an engineering geological characterisation of tropical clays - GBV

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90 Plasticity Index (PI) 0,5 0,4 0,3 0,2 0,1 0,0 Soft Sand Low plasticity (L) Clays CL ML Mediu m plastici y High plasticity (H) CH CI Silt MI MH BS A-line: PI = 0,73(LL-20) 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 Liquid Limit (LL) Very high plasticity (V) CV MV Organic Silt ME Black clays Red soils Figure 7.1. A plasticity chart classification of black clays and red soils of this study. On the standard plasticity chart (BS 1377:1975), the A-line has been defined by the relationship PI = 0,73(LL-20) (7.6) based on experience with British soils (Fig. 7.1). It was originally drawn from experimental data and serves as a tentative boundary between clayey and silty varieties of cohesive soils which generally plot above and below it, respectively. For the black clays and red soils of the present study area, however, the plasticity index is related to liquid limit through a new relationship derived in this study, i.e., PI = 0,79(LL-25) (7.7) Classification of black clays and red soils based on their activity (after Skempton, 1953; Mitchell, 1976) is presented in Table (7.3). The black clays fall in the class of normal active to highly active clays, their relatively higher activity being attributed to the higher content of smectites (montmorillonites). According to Holtz and Kovacs (1981), montmorillonite has the highest activity of clay minerals and would often plot above the A-line of the plasticity chart. The red soils are classified as essentially normal active clays, and this most probably due to the predominant presence of kaolinite. Kaolinite is a low activity clay mineral and would often plot below the A-line (Holtz and Kovacs, 1981). The activity state/ level and classification of black clays and red soils is also summarised diagrammatically in Fig. (7.2). From the activity chart, the black clays are found to harbour medium to very high activity levels (i.e. normal to highly active) while the red soils occur in a medium activity state ( i.e. normal active).
91 Activity chart Plasticity index PI (%) of whole sample 100 50 0 LOW Activity = 2,0 HIGH MEDIUM VERY HIGH Activity = 1,0 Activity= 0,5 0 50 100 Black clays Red soils % clay in whole sample Figure 7.2. Activity level classification of black clays and red soils in this study (After, NAVFAC, 1982). Results of Atterberg limits and /or activity values have also been used to asses the probable type of clay minerals present in the soils, based on the classification by Skempton (1953) and Day (2001), i.e. Table (7.1). It is therefore anticipated that black clays which exhibit activity values of 1,10–3,38 (mean: 1,87) are mainly smectites (Ca-montmorillonites mainly); while the red soils with activity values of 0,83-1,03 (mean: 0,94) would be predominated by kaolinite and / or illite (Table 7.3). This prediction has been confirmed by actual clay mineralogical studies on the soils which showed the black clays to be over 90% smectites in composition; while the red soils are mainly kaolinite (80% and over). In addition, results of Atterberg limits obtained during first stage tests served to aid in the selection of samples for chemical/ mineralogical analyses as well as more detailed soil studies involving shear strength and consolidation-settlement tests. 7.1.4 Free swell test (after Gibbs & Holtz, 1956) 7.1.4.1 Scope This test was carried out to determine the increase in volume of clay soils from a loose dry powder-form condition when poured into water, expressed as a percentage of the original volume. According to Gibbs and Holtz (1956), the amount of free swell serves to be indicative of the probable swelling and/ or expansive behaviour of clay soils, especially when wetted from relatively dry conditions under light structural loads. A classification of soils based on free swell is summarised in Table (7.4).
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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
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 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: 89 Plate 7.2a. Apparatus for liquid
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
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
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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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