an engineering geological characterisation of tropical clays - GBV

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7.4 Oedometer consolidation tests
7.4.1 Scope
Oedometer consolidation tests were carried out in this study to investigate and establish
consolidation-settlement characteristics of normally consolidated as well as overconsolidated
clay soils found, in terms of estimating the amount and rate of settlement when externally
loaded. The tests were carried out on saturated clays, and were also aimed at determining and
establishing percentage swelling and swelling pressures of the clay soils.
Determined parameters of coefficient of volume compressibility and coefficient of
consolidation were used to describe the consolidation-settlement characteristics of soils. The
coefficient of volume compressibility (i.e. modulus of volume change) was used to indicate
the compressibility of soils in terms of the amount by which they would compress when
loaded and allowed to consolidate. The coefficient of consolidation is a time related parameter
(Head, 1984), and served to indicate the rate of compression and hence the time period over
which consolidation -settlement would take place.
Results of the tests and parameters derived thereof would be usefully applied in foundation
design in limiting settlements to within tolerable limits when structural foundation loads are
imposed on the soils. In addition, the rate of settlements and the time within which such
settlements could be virtually complete would be estimated.
The theory of consolidation (Terzaghi, 1925; Terzaghi and Peck, 1948; Taylor, 1948) is based
on the fact that, consolidation process of soils subjected to external loading mainly entails
escape of water from the voids between the skeleton of solid grains. The loss of pore water is
generally rapid in free-draining materials, but is limited in soils of impeded drainage. As a
result, settlements in high permeability materials of sands and gravels generally take place in
a short time, usually as construction work proceeds, thereby causing no major problems. On
the other hand, low permeability clays could undergo settlements over much longer periods of
time after completion of construction, thereby subjecting constructed structures to a prolonged
state of instability (Nelson and Miller, 1992; Johnson and DeGraff, 1988; Head, 1984).
In this study, the amounts and rates of consolidation settlement of soils found under various
loading conditions have been studied, analysed and established. The spatial distribution and
variation of consolidation parameters across the study area have also been investigated and
established.
7.4.2 Theory of consolidation
According to Terzaghi (1926), saturated clays which are externally loaded have the total
applied stress, σ, initially supported by the excess pore water pressure, µ, which is induced.
As water drains out of the soil, the pore pressure correspondingly falls so that an increasing
proportion of the applied load is transferred to the grains forming the soil skeleton. The
consolidation process therefore involves gradual transfer of stress from pore water to soil
skeleton. The degree of consolidation, U, is a measure of the extent of this stress transfer at
any instant during consolidation; and is given by the equation
U (%) = ((µo – µ)/ µo) * 100 (7.21)
where µo = initial excess pore pressure