%) also occur. The kaolinite is
depleted in Al2O3 and this is probably caused by low PH conditions imparted to the soils by
weak carbonic acids formed by processes of organic matter decomposition.
The red soils encountered in this study are polygenetic; and have been derived, developed and
formed under humid conditions, high temperatures and presence of carbon dioxide by
weathering and alteration of Nairobi trachytes, volcanic tuff and ash. This suggestion is
supported by comparing results of chemical analyses obtained for the red soils in this study
with similar results previously obtained for the underlying volcanic materials; and both of
which generally fall into close agreement. A limited contribution by eroded and watertransported
detrital materials is confirmed by results of scanning electron microscope which
reveal inclusions of sub-rounded to rounded quartz grains. Leaching effects in the red soils
and consequent removal of silica and soluble bases have caused the soils to exhibit lower
contents of SiO2, MgO, CaO, Na2O, K2O and correspondingly higher contents of Fe2O3 and
Al2O3 relative to the volcanic materials. This is evidenced by analysis results of scanning
electron microscope for the red soils which show Fe-rich crusts as part of the matrix while
quartz and feldspars are in limited availability and/or virtually absent.
On the other hand, results of X-ray diffraction and scanning electron microscope analyses
show the clay minerals composition of black clays to be mainly smectites and/ or
montmorillonite (over 90%) and some kaolinite (less than 10%). Occurrence of illite is
occasional and only in trace form. Quartz (4-9%) and accessories of K-feldspars (sanidine,
orthoclase, microcline), haematite and carbonates (calcite, dolomite, ankerite, siderite) also
occur.
Results of chemical analyses together with current field observations and evidences related to
the nature of occurrence of black clays show that these soils are not wholly developed in situ;
and are not derived from the underlying practically impermeable Nairobi and Kapiti
phonolites through weathering and alteration. Rather, the black clays have been derived from
the gradual conversion of volcanic ash as well as colluvium, alluvium and soft Pleistocene
materials, all previously occurring as lacustrine deposits in a basin-like lake and swampy
environment during a pluvial period. Further confirmation is provided by results of scanning
electron microscope which show presence of detrital components of biotite and sub-rounded
to rounded quartz grains as well as heavy minerals of Ti and Zn in the black clays, a result of
erosion processes and water transport of soil/rock materials from surrounding areas. The black
clays generally exhibit higher concentrations of soluble bases and silica than the red soils; and
this confirms the suggestion that leached components of soluble bases and silica derived from
areas of red soils occurring on higher grounds serve to further contribute to the formation of
black clays in the plains under alkaline and impeded drainage conditions.

202
The black clays and red soils generally exhibit low total carbon contents of less than 2,5%.
The black clays show 0,50-1,48% total carbon (mean: 1,17%), 0,22-1,19% organic carbon
(mean: 0,93%) and 0,04-0,74% inorganic carbon (mean: 0,24%); while the red soils exhibit
0,45-2,47% total carbon (mean: 1,12%), 0,35-1,90 organic carbon (mean: 0,85%) and 0,10-
0,57% inorganic carbon (mean: 0,27%). The black clays also show a general decrease of total
carbon and organic carbon, and therefore, organic matter content of the soils with depth; the
variation being best described and represented by a polynomial (R = 0,56) and exponential (R
= 0,85) relationship, respectively. The inorganic carbon component generally increases with
depth through in situ conversion of underlying volcanic tuffs to secondary limestone; and this
variation tends to fit a polynomial distribution (R = 0,63). On the other hand, the red soils
show decreasing total carbon as well as organic and inorganic carbon contents with depth, the
three variations closely approximating a potential relationship with fairly strong correlations
of R = 0,94. The decrease of the organic carbon is in reflection of decreasing organic matter
content of the soils with depth; while decreasing inorganic carbon could be attributed to low
pH conditions and leaching of carbonates and other soluble components of Mg, Ca, K and Na
from the soils during rainfall.
Very soft and sensitive clays were also encountered in the present area. They occur in terms
of black clays in peaty/ swampy environs and/ or those of impended drainage; as well as red
soils in densely forested and uncultivated zones. The soils generally exhibit a very soft to firm
consistency, with vane shear strength values of less than 100 kPa. The undrained shear
strength ranges from 30,94 – 41,50 kPa for black clays; and 10,92 – 87,36 kPa for red soils,
and generally increases with increased soil depths. The variation of vane shear strength with
soil depth closely fits a logarithmic relationship with a strong correlation (R = 0,94 for black
clays; and R = 0,97 – 1,0 for red soils). The variation of vane shear strength with natural
moisture content of soils is best described by a polynomial relationship with a very strong
correlation (R = 1.0 for black clays and R = 0,91 - 0,99 for red soils). Variation of vane shear
strength could also be predicted from bulk densities of soils through a polynomial relationship
with a very strong correlation (R = 0,97 for black clays; R = 0,99-1,0 for red soils).
A soils classification based on index properties and grain sizes places the black clays into very
high to extremely high plasticity clays, silty clays and/ or silty clays with sand, as well as
clayey silts; all of which harbour medium to very high levels of activity (normal to highly
active) so that they exhibit high swelling capabilities and potential expansiveness on wetting
from a dry condition. This engineering behaviour has been explained by the high content of
the more expansive clay minerals, i.e. smectites (90% and over) in the clay fraction. On the
other hand, the red soils are medium to high plasticity clayey silts and silty clays with sand;
having at most only medium activity levels (normal active) and therefore exhibiting generally
low swelling capabilities and potential expansiveness when allowed free access to water.
These characteristics have been attributed to the high content (80% and over) of the rather less
expansive clay mineral, kaolinite, in the clay fraction of the soils.
The distribution and variation of index properties of black clays across the study area at
depths of less than 0,50m and those at 0,50m and greater, were found to be generally similar,
comparable and in good agreement, implying a generally homogeneous engineering character
of the soils. Index properties of red soils point to a comparatively more stable and reliable
engineering character of the soils.
A new relationship has been derived, developed and established in this study for the
estimation of the plasticity index (PI) of black clays from laboratory determined linear
shrinkage (LS) of the soils, i.e.

