Experimental studies on mechanical behaviour of rock joints with varying matching degrees

Besides surface roughness, the matching degree between the joint upper and lower blocks is another important factor affecting the shear behavior of a rough rock joint. A simplified way, by imposing varying magnitudes of horizontal dislocation along the shear direction between the joint upper and lower blocks, is used to model the different matching degrees of a joint that is made by cement. A large number of direct shear tests are then performed under constant normal stress condition to investigate the shear behavior of rock joints under the varying matching degrees. The experimental observations indicate that with the increase of dislocation, the peak shear strength decreases and has a larger reduction rate under the lower normal stress level. With the increasing dislocation, the peak shear displacement increases, and the shear stiffness decreases and gradually approaches a constant. The influence of dislocation on the shear stiffness is more prominent under a higher applied normal stress. Several simple combinations of roughness parameters and normalized dislocation are used to perform regression analysis, and a new empirical peak shear strength criterion is put forward to capture the peak shear strength degradation of rock joints under different matching degrees. A preliminary comparison between the proposed criterion and the existing criteria is also provided. The new parameters for the proposed criterion can be easily determined in the laboratory.