A Peak Shear Strength Criterion of Rock Joints Based on Brightness Area Percentage

The stability of rock masses is intricately tied to the peak shear strength (PSS) of rock joints. In this study, a novel criterion for accessing the PSS of rock joints is proposed based on a three-dimensional (3D) parameter called bright area percentage (BAP). This parameter serves to estimate the rock joint roughness and to delineate regions in rock joints in which shear failure is likely to occur. The research journey involved laboratory experiments and model fitting, culminating in the establishment of a robust correlation between the BAP and the PSS. 3D laser scanning was employed to obtain laboratory-scale point clouds on the surface of 11 groups of rock joints in the Majiagou landslide area, encompassing both upper and lower surfaces. Subsequently, PSS, joint compressive strength, and basic friction angle of rock joints were determined by performing laboratory experiments. The reliability of the BAP method in predicting potential contact areas during shear was confirmed through receiver operating characteristic curves analysis on four selected rock joints. A new criterion was proposed using BAP based on the structure of Kulatilake’s shear strength criterion. This model distinguishes itself through its succinct form and clear physical interpretation. The corresponding model coefficients were obtained by applying data from eight groups of rock joints; the correlation coefficient R2 of the new fitting model was 0.86. The validity of the new model was assessed by applying it to three sets of rock joint samples. Research indicated that the estimates provided by the new PSS model had an error percentage of less than 10. Finally, the new proposed model demonstrates a lower mean error percentage of 13.0 compared with the 21.2% mean error resulting from the joint roughness coefficient–joint compressive strength model. The model demonstrates better performance under a constant normal stress range of 0.5–2.5 MPa.