an engineering geological characterisation of tropical clays - GBV

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The reliability of the estimations has been demonstrated by the close relationship and strong
agreement between laboratory measured and so calculated free swell values, i.e.
FSmeasured = 1,01FSLL
FSmeasured = 0,99FSPI
FSmeasured = 0,98FSLS
Correlations between clay fraction/ content and other index properties such as linear
shrinkage and free swell were found to be weak and poor, most probably due to the effects of
soil fabric/ structure, clay mineralogy and other factors in controlling the activity and
physical/ engineering behaviour of the soils.
Laboratory shear strength investigations showed the black clays to be limited in their strength;
and are characterised by relatively lower values of angles of shear resistance (φ`) of between
11° and 30°, giving an average value of 18°. The cohesive nature of the clays is reflected in
their large cohesion (c´) values of 12 – 48 kN/m² (mean value: 35 kN/m²). On the other hand,
the red soils were found to be relatively more stable, with angles of shear resistance of 28° to
29°. However, the cohesive character of these soils is negligible due to their generally loose
and friable nature.
Correlations between shear angles determined on undisturbed samples and results of index
tests of black clays were only of poor to fair or moderate strength. A lot of uncertainty would
therefore be encountered in attempting to assess and characterise shear strength of the clays
on the basis of results of index properties obtained by tests carried out on disturbed and
fractioned samples.
Results of laboratory oedometer consolidation tests and compression coefficients derived
thereof show the red soils to be medium to very highly compressible (mv =0,16-2,94 m²/MN )
when externally loaded in the range of 25-800kPa; and this is a probable result of their loose
and friable nature. The soils also exhibit medium to high rates of consolidation-settlement (cv
= 1,18-12,14 m²/year), a result of the relatively high porosity (voids ratio e = 0,59-1,42) and
permeability (K = 7,57E-11 to 7,33E-9) which facilitate faster drainage and rapid pore water
pressure dissipation on external loading. In practice therefore, these soils would be expected
to undergo rapid consolidation – settlements, especially during the construction stage, without
posing long-term instability problems to constructed structures. The soils fall in the class of
normally consolidated to slightly overconsolidated clays.
The black clays would exhibit slightly lower compressibility, i.e. medium to high (mv = 0,03-
1,62 m²/MN) when externally loaded over a 25 –800kPa range, as a result of their generally
dense, compact and cohesive nature. The clays would also exhibit low rates of consolidationsettlement
(cv = 0,05-5,08 m²/year) due to their relatively low porosity (e = 0,23-1,06) and
permeability (K = 1,67E-12 to 1,20E-9) that tend to limit drainage and dissipation of porewater
pressures. Settlement and instability of structures located on these clays would therefore
be expected to persist beyond the construction stage. The clays are classified as normally
consolidated.
The following relationships have been derived in this study for the approximate estimation of
compressibility indices, cc, of the soils using laboratory determined liquid limits, LL, i.e.