an engineering geological characterisation of tropical clays - GBV

129
Ranges of values of coefficient of volume compressibility, mv, obtained for black clays and
red soils are given in Table (7.13), where they have been used to classify the soils based on
compressibility. Ranges of values of mv for other types of soil are also included for
comparison purposes. Results in Table (7.13) show that black clays would generally exhibit
medium to high compressibility over the normal loading range of 25-800kPa; however, cases
of low to medium compressibility are also to be expected. The red soils would be
characterised by medium to very high compressibility over the same loading range.
Comparatively, therefore, the red soils tend to be more compressible than the black clays.
This difference could be attributed to the generally loose and friable nature of the red soils
giving rise to higher porosities, as evidenced by voids ratios (e) of 0,59-1,42 (Table 7.12).
Lower compressibility of the black clays could be attributed to lower porosities (e = 0,23-
1,06), a most probable result of the cohesive nature and dense compact structure of these
clays.
Table 7.13. Compressibility classification of black clays and red soils in this study.
Coeff. of volume
Category
Clay type/ Sample No.
compressibility mv
(m²/MN)
Compressibility
classification
Typical Very organic alluvial clays > 1,5 very high
examples (after and peats
Lambe & Whitman Normally consolidated 0,3-1,5 high
-1979) alluvial/ estuarine clays
Fluvio-glacial clays and 0,1-0,3 medium
lake clays
Boulder clays 0,05-0,1 low
Heavily overconsolidated < 0,05 very low
boulder clays
SA2-70cm 0,03-0,26 very low to medium
This study: SA37-50cm 0,18-1,62 medium to very high
Black clays SB1-70cm 0,09-0,74 low to high
SB42-50cm 0,22-0,80 medium to high
SC17-50cm 0,17-0,60 medium to high
SC29-50cm 0,16-0,44 medium to high
RD1-30cm 0,19-2,0 medium to very high
This study: RD1-100cm 0,21-2,94 medium to very high
Red soils RD1-400cm 0,16-1,11 medium to high
Ranges of values of coefficient of consolidation, cv, obtained for the black clays and red soils
are given in Table (7.14). Typical ranges for other inorganic soils are included for comparison
purposes. From the results, the red soils would undergo medium to high rates of consolidation
–settlement when externally loaded in the range of 25-800 kPa. The rates of consolidationsettlement
for the black clays would be generally low over the same loading range. The high
rates of consolidation by the red soils could be attributed to their relatively high porosity and
high permeability (K = 7,57*E-11 to 7,33*E-9 m/s ) which facilitate faster drainage and rapid
dissipation of pore water pressures. On the other hand, the black clays are characterised by
relatively lower porosity and permeability (K = 1,67*E-12 to 1,2*E-9 m/s) so that drainage
and dissipation of pore pressures on external loading would be less rapid, causing slower
consolidation –settlements.