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