54 Table 5.8. Results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> black <strong>clays</strong> obtained in this study. % SA2- SA2- SA41- SB1- SB1- SB41- SB41- SC17- content <strong>of</strong>: 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 <strong>an</strong>d SC41-50cm are black clay samples. The black to dark grey <strong>clays</strong> are characterised by a number <strong>of</strong> distinctive properties which serve to point to their possible genetic origin, as discussed below. Results <strong>of</strong> chemical <strong>an</strong>alyses performed on these soils in this study (Table 5.8) show the common exch<strong>an</strong>geable bases <strong>of</strong> Ca, Mg, K <strong>an</strong>d Na to be all <strong>of</strong> the same order irrespective <strong>of</strong> the type <strong>of</strong> rock underlying the soil. This serves to show that the black <strong>clays</strong> are most probably not derived from the underlying rocks by weathering. This is further supported by results <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> the soils, which generally do not agree with <strong>an</strong>d/ or are not <strong>of</strong> the same order as those for the main underlying rocks <strong>of</strong> Nairobi <strong>an</strong>d Kapiti phonolites (Tables 3.1, 3.2 <strong>an</strong>d 3.3). The black <strong>clays</strong> exhibit a generally uniform depth, varying from 0,90 to 1,50m. Depths <strong>of</strong> up to 1,80 to 2,10m are also registered in the swampy areas <strong>of</strong> Madaraka West <strong>an</strong>d environs. The <strong>clays</strong> generally show pronounced cracking in the form <strong>of</strong> strong shrinkage cracks on drying from a wet condition. The black <strong>clays</strong> were also observed harbouring occasional s<strong>an</strong>dier vari<strong>an</strong>ts in some places. Results <strong>of</strong> X-ray diffraction <strong>an</strong>alyses performed in this study as well as earlier results <strong>of</strong> chemical/ mineralogical studies by Stephen, Bellis <strong>an</strong>d Muir (1956) show the black <strong>clays</strong> as containing a number <strong>of</strong> heavy minerals usually characteristic <strong>of</strong> metamorphic rocks <strong>of</strong> the gneissic type. The heavy minerals include sillim<strong>an</strong>ite, ky<strong>an</strong>ite, garnet <strong>an</strong>d staurolite; <strong>an</strong>d were traced in soil samples collected from Embakasi area <strong>an</strong>d its environs which are underlain by Nairobi phonolite. The presence <strong>of</strong> the s<strong>an</strong>dier vari<strong>an</strong>ts <strong>an</strong>d heavy minerals serves to point to the possibility <strong>of</strong> portions <strong>of</strong> materials forming the black <strong>clays</strong> as having originated from
55 neighbouring metamorphic areas <strong>an</strong>d deposited by rivers <strong>an</strong>d/ or surface run-<strong>of</strong>f into the present area. Alteration <strong>of</strong> kaolinitic red soils deficient in alkali <strong>an</strong>d alkali earth metals (Na, K, Mg, Ca) could have also caused the formation <strong>of</strong> the respective aluminiumsilicates <strong>of</strong> the heavy minerals. The altered soils were then eroded, tr<strong>an</strong>sported <strong>an</strong>d deposited in the lower areas <strong>of</strong> black <strong>clays</strong>. The black <strong>clays</strong> generally exhibit sharp to fairly sharp contacts with the underlying volc<strong>an</strong>ic rocks. Cases <strong>of</strong> patches <strong>of</strong> black <strong>clays</strong> overlying red soils were observed in the northern <strong>an</strong>d north-western sections <strong>of</strong> the study area. In other parts <strong>of</strong> the study area, laterite was commonly found underlying the black <strong>clays</strong>. In addition, areas with black <strong>clays</strong> having been removed by erosion show the underlying rock weathering through red. The above four observations contradict the suggestion that the black <strong>clays</strong> could have been derived from the underlying volc<strong>an</strong>ic rocks by weathering. The neighbouring areas further eastwards are underlain by metamorphic rocks, <strong>an</strong>d commonly exhibit a line <strong>of</strong> quartz stones between the black soils <strong>an</strong>d rock (Scott, 1963; Saggerson, 1991). However, no quartz veins have been observed within the black <strong>clays</strong>; <strong>an</strong>d this is contrary to what would be expected if the soils had actually developed in situ, suggesting once again that the black <strong>clays</strong> may have not derived from the weathering <strong>of</strong> the underlying rocks. In the present study area underlain by volc<strong>an</strong>ic rocks, pieces <strong>of</strong> quartz stones are found r<strong>an</strong>domly imbedded in the black <strong>clays</strong>. This could point to the possibility <strong>of</strong> the quartz stones <strong>an</strong>d other materials as having been tr<strong>an</strong>sported from the surrounding areas into the present area by erosive <strong>an</strong>d depositional processes. The general morphology <strong>of</strong> the area <strong>of</strong> occurrence <strong>of</strong> black <strong>clays</strong> in the present study area <strong>an</strong>d neighbouring areas further eastwards is roughly basin-like. The eastern <strong>an</strong>d western margins <strong>of</strong> this basin tend to follow the 5200 feet contour, while the northern margin is at <strong>an</strong> altitude <strong>of</strong> about 