203
PI = 1,88*LS
A comparison of plasticity index values calculated using the above relationship (PILS) with
those actually measured in the laboratory (PImeasured) has shown and confirmed a strong
agreement in the two sets of data in the form of
PImeasured = 1,005PILS
The plasticity index of red soils could also be estimated from laboratory measured linear
shrinkage using a new relationship developed for the soils, i.e.
PI = 1,84LS
The so calculated and/ or estimated plasticity indices have also been found to be practically
identical to actual laboratory measured values, i.e.
PImeasured = 1,00PILS
Another new relationship has been developed for the estimation of plasticity index (PI) of
black and red soils from laboratory measured values of liquid limit (LL) with a high degree of
approximation, i.e.
PI = 0,79(LL-25)
The so calculated and laboratory measured plasticity indices have been found to be in near
perfect agreement, i.e.
PImeasured = 0,99PILL
Linear shrinkage values could also be estimated by calculation from laboratory determined
liquid limits of black clays and red soils using the newly developed relationship below, i.e.
LS = 0,39(LL-21)
The calculated and laboratory measured linear shrinkage values have been found to be in very
strong agreement, i.e.
LSmeasured = 0,997LSLL
The swelling capability of black clays and red soils, in terms of free swell (FS), could be
estimated with a high degree of approximation by calculation from laboratory determined
Atterberg limits (LL, PI) and linear shrinkage (LS). The new derived relationships take the
form of
FS = 3,17(LL-45),
FS = 3,79(PI-13) and
FS = 7,29(LS-7)

204
The reliability of the estimations has been demonstrated by the close relationship and strong
agreement between laboratory measured and so calculated free swell values, i.e.
FSmeasured = 1,01FSLL
FSmeasured = 0,99FSPI
FSmeasured = 0,98FSLS
Correlations between clay fraction/ content and other index properties such as linear
shrinkage and free swell were found to be weak and poor, most probably due to the effects of
soil fabric/ structure, clay mineralogy and other factors in controlling the activity and
physical/ engineering behaviour of the soils.
Laboratory shear strength investigations showed the black clays to be limited in their strength;
and are characterised by relatively lower values of angles of shear resistance (φ`) of between
11° and 30°, giving an average value of 18°. The cohesive nature of the clays is reflected in
their large cohesion (c´) values of 12 – 48 kN/m² (mean value: 35 kN/m²). On the other hand,
the red soils were found to be relatively more stable, with angles of shear resistance of 28° to
29°. However, the cohesive character of these soils is negligible due to their generally loose
and friable nature.
Correlations between shear angles determined on undisturbed samples and results of index
tests of black clays were only of poor to fair or moderate strength. A lot of uncertainty would
therefore be encountered in attempting to assess and characterise shear strength of the clays
on the basis of results of index properties obtained by tests carried out on disturbed and
fractioned samples.
Results of laboratory oedometer consolidation tests and compression coefficients derived
thereof show the red soils to be medium to very highly compressible (mv =0,16-2,94 m²/MN )
when externally loaded in the range of 25-800kPa; and this is a probable result of their loose
and friable nature. The soils also exhibit medium to high rates of consolidation-settlement (cv
= 1,18-12,14 m²/year), a result of the relatively high porosity (voids ratio e = 0,59-1,42) and
permeability (K = 7,57E-11 to 7,33E-9) which facilitate faster drainage and rapid pore water
pressure dissipation on external loading. In practice therefore, these soils would be expected
to undergo rapid consolidation – settlements, especially during the construction stage, without
posing long-term instability problems to constructed structures. The soils fall in the class of
normally consolidated to slightly overconsolidated clays.
The black clays would exhibit slightly lower compressibility, i.e. medium to high (mv = 0,03-
1,62 m²/MN) when externally loaded over a 25 –800kPa range, as a result of their generally
dense, compact and cohesive nature. The clays would also exhibit low rates of consolidationsettlement
(cv = 0,05-5,08 m²/year) due to their relatively low porosity (e = 0,23-1,06) and
permeability (K = 1,67E-12 to 1,20E-9) that tend to limit drainage and dissipation of porewater
pressures. Settlement and instability of structures located on these clays would therefore
be expected to persist beyond the construction stage. The clays are classified as normally
consolidated.
The following relationships have been derived in this study for the approximate estimation of
compressibility indices, cc, of the soils using laboratory determined liquid limits, LL, i.e.

