The critical state line
Parry (1960) published a comprehensive set of results obtained from drained and undrained triaxial tests carried out on normally and overconsolidated samples of Weald clay. A few of his results of tests on normally consolidated samples are reproduced in the first four columns of Table 13.1 (converted into SI units). With this information and taking Gs — 2.75, the tabulated values of q, p' and v were calculated. The (p', q) points obtained from each of the test results are plotted in Fig. 13.6a and the (p', v) points are plotted in Fig. 13.6b.
We can deduce from these diagrams that there must be a single line of failure points within the p'qv space which projects as a straight line on to the qp' plane and projects as a curved line, close to the normal consolidation line, on to the vp' plane. This line is known as the critical state line and its position is illustrated in Fig. 13.7.
The equation of the critical state line
The line's projection on to the qp' plane is a straight line with the equation q = Mp', where M is the slope of the line.
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