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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27<br />
Table 3.2 Chemical <strong>an</strong>alyses <strong>of</strong> strongly alkaline lavas <strong>of</strong> Nairobi phonolite<br />
<strong>an</strong>d other rocks.<br />
186 157 135 132 118 W64 112 99 108<br />
SiO2 38,80 45,65 49,62 51,59 54,81 56,56 57,05 57,94 59,14<br />
Al2O3 14,22 16,12 21,23 14,48 19,10 16,77 19,73 17,00 15,75<br />
Fe2O3 7,28 3,07 3,77 7,77 1,57 3,17 1,85 4,03 6,65<br />
FeO 6,34 9,08 2,02 - 3,52 2,74 4,31 1,94 1,35<br />
MgO 5,65 7,38 0,65 2,90 0,96 1,36 0,34 1,22 0,56<br />
CaO 12,35 9,70 3,40 4,53 1,81 1,18 1,56 0,99 1,42<br />
Na2O 5,00 2,74 9,00 - 6,88 7,53 7,54 7,63 7,23<br />
K2O 2,19 1,42 5,45 - 5,41 5,69 4,85 5,13 4,66<br />
H2O+ 1,80 0,33 1,81 3,75 3,63 1,70 0,54 1,15 1,78<br />
H2O- 1,40 0,68 1,75 3,75 0,57 1,55 0,68 0,94 0,25<br />
TiO2 4,30 3,30 0,83 0,72 0,82 1,07 0,82 1,08 0,50<br />
P2O5 0,72 0,43 0,11 - 0,22 0,13 0,23 0,18 0,09<br />
MnO 0,14 0,13 0,18 - 0,27 0,33 0,35 0,30 0,35<br />
CO2 - - - - 0,14 0,05 - 0,09 0,11<br />
SO3 - - - - - - Tr - -<br />
Cl - - 0,21 - 0,01 - - 0,13 0,02<br />
F - - - - 0,09 - - 0,11 -<br />
S - - Nil - 0,02 - - 0,02 -<br />
Loss on - - - 1,23 - - - - -<br />
ignition<br />
-O eqv - - 0,05 - 0,04 - - 0,08 0,00<br />
Total 100,10 100,03 99,88 88,97 99,79 99,83 99,85 99,80 99,86<br />
186. Nephelinite (augitite). West <strong>of</strong> Ngong Bazaar (W. C. Smith, 1931).<br />
157. Alkali basalt (bas<strong>an</strong>ite). Nyeri road, Aberdare r<strong>an</strong>ge (W. C. Smith, 1931).<br />
135. Phonolite. Ol Esakut (Tilley, 1956).<br />
132. Nairobi phonolite. Nairobi (Analysed by E.A.I.R.B.).<br />
118. Phonolite (Kapiti-type). Nairobi-Nyeri road, Nairobi (Bowen, 1937).<br />
W64. Nairobi phonolite. Nairobi (Analysed by H. LIoyd).<br />
112. Fayalite-bearing phonolite. Nyeri hill (W. C. Smith, 1931).<br />
99. Phonolitic trachyte. Athi plains (Bowen, 1937).<br />
108. Phonolite (Kenya-type). North slope, Suswa (McCall, 1964).<br />
3.3.3 Nairobi trachytes<br />
The Nairobi trachytes exhibit a wide distribution, extending from the Dagoretti – Karen area<br />
located to the west <strong>of</strong> the study area, <strong>an</strong>d eastwards to underly Nairobi city <strong>an</strong>d the present<br />
area. They also extend farther northwards, beyond the present area, to underly areas <strong>of</strong><br />
Kiambu <strong>an</strong>d south Githunguri ( Figure 1.1).<br />
A stratigraphic relationship is revealed by a number <strong>of</strong> borehole records <strong>an</strong>d the m<strong>an</strong>y<br />
outcrops in stream courses found in the immediate vicinity <strong>of</strong> Nairobi city.The Nairobi<br />
trachytes are shown to underly tuffs <strong>an</strong>d a number <strong>of</strong> other trachytes, while they overly the<br />
Kiambu trachyte, Mbagathi phonolitic trachyte <strong>an</strong>d Nairobi phonolite (Saggerson, 1991).<br />
Outcrops <strong>of</strong> the rock are particularly observable in the south-western part <strong>of</strong> the present area,<br />
where the trachytes form a flat-topped plateau terminating in a low but prominent escarpment