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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49<br />
The red friable <strong>clays</strong> on the whole are relatively deeper th<strong>an</strong> the black <strong>clays</strong>, mottled <strong>clays</strong> <strong>an</strong>d<br />
shallow yellow-brown to yellow-red friable <strong>clays</strong>. This may possibly be attributed to the<br />
higher rainfall received in areas <strong>of</strong> red soils as well as better drainage conditions <strong>of</strong> these soils<br />
both <strong>of</strong> which favour a relatively faster weathering <strong>of</strong> the underlying rock. However,<br />
development <strong>of</strong> the red soils on the concave slopes <strong>of</strong> the ridges is limited by the formation <strong>of</strong><br />
a laterite cap over parts <strong>of</strong> the rock surface; <strong>an</strong>d this would protect the rock from further<br />
weathering. The formation <strong>of</strong> laterite is favoured by the generally concave shape <strong>of</strong> the slopes<br />
<strong>of</strong> the ridges that results in ch<strong>an</strong>ging drainage conditions downslope.<br />
Table 5.1. Pr<strong>of</strong>ile description <strong>of</strong> a red friable clay.<br />
DEPTH<br />
(M)<br />
SAMPLE<br />
NO.<br />
DESCRIPTION<br />
0,0-0,30 Rd1-30cm Dark reddish brown, slightly<br />
humic, weak crumbly friable<br />
clay<br />
0,30-1,0 Rd1-100cm Dark red to red weak<br />
sub<strong>an</strong>gular blocky friable clay<br />
1,0-2,0 Rd1-200cm Red weak sub<strong>an</strong>gular blocky<br />
friable clay with slight clay skin<br />
development<br />
2,0-4,0 Rd1-400cm Red weak sub<strong>an</strong>gular blocky<br />
friable clay with slight clay skin<br />
development<br />
SILT<br />
(%)<br />
CLAY<br />
(%)<br />
C (%)<br />
73 18 2,47<br />
74 20 0,73<br />
69 24 0,83<br />
72 20 0,45<br />
The boundary lines between the red <strong>clays</strong> <strong>an</strong>d other clay soils mentioned above were first<br />
fixed along roads <strong>an</strong>d/ or pl<strong>an</strong>ned traverses by ground inspection; <strong>an</strong>d then extended <strong>an</strong>d<br />
joined by interpolation. On the aerial photographs, the red <strong>clays</strong> show up as darker tones<br />
relative to the other adjacent soil types mentioned above. A typical red friable clay from the<br />
present study area is described in Table 5.1. The amount <strong>of</strong> total carbon was found to decrease<br />
with depth, <strong>an</strong>d this is most likely also indicative <strong>of</strong> decreasing org<strong>an</strong>ic matter (humus)<br />
content <strong>of</strong> the soils with depth.<br />
Table 5.2. Soluble base content in soils <strong>of</strong> the study area.<br />
Type <strong>of</strong> soil Depth<br />
(m)<br />
Sample<br />
No.<br />
MgO<br />
(%)<br />
CaO<br />
(%)<br />
MnO<br />
(%)<br />
Na2O<br />
(%)<br />
K2O<br />
(%)<br />
Total<br />
(%)<br />
Red friable 0,0-0,30 Rd1-30cm 0,12 0,26 0,96 - 0,96 2,30<br />
<strong>clays</strong> 0,30-1,0 Rd1-100cm 0,097 0,24 0,53 - 0,66 1,53<br />
1,0-2,0 Rd1-200cm 0,14 0,16 0,61 - 0,68 1,59<br />
2,0-4,0 Rd1-400cm 0,13 0,26 0,52 - 0,60 1,51<br />
Shallow yellow 0,0-0,30 SC17-30cm 1,17 1,28 0,529 0,74 1,33 5,05<br />
- brown <strong>clays</strong> 0,30-0,50 SC17-50cm 1,28 1,47 0,488 0,82 1,41 5,47<br />
on laterite >0,50 - - - - - - -<br />
0,0-0,30 SA2-30cm 1,07 1,25 0,298 0,63 1,30 4,55<br />
Black <strong>clays</strong> 0,30-0,70 SA2-70cm 1,25 1,28 0,312 1,06 1,78 5,68<br />
0,70-1,05 SA2-105cm 1,22 4,29 0,363 1,04 1,73 8,64<br />
Black <strong>clays</strong> 0,0-0,30 SB1-30cm 1,02 1,26 0,44 0,70 1,35 4,77<br />
0,30-0,50 SB1-50cm 1,16 1,54 0,455 0,71 1,27 5,14<br />
0,50-0,70 SB1-70cm 1,23 2,01 0,439 0,76 1,34 5,78