Experimental investigation of the shear behavior of soft-hard joints under constant normal stress

The shear behavior of soft-hard joints is important in the stability analysis of rock structures. However, available experimental studies on the shear behavior of soft-hard joints are relatively limited. In this study, a series of direct shear tests were conducted to evaluate the shear stress-displacement curves and the dilatancy of soft-hard joints with different joint roughness coefficient JRC, wall strength ratios, and normal stresses. Acoustic emission technique was applied to investigate the mesoscopic damage evolution of soft-hard joints with shear displacement. Results showed that the shear stress-displacement curves of soft-hard joints were divided into elastic, damage, softening, and residual stages. Peak and residual shear strengths increased with JRC, wall strength ratio, and normal stress. The peak shear displacement increased with wall strength ratio and normal stress, while it decreased with JRC. The damage coefficient increased with JRC, wall strength ratio, and normal stress, and the soft wall surface exhibited a higher damage coefficient compared to the hard wall. The damage spots on the soft wall surface progressively enlarged with wall strength ratio and normal stress. The percentage of tensile cracks was typically higher than 50%. The proposed empirical model based on Barton model and Asadollahi model could accurately predicted the entire shear stress-displacement curves of soft-hard joints obtained from direct shear tests. The findings in this study should be beneficial for estimating the shear response of soft-hard joints under constant normal stress.