205
cc = 0,0099(122-LL)
for black clays, and
cc = 0,0016(308-LL)
for both black clays and red soils.
Correlation between so calculated compression indices and laboratory measured compression
indices were found to be strong (R = 0,85) for the black clays alone; and moderate (R = 0,58)
for combined black clays and red soils.
Correlations between swell indices, Cs, and index properties (Atterberg limits, free swell) of
black clays and red soils were found to be generally poor. The black clays exhibit significant
swelling pressures of 49-104 kPa. However, strengths of correlation between swelling
pressures and index properties were found to be generally weak. The above findings serve to
highlight uncertainities that may be encountered in attempting to assess and predict expansion
tendencies (on unloading in situ) as well as swelling pressures imposed by clay soils on their
saturation, based on results of laboratory index tests carried out on disturbed and fractioned
soils.
Swelling test results have facilitated derivation of two methods and/ or relationships that serve
to summarise the swelling characteristics of black clays. Both relationships are logarithmic in
representing the variation of percentage swelling (S%) with extent of external loading, P%
(i.e., load decrements, P kPa, expressed as a percentage of swelling pressure, SP kPa). The
first relationship involves deriving percentage swelling by expressing amount of swelling (S
mm) as a fraction of the ultimate swelling (Smax mm) which occurs under zero external
loading conditions. This relationship exhibits a very strong correlation (R = 0,995), i.e.
P = -22,3Ln(S) + 100,80, where
S (%) = percentage swelling (S mm/Smax mm)
P (%) = imposed loads (P kPa) expressed as a percentage of swelling pressure (SP kPa)
Based on this first relationship, therefore, it has been shown that black clays would undergo
high percentage swelling (S >75%) when externally loaded to less than 5 kPa; and low
percentage swelling for loads of over 80 kPa. As a result, external loads of well over 100 kPa
would ensure minimal potential destabilisation of constructed light engineering structures
from swelling effects of the clays on wetting. For known swelling pressure (SP) and ultimate
amount of swelling (Smax) under zero loading, the relationship could be possibly used as a
guide to estimate external loading of light engineering structures necessary to give a desired
and/ or permitted percentage swelling, as may be required by design computations. However,
the relationship cannot be reliably used for estimation of percentage swelling from known
values of external loads, since the former is usually underestimated by an average amount of
S = 8%.
The second relationship has percentage swelling derived by relating amount of swelling, S
mm, to initial test specimen thickness, Ho mm. It is also closely logarithmic with a similarly
very strong correlation (R = 0,98), i.e.
P = -15,86Ln(S) + 29,84

206
where P (%) = (P kPa/SP kPa) * 100
S (%) = (S mm/Ho mm) * 100
Unlike the first relationship, this last relationship could be used for estimation of both external
loading conditions necessary to meet an allowable percentage swelling and vice versa, as long
as the swelling pressure of the clays is known. It would therefore be a preferred relationship in
estimating and/ or characterising swelling characteristics of clays.

