an engineering geological characterisation of tropical clays - GBV

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Chapter 5
Types of soil
5.1 Superficial deposits
Superficial deposits of Recent age consist of alluvia and conglomerates exposed in the
principal river courses and their tributaries, especially in the southern and south-eastern
sections of the study area. Accumulations of alluvium occur mainly in the form of old river
sediments. Also included are new sediments currently being added to the flood plain without
developed morphology but have a more humic surface horizon. On the Athi plains, these soils
have also developed from the drying up of papyrus swamps, either as a result of canalisation
of the rivers found and/ or drier climatic conditions. Exposures of a conglomerate band
composed of boulders and pebbles of local lava occur on the north bank of the Mbagathi river,
within the Nairobi National Park. The boulders and pebbles are partly indurated while, on the
other hand, the more modern gravels also found exposed along the course of the Mbagathi
river are largely uncemented.
Deposits of shallow stony soils associated with rock outcrops characterise the courses of most
of the streams draining the high ground east of the Rift, especially in the northern and northwestern
sections of the present area. The soils are variously developed and originated from
steep slope areas where they derived by processes of accelerated erosion of the various soils
found, and which have lost their original characteristics (Saggerson, 1991). They are very
shallow and occur on the main valley sides in the form of pockets on slight shelves and
between boulders.
Isolated areas of impeded drainage also occur across the study area, and are characterised by
development of swamps with black peaty soils as well as dark greyish-brown mottled clays,
both of which form accumulations of poorly-drained soils. The peaty swamps constitute a
very minor soil group in the study area and usually occur in the form of narrow stretches
confined to the banks of streams and/ or rivers flowing through them. The peaty soils found
associated are characterised by the growth of papyrus plants and a typical grass vegetation
cover. These soils also exhibit a high humus content and are usually subjected to seasonal or
permanent water table. They are usually covered by water during flood times (Saggerson,
1991). The swampy areas clearly show up as dark tones on aerial photographs.
The mottled clays commonly exhibit a dark grey to greyish-brown humic (2-3% carbon)
topsoil overlying a greyish-brown angular blocky mottled subsoil. These ill-drained soils
generally occur in the main valley bottoms of old drainage lines and/ or depressions where the
greyish-brown mottled clays commonly fill in areas of fault troughs and/ or occupy the flat
ground at the foot of some fault scarps where drainage is locally impeded. The depressions
are most probably a result of minor fissuring which caused the formation of sinkholes at the
junctions of such fissures. Later on, the sinkholes and old drainage lines could have been
filled in with deposited materials, which also acted as the parent material for the development
and formation of the mottled clays under conditions of impeded drainage. According to Scott
(1963), the mottled clays most probably owe their origin and development from the
weathering and alteration of previously deposited volcanic ash and colluvium, as well as other
relatively soft Pleistocene materials. Seepage of water from the surrounding soil into the
mottled clays occurs during the rainy season, causing the latter soils to be water-logged for a
period of 2 to 3 months, commonly with a water depth of up to 30 cm lying on the soil
surface. The edges of the mottled clays are usually characterised by the formation of a laterite