Shear behavior of two-order asperities in three-dimensional rock joints: Experimental investigation and development of a morphology-based shear strength criterion

Rock instability is predominantly driven by the shear failure of rock joints, with joint morphology playing a critical role in governing shear behavior. Most existing studies emphasize overall joint morphology, often neglecting the distinct contributions of first- and second-order asperities. To address this limitation, this paper systematically investigates the roles of waviness and unevenness in influencing the shear behavior of rock joints. Joint morphology was decomposed using three-dimensional laser scanning and wavelet transformation techniques. Digital carving technology was employed to fabricate rock joint specimens, which underwent parallel direct shear tests. The results indicate that waviness primarily governs peak shear strength, while unevenness contributes to shear behavior during the pre-peak stress accumulation stage. Overestimating the contribution of unevenness results in an inaccurate assessment of roughness effects on peak shear strength. At high normal stresses, increased damage to waviness contributed to shear strength increments. Based on these findings, a shear strength criterion was developed, integrating the differential morphological contributions of waviness and unevenness. The proposed criterion demonstrated superior predictive accuracy when validated against experimental data. This work provides a deeper understanding of the multi-order asperity contributions to shear behavior. • The contribution of unevenness to peak shear strength exhibits an exponential decline with increasing normal stress. • An improved shear strength criterion is proposed that quantifies the differential contributions of two-order asperities. • A novel roughness parameter is developed that considers both the directionality and asperity height of joint morphology. • The roughness failure threshold is investigated and suggested.