Effect of A ai - A0-3 Increase in effective stresses:

Change in volume of soil skeleton, AVC = -CcV(A<7j + 2A<73)

AVv = -CvnAudV and AVC must equal AVv ^ Cc(A<ti — A03) - CcAud = CvnAud or


Generally a soil is not perfectly elastic and the above expression must be written in the form:

where A is a coefficient determined experimentally. The expression is often written in the form:

À and B can be obtained directly from the undrained triaxial test. As has been shown, for a saturated soil B= 1.0 and the above expression must be.

Values of A

For a given soil, A varies with both the stress value and the rate of strain, due mainly to the variation of Aud with the deviator stress. The value of Aud under a particular stress system depends upon such factors as the degree of saturation and whether the soil is normally consolidated or overconsolidated. The value of A must be quoted for some specific point, e.g. at maximum devi-ator stress or at maximum effective stress ratio (a\la'i); at maximum deviator stress it can vary from 1.5 (for a highly sensitive clay) to —0.5 (for a heavily overconsolidated clay).

3.12 The triaxial extension test

In the normal triaxial test the soil sample is subjected to an all-around water pressure and fails under an increasing axial load. This is known as a compression test in which <j\ > a2 =

When the cohesive intercept, c', is equal to zero as is the case for drained granular soils, silts and normally consolidated clays, then the relevant form of the Mohr-Coulomb equation is:

o\ — <73 = <tj sin 4> + cr-} sin <fi where <rif and a^ are the respective stresses at failure.

It is possible to fail the sample in axial tension by first subjecting it to equal pressures o\ and er3 and then gradually reducing a\ below the value of cr3 until failure occurs. This test is known as an extension test and the Mohr-Coulomb expression becomes:

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