9.2 Peak, critical state and residual strengths
As shown in Fig. 9.1, soils initially on the dry side of the critical line reach peak states
before the critical state. The peak state will normally be reached at strains of the order
of 1% while the critical state will be reached after strains greater than 10% (in some
soils the critical states are not reached until the strains have exceeded 50% or so).
Notice that the peak state coincides with the point of maximum rate of dilation (i.e. at
maximum ψ). Soils on the wet side compress throughout, shearing up to the ultimate
state, and there is no peak.