207
Chapter 12
Recommendations
Analysis of short term stability problems during and/ or immediately after construction
involve use of the undrained shear strength or apparent cohesion, cu, in the relevant
computations (Terzaghi and Peck, 1967). The corresponding angle of shear resistance, φ, is
especially useful in the derivation of the necessary earth pressure coefficients and/ or bearing
capacity coefficients. It would be more helpful therefore, that future works involving soils of
the area include laboratory triaxial tests in their studies with the purpose of providing the
necessary undrained shear strength parameters, cu, φ. The triaxial testing procedure is
generally more satisfactory in measuring the shear strength of clay soils in terms of total
stresses, through quick undrained shear tests (Head, 1988). Short-term stability analyses may
include computations related to bearing capacity of footings and foundations for structures on
saturated homogeneous clays, earth pressure on retaining walls, earth pressure against bracing
in temporary excavations, safeguard against heave of the bottom of temporary open
excavations in clays, stability of side-slopes of cuttings, as well as short term stability of
embankments and earth dams.
The black clays in some parts of the study area were found to contain gravelly lateritic
materials of up to 40 mm sizes. Investigation and determination of shear strength
characteristics of such materials by triaxial testing would be impracticable; while use of the
standard shearbox apparatus as employed in this study, would only produce unreliable results.
Future investigations and studies regarding shear strength of these significantly coarse
gravelly clays should therefore employ the large shearbox apparatus such as the one described
by Head (1988). This apparatus could provide more reliable shear strength parameters by
executing slow drained shear tests on large representative and undisturbed block samples of
the clays measuring 305 mm square and 150 mm thick. The results of shear strength
parameters so obtained would be useful at the design stages of a number of structures such as
embankments and earth dams which usually incorporate gravel fill material (Pike, 1973;
Pike, Acott and Leech, 1977). The results would also serve as a basis for discriminating,
subdividing and classifying gravelly soil materials to suit different construction needs and
works (Bishop, 1948; Pike, 1973).
The red soils were found in this study to be generally more permeable and relatively free
draining. They would therefore exhibit rapid drainage and settlement when externally loaded.
The resulting compression/ settlement curves tend to deviate from conventional shapes
derived from one-dimensional theory of consolidation so that determination of relevant
consolidation parameters would also be difficult. Use of a Rowe consolidation cell
(Rowe,1966), capable of accommodating larger soil specimens would be recommended for
oedometer tests on the clayey silt and silty varieties of red soils, so as to obtain a more
definite value of coefficient of consolidation, cv, for use in the more reliable estimation of
rates of settlement under various loading conditions. Alternatively, cv could be derived
empirically from the relation
cv = K/(mv * 0,31*E-9) m²/year
where K (m/s) = coefficient of permeability as measured in situ in the field
mv (MN/m²) = coefficient of volume compressibility derived from laboratory
consolidaton test using the Rowe cell

208
In addition, more reasonable estimates of both the amount and rate of settlement of soils of
the project area could be achieved by taking horizontal drainage into account during
laboratory consolidation tests. This could be accomplished by performing consolidation tests
on larger specimens under hydraulic loading using a larger cell, such as the Rowe
consolidation cell, and designed for provision of horizontal drainage (Rowe, 1966). In the
normal oedometer consolidation tests, however, horizontal drainage could be taken into
account by either fitting a pervious lining inside the consolidation ring and sealing the
specimen ends; or by trimming the specimen in a vertical plane (Head, 1988).
A provision should be made to determine in situ and/ or monitor in the field consolidation
settlements and accompanying increase in effective strength which occur due to dewatering
and/ or partial drying of soft soils (in swamp and peaty environments) caused by a fall in the
groundwater table; and this especially in the transition period from wet to dry months, and
vice versa. This would serve to assess and predict possible destructive implications onto
constructed and/ or projected engineering structures. According to Head (1988), a fall of 1 m
in the groundwater table would increase the effective stress in the whole clay deposit beneath
the water table by about 10 kPa, the actual amount of consolidation depending on the change
in effective stress in the soil.
It has been established in this study that the plasticity index (PI) of black clays and red soils
could be approximately estimated from laboratory measured results of linear shrinkage (LS)
and/ or liquid limit (LL) using newly developed relationships, i.e.
PI = 1,88*LS (for black clays),
PI = 1,84LS (for red soils) and
PI = 0,79(LL-25) for both black clays and red soils.
In addition, the swelling characteristics of black clays could be approximately expressed in
terms of logarithmic relationships derived in the present study, i.e.
P = -22,3Ln(S) + 100,80, where
S (%) = percentage swelling with respect to ultimate swelling (Smax mm) under zero
loading
P (%) = imposed loads (P kPa) expressed as a percentage of swelling pressure (SP kPa)
and P = -15,86Ln(S) + 29,84, for
S (%) = percentage swelling with respect to initial test specimen thickness (Ho mm)
P (%) = imposed loads (P kPa) expressed as a percentage of swelling pressure (SP kPa)
It would be necessary and useful to extend the investigation with the purpose of finding out if
the same relationships for estimating plasticity index and swelling characteristics hold for
similar tropical clays and/ or soils in Africa, and around the world in general.
The following relationships have been derived in this study for the approximate estimation of
compressibility indices, cc, of the soils using laboratory determined liquid limits, LL, i.e.

