an engineering geological characterisation of tropical clays - GBV

50
The variation in the content of total soluble bases in red soils is presented in Table 5.2
(sample Rd1), alongside that for black clays for comparison purposes. The amount of total
soluble base was also observed to decrease with increasing depth of the red soils. This could
be explained by the good drainage conditions of the soils which serve to facilitate removal of
the basic substances in solution during rainfall.
Further eastwards across the study area, the red soils pass into a composite soil group
consisting of shallow yellow-brown friable clays overlying a laterite horizon; and shallow
yellow-red friable clays on rock. This is because of the difficulty in distinguishing between
and separating these two soil units on aerial photographs. However, the two soil components
are genetically different. These soils commonly exhibit a brown (Sample No. SC17-30cm)
relatively low humic (up to 1,5% carbon) topsoil overlying a yellow-brown (Sample No.
SC17-50cm) subangular blocky friable clay with iron concretions passing downwards into
massive laterite (ferricrete). In other places, the soils consist of a brown low humic (0,5-1,5%
carbon) topsoil underlain by a yellow-red subangular blocky friable clay passing into rock.
The soils are usually characterised by a slight seasonal impeded drainage. They also support a
scrub grass vegetation which appears as a light tone on aerial photographs. The shallow soils
over rock are generally redder than their lateritic counterparts. They are found occurring
mainly on the flat lands adjoining and/ or at the edges of the black clays in the plains. They
are apparently youthful soils which have developed most probably after the removal of the
black clays by erosive processes. The shallow soils over laterite occur mainly on the lower
slopes of ridges. According to Scott (1963), they are most probably a result of seepage water
from higher ground whose downslope flow is checked by the change of slope and poorer
drainage conditions at the foot of the slope, causing the deposition of iron and aluminium
compounds from the seepage waters to form a laterite sheet. The development of the laterite
sheet also serves to increase the drainage impedence , thereby favouring further development
and formation of the shallow lateritic soils which extend upslope.
Table 5.3. Profile description of a shallow yellow-brown clay overlying laterite.
Depth (m) Sample No. Description Silt Clay C (%)
(%) (%)
0,0-0,30 SC17-30cm Brown slightly humic,
crumbly friable clay with
occasional rounded murram
concretion
59 29 1,54
0,30-0,50 SC17-50cm Yellow brown subangular
blocky friable clay with
occasional rounded murram
concretion
>0,50 - Mainly murram concretion
passing into massive murram
59 27 1,29
- - -
A typical yellow-brown friable clay overlying laterite in the current study area is described in
Table 5.3. The amount of total carbon is observed to decrease with increasing depth of soils,
and this could as well serve to reflect possible decrease of organic matter content of the soils
with depth. However, the soils exhibit a slight increase in the amount of total soluble base
with increased depths (Table 5.2, Sample SC17), and this is most probably due to the slightly
impeded drainage character of the soils which serves to limit and/ or inhibit removal of the
basic substances in solution.