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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81<br />
The red soils generally exhibit a decrease in their carbon contents with depth (Fig. 6.11). The<br />
decrease <strong>of</strong> the org<strong>an</strong>ic carbon is indicative <strong>of</strong> a general decrease <strong>of</strong> the org<strong>an</strong>ic matter content<br />
<strong>of</strong> the soils with depth. The decrease <strong>of</strong> the inorg<strong>an</strong>ic carbon could be related to the good<br />
drainage conditions <strong>of</strong> the soils which favour leaching <strong>an</strong>d removal <strong>of</strong> carbonates as well as<br />
other soluble components <strong>of</strong> Mg, Ca, Na <strong>an</strong>d K. The variation <strong>of</strong> total carbon, org<strong>an</strong>ic carbon<br />
<strong>an</strong>d inorg<strong>an</strong>ic carbon contents <strong>of</strong> these soils with depth is best described by a potential<br />
relationship with strong correlations (R = 0,94; R = 0,94; <strong>an</strong>d R=0,94, respectively).<br />
Generally, however, the soils exhibit higher contents <strong>of</strong> org<strong>an</strong>ic carbon th<strong>an</strong> inorg<strong>an</strong>ic carbon.<br />
The total carbon <strong>an</strong>d org<strong>an</strong>ic carbon contents <strong>of</strong> black <strong>clays</strong> generally decrease with depth<br />
(Fig. 6.12); <strong>an</strong>d this is most probably due to the decrease <strong>of</strong> org<strong>an</strong>ic matter content <strong>of</strong> the soils<br />
with depth. On the contrary, the inorg<strong>an</strong>ic carbon component is found to generally increase<br />
with depth, <strong>an</strong>d this could be attributed to the weathering <strong>of</strong> the underlying volc<strong>an</strong>ic tuffs<br />
which usually results in the formation <strong>of</strong> secondary limestone in situ. The variation <strong>of</strong> total<br />
carbon with depth could be described by a polynomial relationship, but with a relatively<br />
weaker correlation (R = 0,56); while the variation <strong>of</strong> org<strong>an</strong>ic carbon with depth is best<br />
approximated by <strong>an</strong> exponetial relationship with a strong correlation (R = 0,85). On the other<br />
h<strong>an</strong>d, variation <strong>of</strong> inorg<strong>an</strong>ic carbon content with depth tends to fit a polynomial relationship,<br />
with a moderately strong correlation (R = 0,63).<br />
Black <strong>clays</strong>: carbon content/ depth<br />
1,60<br />
1,40<br />
1,20<br />
Ctotal = 0,5235Dp 2 - 1,3301Dp + 1,7069<br />
total carbon<br />
carbon (%)<br />
1,00<br />
0,80<br />
0,60<br />
0,40<br />
R 2 = 0,3146<br />
Corg = 2,4918e -2,0216Dp<br />
R 2 = 0,7303<br />
CDiff = 1,5224Dp 2 - 1,4352Dp + 0,5028<br />
org<strong>an</strong>ic carbon<br />
inorg<strong>an</strong>ic carbon<br />
Polynomial (total<br />
carbon)<br />
Exponential<br />
(org<strong>an</strong>ic carbon)<br />
Polynomial<br />
(inorg<strong>an</strong>ic carbon)<br />
0,20<br />
R 2 = 0,3943<br />
0,00<br />
0,00 0,20 0,40 0,60 0,80 1,00 1,20<br />
depth Dp (m)<br />
Figure 6.12. Variation <strong>of</strong> carbon content <strong>of</strong> black <strong>clays</strong> with depth.<br />
Combined results <strong>of</strong> black <strong>clays</strong> <strong>an</strong>d red soils also show a general decrease <strong>of</strong> total carbon as<br />
well as org<strong>an</strong>ic carbon with depth (Fig. 6.13). The results also reveal approximating potential<br />
relationships with strong correlations in the variation <strong>of</strong> total carbon (R = 0,77) <strong>an</strong>d org<strong>an</strong>ic<br />
carbon (R = 0,75) with depth. However, no well defined relationship occurs to describe the<br />
variation <strong>of</strong> combined data <strong>of</strong> inorg<strong>an</strong>ic carbon <strong>of</strong> the two types <strong>of</strong> soil with depth.