an engineering geological characterisation of tropical clays - GBV

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rapid dissipation of pore water pressures and faster consolidation settlements (cv = 1,18-12,14
m²/year) when externally loaded. Constructed structures located on red soils are therefore
expected to undergo faster consolidation –settlements, much of them during the construction
stage, with minimum long-term instability implications after construction.
On the other hand, the black clays have been found to be characterised by relatively smaller
voids ratios and lower porosity (eo: 0,709-1,098; e = 0,23-1,06 over 25-800 kPa loading
range). These clays are generally more compact, dense and cohesive, and therefore less
permeable (K = 1,67E-12 to 1,20E-9 m/s), less compressible (mv = 0,03-1,62 m²/MN) and
would exhibit slower dissipation of pore water pressures and, therefore, slower consolidation
settlements (cv = 0,05-5,08 m²/year) on external loading. The low rates of consolidationsettlement
exhibited by the black clays would also mean structural settlements occurring
beyond the construction stage. Long-term instability problems would therefore be expected
for structures constructed on black clays.
Results of oedometer consolidation tests obtained in this study could be used to solve
problems related to foundations for light structures in the current area. The consolidation data
and parameters could be applied in conjuction with classification data, as well as knowledge
of the loading history of the soils to make estimates and predictions as regards probable
behaviour of foundations under various loading conditions. The results would specifically
assist in calculating the amount of settlement which would ultimately take place for the
structure as a whole, while variations in long-term settlements and/ or differential settlements
between individual footings could also be estimated; by applying methods described by
Terzaghi (1939), MacDonald and Skempton (1955) and Skempton and Bjerum (1957).
According to Skempton (1956), Terzaghi (1934), Mitchell, Vivatrat & Lambe (1977),
differential settlements are usually more critical than overall settlement, and can cause tilting
of a structure as a whole and/or distortions within the structure; and should therefore be kept
within acceptable limits to avoid possible structural deterioration and damage. In addition,
possible settlement of piled foundations as a result of the presence of deep-seated weak
compressible clay layers, especially in red soils, could also be estimated.
On the other hand, estimates of the rate of consolidation could be further used to give the
duration of time within which structural settlements would be completed, either during or
after construction. In situations of long-term settlements as may occur in black clays,
settlement/ time graphs could serve to show the duration of the most significant part of
settlements, and this could be compared with the economic life of the structure. The
settlement/ time relationship would also assist in ascertaining possible development of
unacceptable differential settlements in the long-term, after construction.
Areas of soft ground and clays in the form of alluvial and / or swampy environments are
common in the present study area. The shear strength and capacity of these grounds to support
increased foundation loads could be increased and improved by consolidation through preloading
of the ground with a surcharge of temporary fill. In this case, results of laboratory
consolidation tests would be useful in estimating the extent and rate of the resulting
settlement; and thereby indicate whether a provision for accelerating the consolidation, such
as installation of sand drains, is justifiable.
In situations where piles could be driven through the pre-consolidated soft strata to transmit
structural loads to a deeper firm stratum, the tendency of continued settlement of the fill and
soft strata would have the effect of throwing additional loads onto the piles. Here also,