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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In addition, more reasonable estimates <strong>of</strong> both the amount <strong>an</strong>d rate <strong>of</strong> settlement <strong>of</strong> soils <strong>of</strong><br />
the project area could be achieved by taking horizontal drainage into account during<br />
laboratory consolidation tests. This could be accomplished by performing consolidation tests<br />
on larger specimens under hydraulic loading using a larger cell, such as the Rowe<br />
consolidation cell, <strong>an</strong>d designed for provision <strong>of</strong> horizontal drainage (Rowe, 1966). In the<br />
normal oedometer consolidation tests, however, horizontal drainage could be taken into<br />
account by either fitting a pervious lining inside the consolidation ring <strong>an</strong>d sealing the<br />
specimen ends; or by trimming the specimen in a vertical pl<strong>an</strong>e (Head, 1988).<br />
A provision should be made to determine in situ <strong>an</strong>d/ or monitor in the field consolidation<br />
settlements <strong>an</strong>d accomp<strong>an</strong>ying increase in effective strength which occur due to dewatering<br />
<strong>an</strong>d/ or partial drying <strong>of</strong> s<strong>of</strong>t soils (in swamp <strong>an</strong>d peaty environments) caused by a fall in the<br />
groundwater table; <strong>an</strong>d this especially in the tr<strong>an</strong>sition period from wet to dry months, <strong>an</strong>d<br />
vice versa. This would serve to assess <strong>an</strong>d predict possible destructive implications onto<br />
constructed <strong>an</strong>d/ or projected <strong>engineering</strong> structures. According to Head (1988), a fall <strong>of</strong> 1 m<br />
in the groundwater table would increase the effective stress in the whole clay deposit beneath<br />
the water table by about 10 kPa, the actual amount <strong>of</strong> consolidation depending on the ch<strong>an</strong>ge<br />
in effective stress in the soil.<br />
It has been established in this study that the plasticity index (PI) <strong>of</strong> black <strong>clays</strong> <strong>an</strong>d red soils<br />
could be approximately estimated from laboratory measured results <strong>of</strong> linear shrinkage (LS)<br />
<strong>an</strong>d/ or liquid limit (LL) using newly developed relationships, i.e.<br />
PI = 1,88*LS (for black <strong>clays</strong>),<br />
PI = 1,84LS (for red soils) <strong>an</strong>d<br />
PI = 0,79(LL-25) for both black <strong>clays</strong> <strong>an</strong>d red soils.<br />
In addition, the swelling characteristics <strong>of</strong> black <strong>clays</strong> could be approximately expressed in<br />
terms <strong>of</strong> logarithmic relationships derived in the present study, i.e.<br />
P = -22,3Ln(S) + 100,80, where<br />
S (%) = percentage swelling with respect to ultimate swelling (Smax mm) under zero<br />
loading<br />
P (%) = imposed loads (P kPa) expressed as a percentage <strong>of</strong> swelling pressure (SP kPa)<br />
<strong>an</strong>d P = -15,86Ln(S) + 29,84, for<br />
S (%) = percentage swelling with respect to initial test specimen thickness (Ho mm)<br />
P (%) = imposed loads (P kPa) expressed as a percentage <strong>of</strong> swelling pressure (SP kPa)<br />
It would be necessary <strong>an</strong>d useful to extend the investigation with the purpose <strong>of</strong> finding out if<br />
the same relationships for estimating plasticity index <strong>an</strong>d swelling characteristics hold for<br />
similar <strong>tropical</strong> <strong>clays</strong> <strong>an</strong>d/ or soils in Africa, <strong>an</strong>d around the world in general.<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.