70C to Y is undrained shear (cell pressure constant, drainage taps cloase, deviator stress applied) within the initial yield locus set up by isotropic compression to C. Behavious is “elastic” within the initial yield locus, so as there is no change in specific volume there can be no change in average effective stress and p' = constant. At Y, the sample reaches the initial yield locus and yields. This is the transition point to plastic behaviour. From Y to F, the sample is sheared at constant specific volume. To move the state of the sample to the appropriate point on the critical state line, the average effective stress must decrease and the pore water pressure increases to achieve this. At F, the sample reaches the critical state appropriate to the specific volume as tested, at which continued deformation can take place at constant q, p' and v. Assuming that A to B is on the isotropic normal compression line and that F is on the critical state line, the critical state paramaters Μ, λand κmay be calculated as follows.
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