209
cc = 0,0099(122-LL)
for black clays, and
cc = 0,0016(308-LL)
for both black clays and red soils, where
cc = a dimensionless number, and
LL (%) = liquid limit
Correlation between so calculated compression indices and laboratory measured compression
indices were found to be strong (R = 0,85) for the black clays alone; and moderate (R = 0,58)
for combined black clays and red soils. However, these derivations were based on a limited
amount of data on compression indices (n = 6 for black clays; n = 9 for black clays and red
soils) available in the current study. It would be useful that future works attempt the same
correlations and derivations based on a larger amount of data as regards laboratory results of
liquid limit tests performed on disturbed samples on one hand, and those of compression
indices obtained from oedometer tests on undisturbed specimens on the other. Possible
improvements in the strength of correlations and assessment of compressibility characteristics
of soils using index properties (Atterberg limits) would also be indicated. Investigations could
also be extended to other types of soil world-wide, to find out if the same and/ or similar
relationships hold.
All in all, it may be useful to initiate and establish a field monitoring program for the soils of
the Nairobi area, aimed at monitoring of expansive/ reactive soil movements and moisture
changes. The selected sites would involve soils of varying geographic and lithological
conditions. This would serve the purpose of providing a comprehensive picture of expansive
and/ or reactive soil behaviour, in terms of characterisation of site reactivity (shrink-swell
potential) and unsaturated moisture flow. The scheme my take the form of monitoring and
recording profiles of ground movements and moisture changes as well as providing general
data on soils with time (e.g. on a monthly basis), over a period of some years. A further
analysis in terms of environmental factors would be necessary and helpful.
According to Delaney & Allman (1998), useful instrumentation for a field monitoring
program may include use of surface and sub-surface pegs (surface and sub-surface levels),
neutron probes (soil volumetric moisture content), thermocouples (soil temperature), gypsum
blocks (matrix soil suction), piezometer (groundwater level, especially in alluvial soils),
automatic weather stations (rainfall, humidity, temperature, solar radiation, wind velocity/
direction) etc. Results may include data on design parameters for foundations on expansive
and/ or reactive soils (e.g. active depth, suction change, seasonal heave etc.), as well as
detailed information on field behaviour to compare with models which may be developed in
the laboratory to simulate and characterise possible field behaviour of the soils. The results
would also provide a basis for comparing and assessing the different methods of
characterising site reactivity and/ or shrink-swell behaviour.
The first of the two methods developed in this study to estimate swelling characteristics of
black clays [i.e. Eq. (7.49): P = -22,3Ln(S) + 100,8] was shown to be limited in predicting
percentage swelling that may be realised under selected structural loading conditions. It was
also explained that the underestimation of percentage swelling values was most probably a
result of some degree of plastic deformation (or consolidation of test specimens when initially

210
loaded to their swelling pressure), and which could not therefore be recovered on subsequent
complete unloading. In order to improve on the estimation capability and reliability of this
method, it is recommended that the amount of ultimate swelling (Smax) under zero loading,
and on which calculation of percentage swelling is based, be determined on separate test
specimens from those used in the swelling test. Cumulative swelling values (S mm) obtained
during the swelling test would then be expressed as a percentage of this separately determined
maximun swelling value, Smax (mm).
Adoption of a modified triaxial compression cell (after Abebe, 2002) to investigate expansive
clays could prove useful in providing more accurate information on swelling characteristics
through simultaneous measurement of swelling pressure and associated swelling strain
(volume change).

211
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Appendix A: Oedometer consolidation tests
Complete results for the loading/ unloading stages of black clays and red soils
Table A1. Consolidation parameters for black clays, Sample No. SA2-70cm.
Calculation sheet Sample No.: SA2-70cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 0,709 - - - - - -
100 0,043 0,307 0,704 0,000052 0,031 1,00 5,080 4,837E-11 0,30 0,070 0,0002
200 0,408 2,914 0,659 0,000446 0,262 9,50 0,519 4,210E-11 0,004
400 0,935 6,679 0,595 0,000322 0,194 28,00 0,165 9,914E-12 0,007
800 1,556 11,114 0,519 0,000190 0,119 60,00 0,070 2,597E-12 0,009
1600 2,292 16,371 0,429 0,000112 0,074 60,00 0,063 1,448E-12 0,011
2400 2,708 19,343 0,379 0,000063 0,044 95,00 0,036 4,983E-13 0,007
1600 2,601 18,579 0,392
400 1,942 13,871 0,472
100 1,235 8,821 0,558
Table A2. Consolidation parameters for black clays, Sample No. SA37-50cm.
Calculation sheet
Sample No.: SA37-50cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 1,098 - - - - - -
25 0,258 1,843 1,060 0,001547 0,737 2,90 1,725 3,942E-10 0,003
50 0,813 5,807 0,977 0,003328 1,615 7,00 0,673 3,372E-10 0,40 0,08 0,004
100 1,511 10,793 0,872 0,002092 1,059 12,00 0,357 1,172E-10 0,008
200 2,283 16,307 0,756 0,001157 0,618 27,00 0,141 2,703E-11 0,010
400 3,085 22,036 0,636 0,000601 0,342 45,00 0,074 7,849E-12 0,015
800 3,875 27,679 0,518 0,000296 0,181 55,00 0,052 2,935E-12 0,017
1600 4,290 30,643 0,455 0,000078 0,051 95,00 0,027 4,275E-13
100 3,700 26,429 0,544
25 3,400 24,286 0,589
12 3,087 22,050 0,636
Table A3. Consolidation parameters for black clays, Sample No. SB1-70cm.
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 1,045 - - - - - -
50 0,259 1,850 1,007 0,000757 0,370 0,48 10,421 1,195E-09 0,0002
100 0,770 5,500 0,933 0,001493 0,744 2,00 2,364 5,451E-10 0,002
200 1,585 11,321 0,814 0,001191 0,616 1,80 2,375 4,535E-10 0,3 0,043 0,004
400 2,320 16,571 0,706 0,000537 0,296 11,00 0,343 3,148E-11 0,005
800 2,725 19,464 0,647 0,000148 0,087 18,00 0,190 5,113E-12 0,006
1600 3,303 23,593 0,563 0,000106 0,064 40,00 0,078 1,558E-12 0,012
2400 3,638 25,986 0,514 0,000061 0,039 92,00 0,031 3,802E-13 0,008
1600 3,605 25,750 0,518
400 3,321 23,721 0,560
50 2,666 19,043 0,656

