an engineering geological characterisation of tropical clays - GBV

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5.2.1 Red friable clays
The red soils are found overlying the trachytic rocks in the higher altitude sections of the
study area, located mainly to the north-west. This part of the study area is sub-humid and
characterised by a high rainfall of 760 – 1000 mm per year (Kenya Meteorological
Department, 2001). The soils are well-drained and exhibit a dark reddish-brown (Sample No.
Rd-30cm) and humic (up to 2,5% carbon) topsoil overlying a red subsoil (Sample Nos. Rd-
100cm; Rd-200cm; Rd-400cm) of weak subangular blocky friable clay. Stone lines are
sometimes found associated, occurring just above the metamorphic rocks.
The soils are most probably polygenetic in that, the humid conditions must have favoured
their development by weathering and alteration from the volcanic rocks, tuff and ash found.
Later episodes of violent volcanicity in the Rift region to the west must have contributed
additional showers of volcanic ash that covered already developed red soils of the underlying
land surface (Scott, 1963). Current field observations show the red soils as having profiles
consisting of burried humic horizons; and this serves to indicate that sufficient time must have
elapsed between subsequent ash showers, thereby permitting the development and formation
of a new soil from the previously deposited ash.
The red soils extend further westwards and north-westwards, beyond the present area and
across the Kikuyu highlands, to cover and include Kiambu township. The soils occupy
principally the tops /summits as well as upper and middle slopes of hills and ridges found, and
this serves to indicate the irregular character of the land surface over which the volcanic ash
fell. On the lower slopes of the ridges, the topography becomes generally flatter, with
accompanying poorer drainage conditions, resulting in the formation of shallow yellow-brown
to yellow-red friable clays overlying a laterite horizon or rock. The transition zone between
the red friable clays and the shallow yellow-brown to yellow-red friable clays is characterised
by narrow bands of an intermediate red friable clay with iron concretions, merging into
massive laterite.
Commonly found associated with the red friable clays are intercalations of poorly drained
dark greyish-brown mottled clays observable in parts of the Nairobi municipality; and which
also occur in depressions between ridges, as well as the generally flat land surface overlying
the trachytes to the north and north-west of Nairobi city. The mottled clays are partly a result
of the development of subsidiary drainage in the lowlying land and/ or depressions between
the undulations of ridges.
Further north-westwards, beyond the present area of study, the red soils give way to strong
brown to yellow-red friable clays and dark red friable clays in a humid region covering the
high ground flanking the Rift Valley, and where a rainfall amount of over 1000 mm per year
is characteristic (Kenya Meteorological Department, 2001). These latter types of soil also
overly the trachytic rocks found, and have been derived from the volcanic rocks, tuff and ash
through processes of weathering and alteration. The soils are well-drained and also
characterised by a dark reddish-brown high humic (3-7% carbon) topsoil overlying a dark red
subsoil of subangular blocky friable clay with clay skins. The red friable clays are
comparatively less humic, have a lower total exchangeable base content and are less saturated
than the dark red friable clays. They are also characterised by a much weaker structure and
less marked development of clay skins. The brighter red colour and weaker structure of these
soils may be partly a result of the lower humus status.