an engineering geological characterisation of tropical clays - GBV

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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
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 and 38: 25 and prismatic apatite occur as a
Page 39 and 40: 27 Table 3.2 Chemical analyses of s
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 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
Page 89 and 90: 77 Plate 6.29. Iron concretion with
Page 91 and 92: 79 The CS-225 is a micro-processor
Page 93 and 94: 81 The red soils generally exhibit
Page 95 and 96: 83 Chapter 7 Laboratory soils index
Page 97 and 98: 85 According to Johnson and Degraff
Page 99 and 100: 87 Table 7.2. Results of index test
Page 101 and 102: 89 Plate 7.2a. Apparatus for liquid
Page 103 and 104: 91 Activity chart Plasticity index
Page 105 and 106: 93 Table 7.3 (continued). Atterberg
Page 107 and 108: 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
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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
Page 139 and 140:
127 Cumulative log-time/settlement
Page 141 and 142:
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
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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/
Page 197 and 198:
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
Page 207 and 208:
195 Table 10.6. Calculated and labo
Page 209 and 210:
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
Page 215 and 216:
203 PI = 1,88*LS A comparison of pl
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205 cc = 0,0099(122-LL) for black c
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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.
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213 Galster, R.W., 1977. A system o
Page 227 and 228:
215 Mitchell, J.K., 1993. Fundament
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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
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4000 3000 2000 Distance (m) 1000 We
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Appendix E Geotechnical soil map of
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