an engineering geological characterisation of tropical clays - GBV

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50 The variation in the content of total soluble bases in red soils is presented in Table 5.2 (sample Rd1), alongside that for black clays for comparison purposes. The amount of total soluble base was also observed to decrease with increasing depth of the red soils. This could be explained by the good drainage conditions of the soils which serve to facilitate removal of the basic substances in solution during rainfall. Further eastwards across the study area, the red soils pass into a composite soil group consisting of shallow yellow-brown friable clays overlying a laterite horizon; and shallow yellow-red friable clays on rock. This is because of the difficulty in distinguishing between and separating these two soil units on aerial photographs. However, the two soil components are genetically different. These soils commonly exhibit a brown (Sample No. SC17-30cm) relatively low humic (up to 1,5% carbon) topsoil overlying a yellow-brown (Sample No. SC17-50cm) subangular blocky friable clay with iron concretions passing downwards into massive laterite (ferricrete). In other places, the soils consist of a brown low humic (0,5-1,5% carbon) topsoil underlain by a yellow-red subangular blocky friable clay passing into rock. The soils are usually characterised by a slight seasonal impeded drainage. They also support a scrub grass vegetation which appears as a light tone on aerial photographs. The shallow soils over rock are generally redder than their lateritic counterparts. They are found occurring mainly on the flat lands adjoining and/ or at the edges of the black clays in the plains. They are apparently youthful soils which have developed most probably after the removal of the black clays by erosive processes. The shallow soils over laterite occur mainly on the lower slopes of ridges. According to Scott (1963), they are most probably a result of seepage water from higher ground whose downslope flow is checked by the change of slope and poorer drainage conditions at the foot of the slope, causing the deposition of iron and aluminium compounds from the seepage waters to form a laterite sheet. The development of the laterite sheet also serves to increase the drainage impedence , thereby favouring further development and formation of the shallow lateritic soils which extend upslope. Table 5.3. Profile description of a shallow yellow-brown clay overlying laterite. Depth (m) Sample No. Description Silt Clay C (%) (%) (%) 0,0-0,30 SC17-30cm Brown slightly humic, crumbly friable clay with occasional rounded murram concretion 59 29 1,54 0,30-0,50 SC17-50cm Yellow brown subangular blocky friable clay with occasional rounded murram concretion >0,50 - Mainly murram concretion passing into massive murram 59 27 1,29 - - - A typical yellow-brown friable clay overlying laterite in the current study area is described in Table 5.3. The amount of total carbon is observed to decrease with increasing depth of soils, and this could as well serve to reflect possible decrease of organic matter content of the soils with depth. However, the soils exhibit a slight increase in the amount of total soluble base with increased depths (Table 5.2, Sample SC17), and this is most probably due to the slightly impeded drainage character of the soils which serves to limit and/ or inhibit removal of the basic substances in solution.
51 Results of chemical analyses of the red soils performed in the current study (Table 5.4) show them as lacking and deficient in magnesia (MgO: 0,097-0,14 %). Chemical analyses of the rocks from which these soils are derived also indicate their relatively very low magnesia content (MgO: 0,30-0,89 %), compared with the basic lavas and other volcanic rocks of the Nairobi region (Table 3.4). Farmers in the coffee-growing estates found in the areas of red soils tackle this problem through frequent artificial application of magnesite-rich fertilisers. The red soils generally exhibit lower contents of SiO2, MgO, CaO, Na2O and K2O than the volcanic materials from which they are derived (Tables 3.3 and 5.4). This could also be a result of the good drainage conditions of the soils as well as the high rainfall which favoured leaching and removal of the soluble bases and silica, thereby leaving the soils relatively enriched in Fe2O3 and Al2O3 minerals. Table 5.4. Results of chemical analyses of red soils obtained in this study. % content of: Rd1-30cm Rd1-100cm Rd1-200cm Rd1-400cm SiO2 47,50 47,10 47,20 47,6 Al2O3 32,20 33,40 33,20 33,30 Fe2O3 15,30 15,50 15,50 15,20 FeO - - - - BaO 0,062 0,061 0,071 0,064 MgO 0,12 0,097 0,14 0,13 CaO 0,26 0,24 0,16 0,26 Na2O ‹ ‹ ‹ ‹ K2O 0,96 0,66 0,68 0,60 P2O5 0,19 0,086 0,096 0,062 ZrO2 0,37 0,37 0,37 0,36 TiO2 1,57 1,51 1,51 1,46 MnO 0,96 0,53 0,61 0,52 Loss on ignition - - - - Total 99,49 99,55 99,54 99,56 Rd1-30cm, Rd1-100cm, Rd1-200cm, Rd1-400cm. Samples of red soils collected from Arboretum (Nairobi), at depths of 30cm, 100cm, 200cm and 400cm, respectively. ‹: Concentration less than 0,04 % (400 ppm). Similar results of previous chemical analyses of a number of clays and red soils of Nairobi area are also given in Table 5.5, for comparison purposes. A relation of the results of Tables 5.4 and 5.5 reveals a depletion in the amounts of soluble bases (MgO, CaO, NaO) to have taken place with time, most probably due to effects of leaching in the red soils. Correspondingly, the proportionate amounts of Fe2O3 (free iron oxide, haematite, goethite) and Al2O3 (clay minerals) have increased with time. The more easily leached components from areas of red soils persist or accumulate on lower ground of depressions between ridges and/ or the flat plains, where they contribute to the formation of black soils.
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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 61: 49 The red friable clays on the who
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
Page 109 and 110: 97 A standard classification of soi
Page 111 and 112: 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
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
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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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