Table A4. Consolidation parameters for black clays, Sample No. SB42-50cm.
Calculation sheet Sample No.: SB42-50cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 0,732 - - - - - -
100 1,123 8,021 0,593 0,001389 0,80 9,50 0,494 1,229E-10 0,005
200 2,041 14,579 0,480 0,001136 0,71 20,00 0,200 4,430E-11 0,009
400 3,098 22,129 0,349 0,000654 0,44 24,00 0,142 1,939E-11 0,024
800 4,075 29,107 0,228 0,000302 0,22 50,00 0,056 3,916E-12 0,34 0,03 0,017
1600 4,972 35,514 0,117 0,000139 0,11 70,00 0,033 1,168E-12 0,021
2400 5,404 38,600 0,064 0,000067 0,06 95,00 0,021 3,941E-13 0,009
1600 5,338 38,129 0,072
400 4,732 33,800 0,147
100 4,155 29,679 0,218
25 3,677 26,264 0,277
12 3,075 21,964 0,352
Table A5. Consolidation parameters for black clays, Sample No. SC17-50cm
Calculation sheet Sample No.: SC17-50cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 0,728 - - - - - - - -
100 0,244 1,743 0,698 0,000301 0,17 9,00 0,556 3,006E-11 0,006
200 1,067 7,621 0,596 0,001016 0,60 30,00 0,154 2,862E-11 0,007
400 1,999 14,279 0,481 0,000575 0,36 18,00 0,225 2,508E-11 0,009
800 2,889 20,636 0,371 0,000275 0,19 50,00 0,069 3,991E-12 0,016
1600 3,962 28,300 0,239 0,000166 0,12 65,00 0,045 1,674E-12 0,44 0,040 0,020
2400 4,386 31,329 0,187 0,000065 0,05 120,00 0,021 3,424E-13 0,012
1600 4,317 30,836 0,195
400 3,810 27,214 0,258
70 3,043 21,736 0,352
Table A6. Consolidation parameters for black clays, Sample No. SC29-50cm.
Consolidation test calculation sheet Sample No: SC29-50cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 0,901 - - - - - -
100 0,466 3,329 0,838 0,000633 0,33 2,50 1,971 2,034E-10 0,003
200 1,061 7,579 0,757 0,000808 0,44 5,50 0,828 1,129E-10 0,004
400 1,791 12,793 0,658 0,000496 0,28 15,00 0,274 2,397E-11 0,009
800 2,553 18,236 0,555 0,000259 0,16 35,00 0,104 5,027E-12 0,013
1600 3,418 24,414 0,437 0,000147 0,09 38,00 0,083 2,431E-12 0,014
2400 3,807 27,193 0,384 0,000066 0,05 80,00 0,035 4,995E-13 0,38 0,065 0,008
1600 3,735 26,679 0,394
400 3,256 23,257 0,459
50 2,585 18,464 0,550

Table A7. Consolidation parameters for red soils, Sample No. RD1-30cm
Consolidation test
calculation sheet Sample No.: RD1-30cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 1,560 - - - - - - - -
12 1,205 8,607 1,340 0,018365 7,17 0,30 15,56 3,459E-08 0,002
25 1,537 10,979 1,279 0,004671 2,00 0,35 11,85 7,331E-09 0,003
50 1,770 12,643 1,237 0,001705 0,75 1,05 3,77 8,750E-10 0,003
100 2,338 16,700 1,133 0,002078 0,93 1,25 2,97 8,547E-10 0,004
200 2,990 21,357 1,014 0,001192 0,56 1,50 2,23 3,860E-10 0,004
400 3,720 26,571 0,880 0,000668 0,33 1,60 1,84 1,892E-10 0,004
800 4,490 32,071 0,739 0,000352 0,19 1,85 1,38 7,988E-11 0,004
1600 5,174 36,957 0,614 0,000156 0,09 2,20 0,99 2,769E-11 0,003
2400 5,523 39,450 0,550 0,000080 0,05 3,90 0,50 7,646E-12 0,42 0,030 0,003
1600 5,503 39,307 0,554
400 5,410 38,643 0,571
100 5,323 38,021 0,587
25 5,185 37,036 0,612
12 5,092 36,371 0,629
Table A8. Consolidation parameters for red soils, Sample No. RD1-100cm.
Consolidation test
calculation sheet Sample No.: RD1-100cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 1,607 - - - - - - - -
12 0,513 3,664 1,511 0,007961 3,05 0,16 30,69 2,905E-08 0,003
25 1,029 7,350 1,415 0,007391 2,94 0,98 4,67 4,258E-09 0,005
50 1,816 12,971 1,269 0,005862 2,43 1,00 4,11 3,094E-09 0,007
100 2,781 19,864 1,089 0,003594 1,58 1,11 3,21 1,575E-09 0,008
200 3,787 27,050 0,902 0,001873 0,90 1,40 2,13 5,928E-10 0,008
400 4,693 33,521 0,733 0,000844 0,44 1,50 1,65 2,270E-10 0,006
800 5,462 39,014 0,590 0,000358 0,21 1,75 1,18 7,574E-11 0,005
1600 6,095 43,536 0,472 0,000147 0,09 2,10 0,84 2,404E-11 0,4 0,030 0,004
2400 6,396 45,686 0,416 0,000070 0,05 3,80 0,41 6,071E-12 0,004
1600 6,383 45,593 0,418
400 6,288 44,914 0,436
100 6,164 44,029 0,459
25 6,033 43,093 0,484
12 5,952 42,514 0,499

