Determination of the coefficient of consolidation cv from the consolidation test

If for a particular pressure increment applied during a consolidation test the compression of the test sample is plotted against the square root of time, the result shown in Fig. 10.6 will be obtained.

The curve is seen to consist of three distinct parts: AB, BC and CD.

• AB (initial compression or frictional lag)

A small but rapid compression sometimes occurs at the commencement of the increment and is probably due to the compression of any air present or to the reorientation of some of the larger particles in the sample. In the majority of tests this effect is absent and points A and B are coincident. Initial compression is not considered to be due to any loss of water from the soil and should be treated as a zero error for which a correction is made.

• BC (primary compression)

All the compression in this part of the curve is taken as being due to the expulsion of water from the sample, although some secondary compression will also occur. When the pore pressure has been reduced to a negligible amount it is assumed that 100 per cent consolidation has been attained.

• CD (secondary compression)

The amount by which this effect is evident is a function of the test conditions and can hardly be related to an in situ value.

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