an engineering geological characterisation of tropical clays - GBV

53
Results of previous soil classification tests carried out on a number of clays and red soils in
the neighbourhood of the present study area are summarised in Table 5.7, for comparison
purposes.
It can be observed from the results of Tables 5.6 and 5.7 that the cementing effect of clay
particles by free iron oxide must have progressively reduced the proportion of free active clay
particles in the red soils through geological time. As a result, values of the Atterberg limits
(plastic and liquid limits, plasticity indices) of the soils have also decreased with time.
Table 5.7. Earlier results of soil classification tests on red clays from the Nairobi area (After
Sherwood, 1967).
LOCATION
SOIL
NO.
CLAY
CONTENT
(BS1377)
(%)
LIQUID
LIMIT
(%)
PLASTIC
LIMIT
(%)
PLASTICITY
INDEX
(%)
Kabete 816 82 76 39 37
Limuru 830 83 87 44 43
Nairobi-Limuru
road
639 88 87 48 39
5.2.2 Black to dark grey clays (black cotton soils)
The black to dark grey clays cover much of the present study area that forms a part of the Athi
and Kapiti plains. The soils are characterised by impeded drainage, and are underlain mainly
by the Nairobi and Kapiti phonolites which form a practically impermeable strata, favouring
further development and occurrence of the ill-drained soils. Contributions to the formation of
the black clays also comes from eroded kaolinite-bearing materials as well as leached
components of soluble bases and silica derived from areas of red soils occurring on the higher
grounds forming the Kikuyu highlands to the west and north-west of the present area. This is
evidenced by chemical/ mineralogical results from current studies, which show the black
clays as containing relatively elevated amounts of MgO, CaO, Na2O, K2O and SiO2 (Table
5.8), as well as traces of finely dispersed kaolinite (Table 5.9).
The black to dark grey clays are also known as black cotton soils which are characterised by
both calcareous and non-calcareous variants. The calcareous types occur on a much lesser
extent in the south-eastern sections of the study area, while the non-calcareous varieties cover
the remaining portion of the plains in the present study area.
Commonly found associated are local swampy environments and alluvium, which occur as
isolated patches within the black cotton soils. A limited intercalated occurrence of the dark
greyish-brown mottled clays is also recognised, while shallow stony soils with rock outcrops
are concentrated along the generally easterly/ south-easterly trending stream valleys found.