Table A9. Consolidation parameters for red soils, Sample No. RD1-400cm.
Consolidation test
calculation sheet Sample No.: RD1-400cm
P (kPa) DH (mm) E (%) e av ( m²/kN) mv (m²/MN) t50 (min) Cv (m²/year) K (m/s) Cc Cs Cα
0 0,000 0,000 1,163 - - - - - - -
12 0,233 1,664 1,127 0,003000 1,39 0,33 15,42 6,630E-09 0,003
25 0,432 3,086 1,096 0,002365 1,11 0,40 12,14 4,185E-09 0,002
50 0,634 4,529 1,065 0,001248 0,60 0,56 8,42 1,554E-09 0,002
100 0,940 6,714 1,018 0,000945 0,46 0,85 5,34 7,580E-10 0,003
200 1,374 9,814 0,951 0,000670 0,33 0,90 4,77 4,909E-10 0,005
400 1,986 14,186 0,856 0,000473 0,24 1,10 3,59 2,695E-10 0,006
800 2,744 19,600 0,739 0,000293 0,16 1,30 2,71 1,324E-10 0,007
1600 3,493 24,950 0,623 0,000145 0,08 1,80 1,71 4,410E-11 0,39 0,020 0,006
2400 3,878 27,700 0,564 0,000074 0,05 2,50 1,11 1,571E-11 0,006
1600 3,863 27,593 0,566
400 3,779 26,993 0,579
100 3,675 26,250 0,595
25 3,528 25,200 0,618
12 3,249 23,207 0,661

Results of swelling tests on black clays
Appendix B: Swelling tests
Swelling S (mm) is expressed as percentage of ultimate swelling Smax (mm) at zero loading.
Table B1
Table B2
Swelling test: Sample No. SA2/70cm Swelling test: Sample No.SB1/70cm
Initial sample size
Initial sample size
Diameter (mm) 71,4 Diameter (mm) 71,4
Height Ho (mm) 11 Height Ho (mm) 11
X-Sectional area (m²) 0,004 X-Sectional area (m²) 0,004
Swelling pressure load (Kg) 33 Swelling pressure load (Kg) 15
Swelling pressure SP (kPa) 82,46 Swelling pressure SP (kPa) 37,48
Maximum swelling Smax (mm) 1,036 Maximum swelling Smax (mm) 0,425
Load Swelling
(kg) (kPa) (mm) S (% ) Load Swelling
33 82,46 0 0 (kg) P (kPa) (mm) S (%)
16,5 41,23 0,106 10 15 37,48 0 0
8 19,99 0,253 24 7,5 18,74 0,054 13
4 10,00 0,321 31 4 10,00 0,126 30
2 5,00 0,539 52 2 5,00 0,197 46
1 2,50 0,725 70 1 2,50 0,279 66
0 0,00 1,036 100 0 0,00 0,425 100
Table B3
Table B4
Swelling test: Sample No. SC25/50cm Swelling test: Sample No. SC29/50cm
Initial sample size
Initial sample size
Diameter (mm) 71,4 Diameter (mm) 71,4
Height Ho (mm) 11 Height Ho (mm) 11
X-Sectional area (m²) 0,004 X-Sectional area (m²) 0,004
Swelling pressure load (Kg) 14,5 Swelling pressure load (Kg) 18
Swelling pressure SP (kPa) 36,23 Swelling pressure SP (kPa) 44,98
Maximum swelling Smax (mm) 0,475 Maximum swelling Smax (mm) 0,493
Load Swelling Load Swelling
(kg) P (kPa) (mm) S (%) (kg) P (kPa) (mm) S (%)
14,5 36,23 0,000 0 18 44,98 0,000 0
7 17,49 0,066 14 9 22,49 0,061 12
3,5 8,75 0,160 34 4,5 11,24 0,149 30
2 5,00 0,260 55 2 5,00 0,266 54
1 2,50 0,347 73 1 2,50 0,356 72
0 0,00 0,475 100 0 0,00 0,493 100

