26 The Nairobi phonolite consists <strong>of</strong> a number <strong>of</strong> flows. This is evidenced by age determinations made on feldspars from the rock, <strong>an</strong>d which r<strong>an</strong>ge from 5,2 to 10,2 Ma, suggesting the possibility <strong>of</strong> more th<strong>an</strong> 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 <strong>of</strong> the rock unit to be 79 m (BH 2023) at Embakasi in the south –eastern part <strong>of</strong> the area; <strong>an</strong>d 65 m (BH C201) at Wilson Airport in the western part <strong>of</strong> the area. The base <strong>of</strong> the flow commonly consists <strong>of</strong> pumiceous <strong>an</strong>d dribbly lava overlying a slightly uneven surface <strong>of</strong> agglomeratic tuff (Saggerson, 1991). Borehole evidence <strong>an</strong>d flow directions recorded in the field suggest the lavas <strong>of</strong> Nairobi phonolite having derived from the Muguga area, to the west <strong>of</strong> Nairobi city <strong>an</strong>d present area. The lavas flowed in a south-easterly stream across Nairobi <strong>an</strong>d the present area, before consolidation. Maximum thicknesses <strong>of</strong> the lavas therefore occur at/ <strong>an</strong>d around the Jomo Kenyatta International Airport located in the eastern part <strong>of</strong> the study area. The lavas generally thin out north-westwards, as well as farther south-eastwards <strong>an</strong>d southwards up to Athi River township, located beyond <strong>an</strong>d outside the present area (Saggerson, 1991). Aerial photographs <strong>of</strong> the plain at Jomo Kenyatta International Airport reveal sub-circular patches with crudely linear arr<strong>an</strong>gements beneath the black cotton soil cover. These patches probably represent the areas <strong>of</strong> maximum weathering at intersections <strong>of</strong> master joint-pl<strong>an</strong>es (Scott, 1963). Parts <strong>of</strong> the Nairobi National Park between Bushy Vale <strong>an</strong>d Rocky Valley show the Nairobi phonolite separated from the underlying Mbagathi phonolitic trachytes by some thickness <strong>of</strong> a few feet <strong>of</strong> dark grey tuff, which belongs to the Athi Tuffs <strong>an</strong>d Lake Beds Series. Outcrops generally reveal minor differences in the Nairobi phonolites. They are characterised by a marked fissility <strong>an</strong>d platy texture developed locally at <strong>an</strong>d/ or near the surface, <strong>an</strong>d which is attributed to flow phenomena. They also reveal the vesicular character <strong>of</strong> the upper portion <strong>of</strong> a number <strong>of</strong> flows. However, the rocks are seldom amygdaloidal (Saggerson, 1991). Fresh h<strong>an</strong>d specimens <strong>of</strong> the rock are black or blue-grey to blue in colour. However, deep weathering has altered much <strong>of</strong> the upper portions <strong>of</strong> the rock located near the ground surface. The rock is also lacking in large phenocrysts, the phonolite having undoubtedly resulted from the rapid cooling <strong>of</strong> <strong>an</strong> already viscous lava. Weathering usually converts the Nairobi phonolites to pale brown <strong>an</strong>d reddish ferricrete deposits (murram), which are observed to overly the rocks in much <strong>of</strong> the study area. In other parts, however, especially in the eastern <strong>an</strong>d south-eastern sections <strong>of</strong> the present area, the lava surface has been autobrecciated to form a boulder-bed consisting <strong>of</strong> rounded <strong>an</strong>d <strong>an</strong>gular fragments <strong>of</strong> phonolite cemented in a light brown, weathered base <strong>of</strong> the same material (Saggerson, 1991). Thin sections <strong>of</strong> Nairobi phonolite exhibit long tabular phenocrysts <strong>of</strong> s<strong>an</strong>idine in a finegrained groundmass. The groundmass consists <strong>of</strong> patches <strong>of</strong> soda-amphibole which include cossyrite <strong>an</strong>d kataphorite, while bright green aegirine <strong>an</strong>d/ or aegirine-augite are found associated. Nepheline is found occurring as small micro-phenocrysts, or as crystals in the groundmass; larger crystals <strong>of</strong> nepheline commonly alter into yellowish zeolite, sodalite, natrolite, <strong>an</strong>alcime <strong>an</strong>d calcite. Strongly pleochroic biotite with magnetite inclusions, or<strong>an</strong>gebrown 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 <strong>of</strong> chemical <strong>an</strong>alyses <strong>of</strong> Nairobi phonolite, as well as other rocks for comparison purposes.
27 Table 3.2 Chemical <strong>an</strong>alyses <strong>of</strong> strongly alkaline lavas <strong>of</strong> Nairobi phonolite <strong>an</strong>d 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 <strong>of</strong> Ngong Bazaar (W. C. Smith, 1931). 157. Alkali basalt (bas<strong>an</strong>ite). Nyeri road, Aberdare r<strong>an</strong>ge (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 <strong>of</strong> the study area, <strong>an</strong>d eastwards to underly Nairobi city <strong>an</strong>d the present area. They also extend farther northwards, beyond the present area, to underly areas <strong>of</strong> Kiambu <strong>an</strong>d south Githunguri ( Figure 1.1). A stratigraphic relationship is revealed by a number <strong>of</strong> borehole records <strong>an</strong>d the m<strong>an</strong>y outcrops in stream courses found in the immediate vicinity <strong>of</strong> Nairobi city.The Nairobi trachytes are shown to underly tuffs <strong>an</strong>d a number <strong>of</strong> other trachytes, while they overly the Kiambu trachyte, Mbagathi phonolitic trachyte <strong>an</strong>d Nairobi phonolite (Saggerson, 1991). Outcrops <strong>of</strong> the rock are particularly observable in the south-western part <strong>of</strong> 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
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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