Direct shear tests are usually carried out to find the shear strength parameters, and few attention is paid on the post-peak curve. An experimental and theoretical study on the relationship between the shear strength parameters and the post-peak curve were carried out. Analysis on the shear stress-strain curves showed that when normal stresses sn satisfy certain conditions, the post-peak curve is interrelated to the shear strength parameters by (1) tdrop=td-tr=c and (2) ?=arctan(trsn), where tdrop, td, and tr are the shear stress drop, the peak shear stress, and the residual shear stress respectively. Based on the theoretical analysis of above phenomena, an approximate method was proposed by using post-peak curves to obtain shear strength parameters of rocks, and its concrete calculating process was presented. Finally, through further study on the slope of shear stress-strain curves during the shear stress drop, a shear stress-strain constitutive equation for the whole curve was proposed: t= f(e, sn). It implies visually the physical meaning of post-peak curves: the whole dropping course of shear stress is the releasing course of the cohesion.
Through direct shear tests, the shear stress-strain curves of rocks are drawn. To calculate the parameters, a shear strength curve is depicted by using the shear strengths td at different normal stresses sn. As a simplification, we use a straight line to replace the shear strength curve, which is the core of Mohr-Coulomb criterion [2, 3, 11, 12], to calculate the cohesion c and the internal friction angle ?. It is now the most frequently used method to compute shear strength parameters, which only employs the shear strength td but focuses rarely on post-peak curves. Study on the post-peak constitutive relation of rocks is the basis of the stability analysis of surrounding rock and the support design of anchor bolt.
