Development of a new shear strength criterion for rock joints based on three-dimensional directional roughness parameters

The major aim of the current research is to accurately predict the shear strength of rough joints to properly evaluate the stability of mining structures built on jointed foundations. The roughness of the joint surface is direction-dependent and is a significant factor influencing the peak shear strength of rough joints. Quantitatively evaluating the three-dimensional (3D) roughness of rock joints is a challenge. This study employed a non-contact 3D laser scanning technology to measure the rough joint morphology and adopted the high-precision 3D printing technique to manufacture the photosensitive resin molds. A batch of joints with the same morphology was replicated, and the joint shear tests were performed under five kinds of load conditions. Two new 3D directional roughness metrics were developed to characterize the morphology of rock joints, i.e., the 3D maximum contact area ratio (Am) and the root mean square of the apparent dip that faces the shear direction (θ*2). These two directional roughness parameters have clear physical significance and can capture the anisotropy of surface roughness. A new peak shear strength criterion incorporating the proposed 3D directional roughness parameters was developed. In addition, comparisons were made between the new criterion and the existing criteria in the literature. It was found that the joint roughness coefficient–joint compressive strength model underestimates the shear strength of rock joints. Compared with the 3D models in the literature, the new model has a better agreement with test results. In addition, the anisotropy of shear strength was also investigated. The anisotropy of joint shear strength decreases with increasing applied normal stress.