an engineering geological characterisation of tropical clays - GBV

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The red soils in this study occur mainly within the built-up areas of Nairobi metropolitan.
Extensively open grounds to facilitate more systematic site investigation and sampling were
therefore hard to come by. As a result, only random site studies and sampling of red soils
were effected in this study. However, in an effort to achieve precise assessments in the
variations of soil depths and engineering properties within these soils, results obtained from
current investigation have been supplemented with some results of previous works (Otando,
1996) covering the same aspects; and this especially in the western and northern sections of
the project area.
4.3 Cone penetration depth sounding
Cone penetration sounding technique was used to determine soil depths across the project
area. A combined presentation of the sounding plan and obtained results for both red soils
and black clays is shown in Fig. (4.2) in which values at site investigation points refer to
respective depths. The red soils are mainly confined to the central-western and north-western
parts of the project area, and show depths of just over 4m to 18m. The black clays cover the
rest of the project area and commonly vary in their depths from about 1m to around 4m. In
general, however, both red soils and black clays show progressively decreased soil depths
eastwards and south-eastwards across the study area, just as indicated by the direction of
arrows in Fig. (4.2).
Depth variations of red soils and black clays are also summarised in contour form (Fig. 4.3),
block diagram (Fig. 4.4), as well as contour overlays on a base map (Appendix D: Fig. D1).