an engineering geological characterisation of tropical clays - GBV

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206 where P (%) = (P kPa/SP kPa) * 100 S (%) = (S mm/Ho mm) * 100 Unlike the first relationship, this last relationship could be used for estimation of both external loading conditions necessary to meet an allowable percentage swelling and vice versa, as long as the swelling pressure of the clays is known. It would therefore be a preferred relationship in estimating and/ or characterising swelling characteristics of clays.
207 Chapter 12 Recommendations Analysis of short term stability problems during and/ or immediately after construction involve use of the undrained shear strength or apparent cohesion, cu, in the relevant computations (Terzaghi and Peck, 1967). The corresponding angle of shear resistance, φ, is especially useful in the derivation of the necessary earth pressure coefficients and/ or bearing capacity coefficients. It would be more helpful therefore, that future works involving soils of the area include laboratory triaxial tests in their studies with the purpose of providing the necessary undrained shear strength parameters, cu, φ. The triaxial testing procedure is generally more satisfactory in measuring the shear strength of clay soils in terms of total stresses, through quick undrained shear tests (Head, 1988). Short-term stability analyses may include computations related to bearing capacity of footings and foundations for structures on saturated homogeneous clays, earth pressure on retaining walls, earth pressure against bracing in temporary excavations, safeguard against heave of the bottom of temporary open excavations in clays, stability of side-slopes of cuttings, as well as short term stability of embankments and earth dams. The black clays in some parts of the study area were found to contain gravelly lateritic materials of up to 40 mm sizes. Investigation and determination of shear strength characteristics of such materials by triaxial testing would be impracticable; while use of the standard shearbox apparatus as employed in this study, would only produce unreliable results. Future investigations and studies regarding shear strength of these significantly coarse gravelly clays should therefore employ the large shearbox apparatus such as the one described by Head (1988). This apparatus could provide more reliable shear strength parameters by executing slow drained shear tests on large representative and undisturbed block samples of the clays measuring 305 mm square and 150 mm thick. The results of shear strength parameters so obtained would be useful at the design stages of a number of structures such as embankments and earth dams which usually incorporate gravel fill material (Pike, 1973; Pike, Acott and Leech, 1977). The results would also serve as a basis for discriminating, subdividing and classifying gravelly soil materials to suit different construction needs and works (Bishop, 1948; Pike, 1973). The red soils were found in this study to be generally more permeable and relatively free draining. They would therefore exhibit rapid drainage and settlement when externally loaded. The resulting compression/ settlement curves tend to deviate from conventional shapes derived from one-dimensional theory of consolidation so that determination of relevant consolidation parameters would also be difficult. Use of a Rowe consolidation cell (Rowe,1966), capable of accommodating larger soil specimens would be recommended for oedometer tests on the clayey silt and silty varieties of red soils, so as to obtain a more definite value of coefficient of consolidation, cv, for use in the more reliable estimation of rates of settlement under various loading conditions. Alternatively, cv could be derived empirically from the relation cv = K/(mv * 0,31*E-9) m²/year where K (m/s) = coefficient of permeability as measured in situ in the field mv (MN/m²) = coefficient of volume compressibility derived from laboratory consolidaton test using the Rowe cell
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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
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29 caused by partial segregation of
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32 could otherwise lead to erroneou
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34 The red soils in this study occu
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36 17 16 15 14 13 12 11 10 9 8 7 6
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38 4.4 Vane test 4.4.1 Introduction
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40 Table 4.1 Classification of soft
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42 The variation of vane shear stre
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44 And thirdly, the field vane appa
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46 horizon, most probably a result
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49 The red friable clays on the who
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51 Results of chemical analyses of
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53 Results of previous soil classif
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55 neighbouring metamorphic areas a
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57 Table 5.10. Profile description
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59 The presence of BaO in the red a
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61 resulted most probably from supp
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63 800 Impulse 700 600 500 SC 17 -5
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65 sericite and chlorite (see next
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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
Page 167 and 168: 155 Chapter 8 Distribution of index
Page 169 and 170: 157 the study area, respectively. R
Page 171 and 172: 159 Liquid limit variation; 0,50m a
Page 173 and 174: 161 Liquid limit (LL) variation (>
Page 175 and 176: 163 (8.6). The few isolated patches
Page 177 and 178: 165 Similarly, soil thicknesses of
Page 179 and 180: 167 Free swell variation; 0,50m dep
Page 181 and 182: 169 fraction at the two depth inter
Page 183 and 184: 171 Variation of fines (%), < 0,50m
Page 185 and 186: 173 %coarse % coarse fraction varia
Page 187 and 188: 175 1°19´S 22 Shear angle variati
Page 189 and 190: 177 9.2 Grain size distribution The
Page 191 and 192: 179 rapid dissipation of pore water
Page 193 and 194: Chapter 10 181 Correlation of index
Page 195 and 196: 183 Black clays; plasticity index/
Page 197 and 198: 185 On the other hand, laboratory m
Page 199 and 200: 187 Red clays; measured/ calculated
Page 201 and 202: 189 These two relationships could b
Page 203 and 204: 191 Diagram: PImeasured/ PIcalculat
Page 205 and 206: 193 Table 10.5, continued. Calculat
Page 207 and 208: 195 Table 10.6. Calculated and labo
Page 209 and 210: 197 The swelling capability in term
Page 211 and 212: 199 Free swell/ clay fraction Free
Page 213 and 214: 201 There has also been a decrease
Page 215 and 216: 203 PI = 1,88*LS A comparison of pl
Page 217: 205 cc = 0,0099(122-LL) for black c
Page 221 and 222: 209 cc = 0,0099(122-LL) for black c
Page 223 and 224: 211 References Abebe, S. T., 2002.
Page 225 and 226: 213 Galster, R.W., 1977. A system o
Page 227 and 228: 215 Mitchell, J.K., 1993. Fundament
Page 229 and 230: 217 ------,1964. Long term stabilit
Page 231 and 232: Appendix A: Oedometer consolidation
Page 233 and 234: Table A7. Consolidation parameters
Page 235 and 236: Results of swelling tests on black
Page 237 and 238: Appendix D Distribution/ variation
Page 239 and 240: 4000 3000 2000 Distance (m) 1000 We
Page 241 and 242: 4000 3000 2000 1000 Distance (m) We
Page 259 and 260: Appendix E Geotechnical soil map of
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