an engineering geological characterisation of tropical clays - GBV

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