Cyclic shear mechanical characteristics and shear strength of multi-scale joints

The shear strength of rock joints exhibits strong size dependency. Conducting multi-scale experiments on joints is of great theoretical value and practical significance for studying the mechanical properties of large-scale joints. This study focuses on the less explored issue of size effect on the mechanical properties of joints under cyclic loading, near-natural rock joints with varying roughness were obtained through cubic splitting tests. Three-dimensional scanning technology was then utilized to capture point cloud data of the joints, followed by the production of resin models of different sizes using three-dimensional printing technology. Finally, model replicas were made through mold casting. Experiments were conducted to investigate the size effect of the structural joints under different normal stresses, as well as the influence of roughness and normal stress on the shear strength of the joints. A peak shear strength formula was developed considering the size effect of the joints, utilizing the adhesive friction theory and Barton's empirical formula. The results show that the peak shear strength of rock joints exhibits a noticeable negative size effect. Variations in peak shear strength are more sensitive in smaller-sized surfaces subjected to high normal stresses and larger-sized surfaces under low normal stresses. Moreover, the residual shear strength at low normal stresses demonstrates a distinct negative size effect. Conversely, under high normal stresses, the residual shear strength transitions from a positive size effect to a negative size effect as the joint size increases.