Suitability Of Square Root Time Method Consolidation
(a) Theoretical curve
Fig. 10.7 The square root of time 'fitting' method.
(a) Theoretical curve
Fig. 10.7 The square root of time 'fitting' method.
of suitability. A consolidation test sample is always drained on both surfaces and in the formula H is taken as half the mean thickness of the sample for the pressure range considered. At first glance it would seem that cv could not possibly be constant, even for a fairly small pressure range, because as the effective stress is increased the void ratio decreases and both k and mv decrease rapidly. However, the ratio of k/mv remains sensibly constant over a large range of pressure so it is justifiable to assume that cv is in fact constant.
One drawback of the consolidation theory is the assumption that both Poisson's ratio and the elastic modulus of the soil remain constant whereas in reality they both vary as consolidation proceeds. Owing to this continuous variation there is a continuous change in the stress distribution within the soil which, in turn, causes a continuous change in the values of excess pore water pressures. Theories that allow for this effect of the change in applied stress with time have been prepared by Biot (1941) and extended by others, but the approximations involved (together with the sophistication of the mathematics) usually force the user back to the original Terzaghi equation.
10.7 Determination of the permeability coefficient from the consolidation test
Having established cv, k can be obtained from the formula k = Cvmv7w. It should be noted that since the mean thickness of the sample is used to determine cv, mv should be taken as a/(l + e) where e is the mean void ratio over the appropriate pressure range.
10.8 Determination of the consolidation coefficient from the triaxial test
It is possible to determine the cv value of a soil from the consolidation part of the consolidated undrained triaxial test. In this case the consolidation is threedimensional and the value of cv obtained is greater than would be the case if the soil were tested in the oedometer. Filter paper drains are usually placed around the sample to create radial drainage so that the time for consolidation is reduced. The effect of threedimensional drainage is allowed for in the calculation for cv, but the value obtained is not usually dependable as it is related to the relative permeabilities of the soil and the filter paper (Rowe, 1959).
The time taken for consolidation to occur in the triaxial test generally gives a good indication of the necessary rate of strain for the undrained shear part of the test, but it is not advisable to use this time to determine cv unless there are no filter drains.
The consolidation characteristics of a partially saturated soil are best obtained from the triaxial test, which can give the initial pore water pressures and the volume change under undrained conditions. Having applied the cell pressure and noted these readings, the pore pressures within the sample are allowed to dissipate while further pore pressure measurements are taken; the accuracy of the results obtained is much greater than with the consolidation test as the difficulty of fitting the theoretical and test curves when air is present is largely removed. The dissipation test is described by Bishop and Henkel (1962).
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