an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
an engineering geological characterisation of tropical clays - GBV
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overlooking the Nairobi National Park. Outcrops <strong>of</strong> individual flows are also represented by<br />
small steps in the escarpment immediately to the west <strong>of</strong>/ <strong>an</strong>d overlooking Nairobi city <strong>an</strong>d<br />
the present study area (Saggerson, 1991).<br />
Borehole records show penetrated rocks as generally occurring in a continous sequence <strong>of</strong><br />
lava flows, varying in thickness from 91 m around Nairobi city centre to 61 m at Ruaraka (to<br />
the north <strong>of</strong> present study area). Records also show the flows as having vesicular upper<br />
contact surfaces <strong>an</strong>d/ or to be separated from one <strong>an</strong>other by sediments or tuffs (BH 3001,<br />
Mbagathi). In addition, stratigraphic information obtained from boreholes <strong>an</strong>d exposures in<br />
quarries in the Nairobi municipality <strong>an</strong>d the present study area, show the Nairobi trachyte<br />
separated from the underlying Nairobi phonolite by a few feet thickness <strong>of</strong> agglomeratic tuff<br />
(Saggerson, 1991).<br />
Table 3.3 Chemical <strong>an</strong>alyses <strong>of</strong> mildly alkaline lavas <strong>of</strong> Nairobi trachyte & other volc<strong>an</strong>ic rocks.<br />
170 151 203 95 W15 87 202<br />
SiO2 43,50 46,74 57,42 59,38 62,56 62,54 59,70<br />
Al2O3 18,65 18,88 17,92 10,84 14,96 15,85 19,20<br />
Fe2O3 1,70 3,11 7,50 3,92 3,25 3,66 8,00<br />
FeO 10,48 7,15 7,50 4,28 3,36 2,62 8,00<br />
MgO 7,42 3,16 1,17 0,89 0,36 0,36 0,30<br />
CaO 10,26 9,30 Tr 3,31 1,21 1,46 2,40<br />
Na2O 2,63 4,17 - 5,60 6,42 6,12 6,40<br />
K2O 0,94 1,67 - 5,01 5,64 5,07 2,80<br />
H2O+ 0,54 0,63 - 3,14 0,54 0,59 -<br />
H2O- 0,80 1,07 - 3,14 0,73 0,45 -<br />
TiO2 2,40 2,44 0,60 0,81 0,66 1,07 1,2<br />
P2O5 0,62 1,29 Nd 1,00 0,10 0,13 -<br />
MnO 0,12 0,18 - 0,35 0,19 - -<br />
CO2 - 0,16 3,42 1,30 Nil - -<br />
SO3 - - 0,15 0,04 0,00 0,08 -<br />
Cl - - - 0,01 0,04 0,04 -<br />
F - 0,11 - - - - -<br />
S - - - - - - -<br />
-O eqv - 0,05 - - 0,02 0,01 -<br />
Totals 100,06 100,01 - 99,88 100,00 100,03 100,00<br />
170. Alkali basalt, Rogati river, Kikuyu ( W. C. Smith, 1931).<br />
151. Olivine basalt, Kijabe Hill (Sh<strong>an</strong>d, 1937).<br />
203. Tuff, junction Ngong-Dagoretti roads, Impala Institute, Nairobi (W. C. Smith, 1931).<br />
95. Alkali trachyte (Plateau Trachyte Series), Magadi Hospital (Baker, 1958).<br />
202. Welded tuff/ Nairobi building stone (E.A.I.R.B).<br />
87. Trachyte phonolitique (soda-trachyte), Kikuyu escarpment (Lacroix, 1923).<br />
W15. Nairobi trachyte, Kenyatta Avenue, Nairobi (E.A.I.R.B).<br />
H<strong>an</strong>d specimens <strong>of</strong> Nairobi trachyte are greenish-grey in colour, <strong>an</strong>d occasionally porphyritic<br />
exhibiting tabular phenocrysts <strong>of</strong> feldspar in a predomin<strong>an</strong>tly fine-grained groundmass. Dark<br />
streaks, patches <strong>an</strong>d b<strong>an</strong>ds may also be found <strong>an</strong>d serve to indicate segregation <strong>of</strong> the mafic<br />
constituents. The more homogeneous varieties <strong>of</strong> the rock also show a trachytic texture<br />
characterised by a typical silvery lustre. Lamination <strong>an</strong>d b<strong>an</strong>ding are also characteristic <strong>an</strong>d<br />
are a result <strong>of</strong> flow <strong>an</strong>d flattening within the lava body. The flow is further emphasised by<br />
fluxionally arr<strong>an</strong>ged feldspar laths, as well as variations in rock composition <strong>an</strong>d colour