an engineering geological characterisation of tropical clays - GBV

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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
Page 1 and 2: AN ENGINEERING GEOLOGICAL CHARACTER
Page 3 and 4: i Contents Page Acknowledgements Su
Page 5 and 6: iii Chapter 8. Distribution of inde
Page 7 and 8: v 7.29 - 7.33 Correlation between s
Page 9 and 10: vii 136 139 7.13 Compressibility cl
Page 11 and 12: ix Acknowledgements I would like to
Page 13 and 14: 1 Chapter 1 Introduction 1.1 Scope
Page 15 and 16: Figure1.1 Map of Nairobi region sho
Page 17 and 18: 5 Plates 1.2 (a) & (b) Strong shrin
Page 19 and 20: 7 Available literature in the form
Page 21 and 22: 9 slopes. The northern boundary bet
Page 23 and 24: 11 1.6 Climate The Nairobi area and
Page 25 and 26: 13 marked daily range of relative h
Page 27 and 28: 15 Chapter 2 Previous works 2.1 Sum
Page 29 and 30: 17 Table 2.1 Stratigraphic correlat
Page 31 and 32: 19 Chapter 3 Geology 3.1 Introducti
Page 33: 21 metamorphic minerals sillimanite
Page 37: 25 and prismatic apatite occur as a
Page 41 and 42: 29 caused by partial segregation of
Page 44 and 45: 32 could otherwise lead to erroneou
Page 46 and 47: 34 The red soils in this study occu
Page 48 and 49: 36 17 16 15 14 13 12 11 10 9 8 7 6
Page 50 and 51: 38 4.4 Vane test 4.4.1 Introduction
Page 52 and 53: 40 Table 4.1 Classification of soft
Page 54 and 55: 42 The variation of vane shear stre
Page 56 and 57: 44 And thirdly, the field vane appa
Page 58: 46 horizon, most probably a result
Page 61 and 62: 49 The red friable clays on the who
Page 63 and 64: 51 Results of chemical analyses of
Page 65 and 66: 53 Results of previous soil classif
Page 67 and 68: 55 neighbouring metamorphic areas a
Page 69 and 70: 57 Table 5.10. Profile description
Page 71 and 72: 59 The presence of BaO in the red a
Page 73 and 74: 61 resulted most probably from supp
Page 75 and 76: 63 800 Impulse 700 600 500 SC 17 -5
Page 77 and 78: 65 sericite and chlorite (see next
Page 79 and 80: 67 Q: Quartz (1%) H: Haematite K: K
Page 81 and 82: 69 Plate 6.3. K-feldspar phenocryst
Page 83 and 84: 71 The trachytes generally show a r
Page 85 and 86: 73 Plate 6.16. Organic matter (rema
Page 87 and 88: 75 Plate 6.24. Solution pores/ cavi
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77 Plate 6.29. Iron concretion with
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79 The CS-225 is a micro-processor
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81 The red soils generally exhibit
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83 Chapter 7 Laboratory soils index
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85 According to Johnson and Degraff
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87 Table 7.2. Results of index test
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89 Plate 7.2a. Apparatus for liquid
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91 Activity chart Plasticity index
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93 Table 7.3 (continued). Atterberg
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95 where 7.1.4.4 Results V = volume
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97 A standard classification of soi
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99 Table 7.6. Viscosity and density
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101 Table 7.7, continued. Results o
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103
Page 117 and 118:
105 Plate 7.7a. Shear testing showi
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107 Shear stress / Displacement Cur
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109 Table 7.10. Distribution and/ o
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111 illustrated in Figures 7.6, 7.7
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113 Black clays Cohesion c´ (kN/m
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115 7.4 Oedometer consolidation tes
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117 The compound coefficient, K/ρw
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119 Primary consolidation is a time
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121 where w (%) = moisture content
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123 Relationships between values of
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125 Plate 7.8. Oedometer consolidat
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127 Cumulative log-time/settlement
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129 Ranges of values of coefficient
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131 is most probably due to the ten
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133 The results of correlation show
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135 by differences in lithology, mi
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137 Black clays and red soils Swell
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139 Testing procedure involved cutt
Page 153 and 154:
141 Black clays Swelling pressure S
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143 Black clays: P (kPa) vs S (%) P
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145 The same relationship is repres
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147 Alternatively, percentage swell
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149 Combined sample results: S % vs
Page 163 and 164:
151 Black clays: Greek method P% =
Page 165 and 166:
153 partly produced by effects of w
Page 167 and 168:
155 Chapter 8 Distribution of index
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157 the study area, respectively. R
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159 Liquid limit variation; 0,50m a
Page 173 and 174:
161 Liquid limit (LL) variation (>
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163 (8.6). The few isolated patches
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165 Similarly, soil thicknesses of
Page 179 and 180:
167 Free swell variation; 0,50m dep
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169 fraction at the two depth inter
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171 Variation of fines (%), < 0,50m
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173 %coarse % coarse fraction varia
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175 1°19´S 22 Shear angle variati
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177 9.2 Grain size distribution The
Page 191 and 192:
179 rapid dissipation of pore water
Page 193 and 194:
Chapter 10 181 Correlation of index
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183 Black clays; plasticity index/
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185 On the other hand, laboratory m
Page 199 and 200:
187 Red clays; measured/ calculated
Page 201 and 202:
189 These two relationships could b
Page 203 and 204:
191 Diagram: PImeasured/ PIcalculat
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193 Table 10.5, continued. Calculat
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195 Table 10.6. Calculated and labo
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197 The swelling capability in term
Page 211 and 212:
199 Free swell/ clay fraction Free
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201 There has also been a decrease
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203 PI = 1,88*LS A comparison of pl
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205 cc = 0,0099(122-LL) for black c
Page 219 and 220:
207 Chapter 12 Recommendations Anal
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209 cc = 0,0099(122-LL) for black c
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211 References Abebe, S. T., 2002.
Page 225 and 226:
213 Galster, R.W., 1977. A system o
Page 227 and 228:
215 Mitchell, J.K., 1993. Fundament
Page 229 and 230:
217 ------,1964. Long term stabilit
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Appendix A: Oedometer consolidation
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Table A7. Consolidation parameters
Page 235 and 236:
Results of swelling tests on black
Page 237 and 238:
Appendix D Distribution/ variation
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 1000 Distance (m) We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
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4000 3000 2000 Distance (m) 1000 We
Page 257 and 258:
4000 3000 2000 Distance (m) 1000 We
Page 259 and 260:
Appendix E Geotechnical soil map of
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