4800 feet. The altitude <strong>of</strong> the southern boundary <strong>of</strong> the soils is up to 6200 feet in the vicinity <strong>of</strong> Ngong hills. The soils are found occurring on both flat sites <strong>an</strong>d appreciable slopes. However, isolated patches <strong>of</strong> black <strong>clays</strong> occurring on high grounds are also found; <strong>an</strong>d are possibly part <strong>of</strong> the same earlier <strong>an</strong>d larger formation, which later underwent erosion leaving behind only relics <strong>of</strong> the soils. The properties <strong>an</strong>d characteristics <strong>of</strong> black <strong>clays</strong> discussed above serve to conclusively suggest that these soils are not wholly developed in situ. A more plausible expl<strong>an</strong>ation as to the genetic origin <strong>of</strong> the black <strong>clays</strong> would be based on the exist<strong>an</strong>ce <strong>of</strong> a large basin in their present areas <strong>of</strong> occurr<strong>an</strong>ce, prior to the formation <strong>of</strong> the soils. The basin was bounded by the Machakos hills to the east, the Ngong hills to the south <strong>an</strong>d the high ground <strong>of</strong> the eastern fl<strong>an</strong>ks <strong>of</strong> the Rift Valley to the west. The northern margin <strong>of</strong> the basin was most probably associated with the Ith<strong>an</strong>ga-Kakuzi hills. The lowest point <strong>of</strong> the basin was most probably around Athi River Township. The basin was filled with water during a wet cycle, forming a lake. Materials <strong>of</strong> colluvium <strong>an</strong>d alluvium from the surrounding hills were later deposited into the lake. Portions <strong>of</strong> the basin within <strong>an</strong>d/ or in the vicinity <strong>of</strong> metamorphic rock areas were therefore characterised by more s<strong>an</strong>dier vari<strong>an</strong>ts <strong>of</strong> black <strong>clays</strong> th<strong>an</strong> areas <strong>of</strong> the basin within volc<strong>an</strong>ic rock areas <strong>an</strong>d/ or next to volc<strong>an</strong>ic hill masses. This is also a possible expl<strong>an</strong>ation for the presence <strong>of</strong> s<strong>an</strong>dier vari<strong>an</strong>ts <strong>of</strong> black <strong>clays</strong> in the current study area, which is underlain by volc<strong>an</strong>ic rocks. In addition, there is a high possibility <strong>of</strong> volc<strong>an</strong>ic ash having also been deposited into the lake during this period; <strong>an</strong>d this could account for the high presence <strong>of</strong> smectites in the black <strong>clays</strong>, as detected during mineralogical (X-ray diffraction) <strong>an</strong>alyses performed on the soils in this study (Table 5.9).
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AN ENGINEERING GEOLOGICAL CHARACTER
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i Contents Page Acknowledgements Su
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iii Chapter 8. Distribution of inde
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v 7.29 - 7.33 Correlation between s
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vii 136 139 7.13 Compressibility cl
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ix Acknowledgements I would like to
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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 61 and 62: 49 The red friable clays on the who
- Page 63 and 64: 51 Results of chemical analyses of
- Page 65: 53 Results of previous soil classif
- 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
- Page 109 and 110: 97 A standard classification of soi
- Page 111 and 112: 99 Table 7.6. Viscosity and density
- Page 113 and 114: 101 Table 7.7, continued. Results o
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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
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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
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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% =
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153 partly produced by effects of w
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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
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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
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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
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179 rapid dissipation of pore water
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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
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187 Red clays; measured/ calculated
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189 These two relationships could b
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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
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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
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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
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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
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Results of swelling tests on black
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Appendix D Distribution/ variation
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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