an engineering geological characterisation of tropical clays - GBV

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24 The following section includes a summary of the major volcanic rocks underlying the present study area. 3.3.1 Kapiti phonolite Borehole records from Nairobi city and the current study area show the Kapiti phonolites as underlying associated volcanic rocks. They are also observed to overly a sub-volcanic floor of erosion surface previously cut into Precambriam metamorphic rocks. These rocks therefore represent the oldest unit of a succession of lavas present in the study area (Gregory, 1921). Exposures of Kapiti phonolite are observable in the south-eastern part of the study area, generally in the form of small outcrops confined along some of the stream courses found; and in the southern part of the area in the tributary valleys of the Athi river. Farther south, outside the present area, small outcrops also occur in the valley of the Athi river and in the tributary valleys of Kitengela river; while extensive outcrops are in the valley of the Stony Athi River, which forms the north-western border of the Kapiti plains. The rock unit descends with a slope nearly identical to that of the sub-volcanic floor, from 1585 m above sea-level in the south-eastern part, to1280 m above sea-level in the northern part of the study area and the Nairobi City centre (Saggerson, 1991). The lava originated from an area to the south of the present study area and was most probably derived from vents located on the flanks of the Rift Valley and/ or from the site of the former Olorgasailie central volcano (Baker, 1958). It was a part of the first Miocene flood eruptions and flowed generally northwards onto the eroded surface, which was covered in places by Tertiary conglomerates and grits (Fairburn, 1963). The Kapiti lava also extends westwards up to Lion Drift in the Nairobi National Park by thinning against the higher ground. The Mua hills and the Kanzalu range located to the east of the study area acted as a barrier to the northerly lava flow, and instead, encouraged a north-westerly flow across the area. A further contribution originated from areas to the north of the study area, where the flows had also given rise to the formation of the Yatta plateau phonolite. Borehole records from Nairobi metropolitan area, the study area and around Athi River township (south of the study area) show the presence of sedimentary strata in the form of clays and other sediments occurring intercalated between and separating several lava flows (Gevaerts, 1964). Exposures show parts of the phonolite to be vesicular in nature, commonly giving rise to formation of small patches of amygdales filled with calcite and zeolite (mainly natrolite). The elongated feldspar phenocrysts ( up to 76 mm long) and vesicles frequently display a preferred orientation (Gregory, 1921; Saggerson, 1991). Hand specimens of Kapiti phonolite are distinct, and show large crystals of feldspar and waxy-looking nephelines occurring as phenocrysts set in a fine-grained dark green to black or dark bluish-grey groundmass. The presence of phenocrysts of these two minerals is diagnostic, and serves to identify and distinguish the Kapiti phonolite from other lavas across Kenya (Gregory, 1921). Thin sections of phonolite show phenocrysts of anorthoclase and nepheline set in an interstitial groundmass (Häkli, 1960). Serpentinised pseudomorphs and patches of chloritic material found in the groundmass are probably secondary after olivine, while opaque iron ore
25 and prismatic apatite occur as accessories (Saggerson, 1991). Composition of the groundmass is also summarised in Table (3.4). Table 3.1 Chemical analyses of alkali-feldspars from the Kapiti phonolite and similar rocks. 1 2 3 4 SiO2 65,61 62,79 64,33 64,80 Al2O3 20,48 22,12 20,94 21,14 Fe2O3 0,20 0,36 0,2 0,68 FeO 0,20 0,41 0,58 - MgO 0,00 - - 0,10 CaO 7,03 3,76 2,01 1,18 Na2O 6,26 7,35 7,22 6,94 K2O 0,32 2,98 4,71 5,26 H2O+ 0,03 0,19 0,27 0,20 H2O- 0,00 0,07 0,10 0,20 Totals 99,93 100,03 100,36 100,00 1. Alkali-feldspar phenocryst (anorthoclase). From Kapiti phonolite, Stony Athi River (Häkli, 1960). 2. Loose crystals of anorthoclase (potash-oligoclase). Crater of Mount Erebus, Antarctica (Mountain, 1925). 3. Anorthoclase. Crater slopes, Mount Kenya (Mountain, 1925). 4. Anorthoclase phenocrysts. From Kenyte, Mount Kenya (Spencer, 1937). The thin section descriptions and results of chemical analysis of Kapiti phonolite from the study area and its environs, has been shown by various authors to be similar (Baker, 1954; Bowen, 1937; Smith Campbell,1950; Saggerson, 1991; Prior, 1903). The results of chemical analyses are summarised in Table 3.1, which also shows a comparison of the composition of alkali-feldspar phenocrysts from the Kapiti phonolite; with those of anorthoclase crystals from Mount Kenya and Antarctica. From the table, specimens from the Kapiti phonolite are found to show a marked rise in the ratio of alkalis, Na2O/K2O, with increase in lime (CaO) content. Specimens from Mount Kenya and Antarctica, however, exhibit a steady increase in CaO with increasing Na2O/K2O ratio. 3.3.2 Nairobi phonolite The Nairobi phonolite covers a large part of the Athi plains. It extends from Nairobi National Park located in the western and south-western parts of the study area, and northwards across the Nairobi city centre to Kiambu township, located north of /and outside the present study area. It overlies the Athi Series with a probable disconformity. Outcrops of the phonolite are observable in the southern and south-western parts of the study area in a prominent scarp, which also shows the contact between the rock and the underlying Athi Series (Saggerson, 1991).
Page 1 and 2: 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 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 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
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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
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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
Page 115 and 116:
103
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
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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
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 and 218:
205 cc = 0,0099(122-LL) for black c
Page 219 and 220:
207 Chapter 12 Recommendations Anal
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 243 and 244:
4000 3000 2000 Distance (m) 1000 We
Page 245 and 246:
4000 3000 2000 Distance (m) 1000 We
Page 247 and 248:
4000 3000 2000 Distance (m) 1000 We
Page 249 and 250:
4000 3000 2000 Distance (m) 1000 We
Page 251 and 252:
4000 3000 2000 Distance (m) 1000 We
Page 253 and 254:
4000 3000 2000 Distance (m) 1000 We
Page 255 and 256:
4000 3000 2000 Distance (m) 1000 We
Page 257 and 258:
4000 3000 2000 Distance (m) 1000 We
Page 259 and 260:
Appendix E Geotechnical soil map of
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