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