Results of swelling tests on black clays
Appendix C: Swelling tests
Swelling S (mm) is expressed as percentage of initial specimen thickness Ho (mm).
Table C1
Table C2
Swelling test: Sample No. SA2/70cm Swelling test: Sample No. SB1/70cm
Initial sample size
Initial sample size
Diameter (mm) 71,4 Diameter (mm) 71,4
Height Ho (mm) 11 Height Ho (mm) 11
X-Sectional area (m²) 0,004 X-Sectional area (m²) 0,004
Swelling pressure load (Kg) 33 Swelling pressure load (Kg) 15
Swelling pressure SP (kPa) 82,46 Swelling pressure SP (kPa) 37,48
Maximum swelling Smax (mm) 1,036 Maximum swelling Smax (mm) 0,425
Load
Swelling
(kg) (kPa) (mm) % Load Swelling
33 82,46 0,000 0,00 (kg) (kPa) (mm) %
16,5 41,23 0,106 0,96 15 37,48 0,000 0,00
8 19,99 0,253 2,30 7,5 18,74 0,054 0,49
4 10,00 0,321 2,92 4 10,00 0,126 1,15
2 5,00 0,539 4,90 2 5,00 0,197 1,79
1 2,50 0,725 6,59 1 2,50 0,279 2,54
0 0,00 1,036 9,42 0 0,00 0,425 3,86
Table C3
Table C4
Swelling test: Sample No. SC25/50cm Swelling test: Sample No. SC29/50cm
Initial sample size
Initial sample size
Diameter (mm) 71,4 Diameter (mm) 71,4
Height Ho (mm) 11 Height Ho (mm) 11
X-Sectional area (m²) 0,004 X-Sectional area (m²) 0,004
Swelling pressure load (Kg) 14,5 Swelling pressure load (Kg) 18
Swelling pressure SP (kPa) 36,23 Swelling pressure SP (kPa) 44,98
Maximum swelling Smax (mm) 0,475 Maximum swelling Smax (mm) 0,493
Load Swelling Load Swelling
(kg) (kPa) (mm) % (kg) (kPa) (mm) %
14,5 36,23 0 0,00 18 44,98 0 0,00
7 17,49 0,066 0,60 9 22,49 0,061 0,55
3,5 8,75 0,16 1,45 4,5 11,24 0,149 1,35
2 5,00 0,26 2,36 2 5,00 0,266 2,42
1 2,50 0,347 3,15 1 2,50 0,356 3,24
0 0,00 0,475 4,32 0 0,00 0,493 4,48

Appendix D
Distribution/ variation of index and engineering properties of
black clays in Nairobi area

16000
14000
12000
10000
West
8000
6000
4000
2000
0
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000
South
Distance (m)
Fig. D1 Distribution/ depth variation (m) of soils in Nairobi area (Otando S. W., 2003)
Distance (m)

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D2(a) Distribution/ moisture (Wn%) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
West
1000
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D2(b) Distribution/ moisture (Wn%) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
1000
Distance (m)
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D3(a) Distribution/ liquid limit (LL%) variation of black clays in Nairobi area for depths less
than 0,5m (Otando S. W., 2003).

4000
3000
2000
1000
Distance (m)
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D3(b) Distribution/ liquid limit (LL%) variation of black clays in Nairobi area for depths greater
than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D4(a) Distribution/ plasticity index (PI%) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D4(b) Distribution/ plasticity index (PI%) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D5(a) Distribution/ linear shrinkage (LS%) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D5(b) Distribution/ linear shrinkage (LS%) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D6(a) Distribution/ free swell (FS%) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D6(b) Distribution/ free swell (FS%) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D7(a) Distribution/ fines fraction (%fines) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D7(b) Distribution/ fines fraction (%fines) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D8(a) Distribution/ clay fraction (%clay) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D8(b) Distribution/ clay fraction (%clay) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D9(a) Distribution/ angle of shear resistance (phi °) variation of black clays in Nairobi area for depths less than 0,5m
(Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D9(b) Distribution/ angle of shear resistance (phi °) variation of black clays in Nairobi area for depths greater than 0,5m
(Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D10(a) Distribution/ apparent cohesion (c' kPa)) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D10(b) Distribution/ apparent cohesion (c' kPa)) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D11(a) Distribution/ activity level (A) variation of black clays in Nairobi area for depths less than 0,5m (Otando S. W., 2003).

4000
3000
2000
Distance (m)
1000
West
0
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000
South
Distance (m)
Fig. D11(b) Distribution/ activity level (A) variation of black clays in Nairobi area for depths greater than 0,5m (Otando S. W., 2003).

Appendix E
Geotechnical soil map of Nairobi area, Kenya

15000
10000
5000
Distance (m)
0
0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000
m
Fig. E1 Geotechnical soils map of Nairobi area, Kenya (prepared by Otando, S. W., 2003).