an engineering geological characterisation of tropical clays - GBV

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114 The combined influence of relative consistency and plasticity indices on the shear angles and shear strength of clays is illustrated in Fig. (7.9a). A fair or moderate correlation (R = 0,54) showing a general increase in shear angles and / or shear strength with an increased ratio of Cr/PI is observed. There is also an improvement in correlation over that based on plasticity index (Figs. 7.7a & 7.8a) alone. Black clays Angle of shear resistance φ´ (°) 35 n = 41 30 25 20 R 2 = 0,29 15 10 0,015 0,025 0,035 0,045 Ratio Cr/PI Shear angle vs Cr/PI Figure 7.9a. Correlation between angle of shear resistance (φ´ ) and ratio Cr/PI for black clays. Black clays Cohesion c´ (kN/m²) 60,00 50,00 n = 41 40,00 30,00 20,00 10,00 0,00 0,016 0,021 0,026 0,031 0,036 Cr/PI Cohesion vs Cr/PI Figure 7.9b. Correlation between cohesion (c´ ) and ratio Cr/PI for black clays. On the other hand, correlations between values of cohesion and those of index parameters (natural moisture content, relative consistency, Atterberg limits) are generally weak, with the resulting plots giving more spread out data points (Figures 7.6b & c, 7.7b, 7.8b, 7.9b). All in all, however, the strengths of correlations between cohesion and shear angles on one hand and relative consistency and plasticity indices on the other are generally poor and, at their best, only moderate. This serves to point to the uncertainities accompanying assessment and characterisation of shear strengths of clays based on results of index tests performed on disturbed and fractioned / sieved soil samples.
115 7.4 Oedometer consolidation tests 7.4.1 Scope Oedometer consolidation tests were carried out in this study to investigate and establish consolidation-settlement characteristics of normally consolidated as well as overconsolidated clay soils found, in terms of estimating the amount and rate of settlement when externally loaded. The tests were carried out on saturated clays, and were also aimed at determining and establishing percentage swelling and swelling pressures of the clay soils. Determined parameters of coefficient of volume compressibility and coefficient of consolidation were used to describe the consolidation-settlement characteristics of soils. The coefficient of volume compressibility (i.e. modulus of volume change) was used to indicate the compressibility of soils in terms of the amount by which they would compress when loaded and allowed to consolidate. The coefficient of consolidation is a time related parameter (Head, 1984), and served to indicate the rate of compression and hence the time period over which consolidation -settlement would take place. Results of the tests and parameters derived thereof would be usefully applied in foundation design in limiting settlements to within tolerable limits when structural foundation loads are imposed on the soils. In addition, the rate of settlements and the time within which such settlements could be virtually complete would be estimated. The theory of consolidation (Terzaghi, 1925; Terzaghi and Peck, 1948; Taylor, 1948) is based on the fact that, consolidation process of soils subjected to external loading mainly entails escape of water from the voids between the skeleton of solid grains. The loss of pore water is generally rapid in free-draining materials, but is limited in soils of impeded drainage. As a result, settlements in high permeability materials of sands and gravels generally take place in a short time, usually as construction work proceeds, thereby causing no major problems. On the other hand, low permeability clays could undergo settlements over much longer periods of time after completion of construction, thereby subjecting constructed structures to a prolonged state of instability (Nelson and Miller, 1992; Johnson and DeGraff, 1988; Head, 1984). In this study, the amounts and rates of consolidation settlement of soils found under various loading conditions have been studied, analysed and established. The spatial distribution and variation of consolidation parameters across the study area have also been investigated and established. 7.4.2 Theory of consolidation According to Terzaghi (1926), saturated clays which are externally loaded have the total applied stress, σ, initially supported by the excess pore water pressure, µ, which is induced. As water drains out of the soil, the pore pressure correspondingly falls so that an increasing proportion of the applied load is transferred to the grains forming the soil skeleton. The consolidation process therefore involves gradual transfer of stress from pore water to soil skeleton. The degree of consolidation, U, is a measure of the extent of this stress transfer at any instant during consolidation; and is given by the equation U (%) = ((µo – µ)/ µo) * 100 (7.21) where µo = initial excess pore pressure
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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
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
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: 113 Black clays Cohesion c´ (kN/m
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
Page 143 and 144: 131 is most probably due to the ten
Page 145 and 146: 133 The results of correlation show
Page 147 and 148: 135 by differences in lithology, mi
Page 149 and 150: 137 Black clays and red soils Swell
Page 151 and 152: 139 Testing procedure involved cutt
Page 153 and 154: 141 Black clays Swelling pressure S
Page 155 and 156: 143 Black clays: P (kPa) vs S (%) P
Page 157 and 158: 145 The same relationship is repres
Page 159 and 160: 147 Alternatively, percentage swell
Page 161 and 162: 149 Combined sample results: S % vs
Page 163 and 164: 151 Black clays: Greek method P% =
Page 165 and 166: 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
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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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