an engineering geological characterisation of tropical clays - GBV

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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
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: 17 Table 2.1 Stratigraphic correlat
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 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
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69 Plate 6.3. K-feldspar phenocryst
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71 The trachytes generally show a r
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73 Plate 6.16. Organic matter (rema
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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
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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 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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