an engineering geological characterisation of tropical clays - GBV

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80 Table 6.6. Carbon content of soils of the study area. Sample No. Total carbon; Ctotal (%) Organic carbon; Corganic (%) Inorganic carbon; Cdiff. (%) SA1-30cm 1,16 1,04 0,12 SA1-50cm 1,48 0,81 0,67 SA2-70cm 0,50 0,46 0,04 Black clays SA2-105cm 0,96 0,22 0,74 SA41-50cm 1,14 1,02 0,12 SB1-50cm 1,13 1,00 0,13 SB1-70cm 1,19 1,08 0,11 SB42-30cm 1,34 1,19 0,15 SB42-50cm 1,30 1,17 0,13 SC17-50cm 1,29 1,08 0,21 SC33-30cm 1,39 1,19 0,20 Rd1-30cm 2,47 1,90 0,57 Red soils Rd1-100cm 0,73 0,54 0,19 Rd1-200cm 0,83 0,61 0,22 Rd1-400cm 0,45 0,35 0,10 6.6.3 Analysis of results The variation of carbon content of soils with depth, Dp, is represented diagrammatically in Figures 6.11, 6.12 and 6.13. Red soils: carbon content/ depth 3,00 carbon (%) 2,50 2,00 1,50 1,00 0,50 Ctotal = 1,0356Dp -0,6115 R 2 = 0,8831 Corg = 0,7821Dp -0,6116 R 2 = 0,8766 total carbon organic carbon inorganic carbon Potential (total carbon) Potential (organic carbon) Potential (inorganic carbon) Cdiff = 0,2526Dp -0,6128 0,00 R 2 = 0,8871 0,00 1,00 2,00 3,00 4,00 5,00 depth Dp (m) Figure 6.11. Variation of carbon content of red soils with depth.
81 The red soils generally exhibit a decrease in their carbon contents with depth (Fig. 6.11). The decrease of the organic carbon is indicative of a general decrease of the organic matter content of the soils with depth. The decrease of the inorganic carbon could be related to the good drainage conditions of the soils which favour leaching and removal of carbonates as well as other soluble components of Mg, Ca, Na and K. The variation of total carbon, organic carbon and inorganic carbon contents of these soils with depth is best described by a potential relationship with strong correlations (R = 0,94; R = 0,94; and R=0,94, respectively). Generally, however, the soils exhibit higher contents of organic carbon than inorganic carbon. The total carbon and organic carbon contents of black clays generally decrease with depth (Fig. 6.12); and this is most probably due to the decrease of organic matter content of the soils with depth. On the contrary, the inorganic carbon component is found to generally increase with depth, and this could be attributed to the weathering of the underlying volcanic tuffs which usually results in the formation of secondary limestone in situ. The variation of total carbon with depth could be described by a polynomial relationship, but with a relatively weaker correlation (R = 0,56); while the variation of organic carbon with depth is best approximated by an exponetial relationship with a strong correlation (R = 0,85). On the other hand, variation of inorganic carbon content with depth tends to fit a polynomial relationship, with a moderately strong correlation (R = 0,63). Black clays: carbon content/ depth 1,60 1,40 1,20 Ctotal = 0,5235Dp 2 - 1,3301Dp + 1,7069 total carbon carbon (%) 1,00 0,80 0,60 0,40 R 2 = 0,3146 Corg = 2,4918e -2,0216Dp R 2 = 0,7303 CDiff = 1,5224Dp 2 - 1,4352Dp + 0,5028 organic carbon inorganic carbon Polynomial (total carbon) Exponential (organic carbon) Polynomial (inorganic carbon) 0,20 R 2 = 0,3943 0,00 0,00 0,20 0,40 0,60 0,80 1,00 1,20 depth Dp (m) Figure 6.12. Variation of carbon content of black clays with depth. Combined results of black clays and red soils also show a general decrease of total carbon as well as organic carbon with depth (Fig. 6.13). The results also reveal approximating potential relationships with strong correlations in the variation of total carbon (R = 0,77) and organic carbon (R = 0,75) with depth. However, no well defined relationship occurs to describe the variation of combined data of inorganic carbon of the two types of soil with depth.
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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
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Figure1.1 Map of Nairobi region sho
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5 Plates 1.2 (a) & (b) Strong shrin
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7 Available literature in the form
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9 slopes. The northern boundary bet
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11 1.6 Climate The Nairobi area and
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13 marked daily range of relative h
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15 Chapter 2 Previous works 2.1 Sum
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17 Table 2.1 Stratigraphic correlat
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19 Chapter 3 Geology 3.1 Introducti
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21 metamorphic minerals sillimanite
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25 and prismatic apatite occur as a
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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
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: 79 The CS-225 is a micro-processor
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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Page 117 and 118: 105 Plate 7.7a. Shear testing showi
Page 119 and 120: 107 Shear stress / Displacement Cur
Page 121 and 122: 109 Table 7.10. Distribution and/ o
Page 123 and 124: 111 illustrated in Figures 7.6, 7.7
Page 125 and 126: 113 Black clays Cohesion c´ (kN/m
Page 127 and 128: 115 7.4 Oedometer consolidation tes
Page 129 and 130: 117 The compound coefficient, K/ρw
Page 131 and 132: 119 Primary consolidation is a time
Page 133 and 134: 121 where w (%) = moisture content
Page 135 and 136: 123 Relationships between values of
Page 137 and 138: 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
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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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