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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20<br />
River). The dating also involved rock specimens from Nairobi metropolit<strong>an</strong> <strong>an</strong>d present study<br />
areas, <strong>an</strong>d included Nairobi trachyte (3,17-3,45 Ma), Nairobi stone (4,84-5,67 Ma), Nairobi<br />
phonolite (5,2-10,22 Ma) <strong>an</strong>d Kapiti phonolite (12,90-13,40 Ma). There is also the likelihood<br />
that volc<strong>an</strong>ic activity continued into Recent times in areas to the west <strong>of</strong> Nairobi. This is<br />
evidenced by the occurrence <strong>of</strong> weathered tuffs <strong>an</strong>d ash layers within soil pr<strong>of</strong>iles,<br />
representing the eruptive periods <strong>of</strong> the later times (Saggerson, 1991).<br />
A summarised discussion <strong>of</strong> the major rock types underlying the present study area is given in<br />
the following sections.<br />
3.2 Precambri<strong>an</strong> rocks <strong>of</strong> the Mozambique Belt<br />
The crystalline metamorphic rocks <strong>of</strong> Precambri<strong>an</strong> age outcrop in areas where the volc<strong>an</strong>ic<br />
cover has been removed by erosion. They are especially noticeable in the southern <strong>an</strong>d southwestern<br />
parts <strong>of</strong> the study area where they occur as isolated exposures in shallow stream<br />
courses, or as float on the stream edges. They are generally small highly weathered outcrops<br />
consisting mainly <strong>of</strong> layered fine-grained schists <strong>an</strong>d coarse gneisses that have been invaded<br />
in some places by pink quartzo-feldspathic pegmatites. Also found but to a lesser extent are<br />
biotite-garnet-epidote gneisses, hornblende gneisses <strong>an</strong>d quartz-feldspar gneisses.<br />
The structures revealed in the metamorphic rocks generally indicate north-south trending<br />
folds. However, the rocks also show some variations in strike <strong>an</strong>d dip due to generally smallscale<br />
folds. They also exhibit lineations which plunge at approximately 14° - 21° to the southwest;<br />
<strong>an</strong>d which are most probably related to the north-east to south-west fold trends that have<br />
been recognised in neighbouring areas (Saggerson et al, 1960). The north-south <strong>an</strong>d north-east<br />
to south-west fold trends are typical <strong>of</strong> much <strong>of</strong> southern Kenya, including the Nairobi area,<br />
<strong>an</strong>d serve to point to the fact that the underlying Precambri<strong>an</strong> rocks have been subjected to a<br />
minimum <strong>of</strong> two phases <strong>of</strong> deformation (Saggerson, 1991).<br />
In other areas, the metamorphic rocks are unexposed <strong>an</strong>d separated from the overlying<br />
volc<strong>an</strong>ic rocks by a pl<strong>an</strong>e <strong>of</strong> unconformity which represents <strong>an</strong> erosional surface formed in<br />
early Miocene. Evidences <strong>of</strong> the erosional surface occur in the form <strong>of</strong> thin layers <strong>of</strong><br />
weathered metamorphic materials separating the Precambri<strong>an</strong> <strong>an</strong>d volc<strong>an</strong>ic rocks, <strong>an</strong>d<br />
exposed in some <strong>of</strong> the streams in the southern part <strong>of</strong> the study area. However, cases <strong>of</strong> soil<br />
materials (<strong>clays</strong>, s<strong>an</strong>ds, detritus) directly overlying the Precambri<strong>an</strong> rocks also occur, <strong>an</strong>d is<br />
evidenced by a number <strong>of</strong> borehole records in the present area. According to Sikes (1939), the<br />
presence <strong>of</strong> a former topography across the study area is also revealed by borehole records,<br />
which show the crystalline rocks as occurring at different elevations above sea-level.<br />
Borehole records <strong>of</strong> the Nairobi Station (BH 1937c; 37° 00`E, 1° 24`S), for inst<strong>an</strong>ce, show the<br />
erosional surface to have been penetrated at <strong>an</strong> elevation <strong>of</strong> about 838 m above sea-level. In<br />
other places, the crystalline rocks were intercepted at elevations <strong>of</strong> 1519 m above sea-level<br />
(BH 2655) in the south-eastern part <strong>of</strong> the area; 91 m depth below ground surface at Wattle<br />
Blossom Farm, southern part <strong>of</strong> the area; <strong>an</strong>d 1509 m above sea-level in the area around<br />
Nairobi National Park. Records also reveal the crystalline metamorphic rocks to have<br />
undergone cyclic penepl<strong>an</strong>ations in early Tertiary. Evidence <strong>of</strong> earlier sedimentation is,<br />
however, absent. The latest metamorphic events to affect the rocks are dated at 480-530 Ma,<br />
based on the age <strong>of</strong> Phlogopite in crystalline limestone from Turoka, in southern Nairobi area<br />
(Saggerson, 1991).<br />
Early studies on soil samples collected from around Embakasi in the south-eastern part <strong>of</strong> the<br />
present area (Saggerson et al, 1968; Stephen et al, 1956) showed them to contain typical