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