an engineering geological characterisation of tropical clays - GBV

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