The values of M, N, T and A vary with the type of soil. From Figs 13.6 and 13.8 we see that the values for remoulded Weald clay are approximately M =0.85; N = 2.13; T = 2.09 and A = 0.10.

13.7 Representation of triaxial tests in p'-q-v space

The results of drained and undrained triaxial compression tests can be represented in the three-dimensional stress space p'-q-v. For both tests the sample is first consolidated to some point A on the normal consolidation curve corresponding to some particular value of specific volume, v0. From this stage the two tests must be considered separately.

13.7.1 The undrained test

If the sample is now sheared undrained the stress path will move upwards from A until it meets the critical state line at point B where failure occurs. As the test is undrained the value of the specific volume remains constant at v0 so that the stress path, no matter how it wanders, is restricted to a plane passing through A and parallel to the p'-q plane.

This plane, for a normally consolidated clay, will not be bigger than the area shown in Fig 13.9 and can be referred to as the undrained plane passing through A or, alternatively, as the undrained plane at distance v0 from the origin. Hence, knowing the position of A, we can obtain the position of the failure point, B, by drawing the undrained plane through A and noting where it intersects the critical state line.

Expressions for p' and q can easily be obtained if we remember that the void ratio at failure, ef, is equal to the void ratio immediately after consolidation, eo, i.e. vq = Vf. Now q = Mp' = M exp —-— A

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