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