Laboratory and numerical studies on the shear behaviors and failure mechanisms of discontinuities with different joint wall strengths

Understanding the shear behaviors and failure mechanisms of discontinuities with different joint wall strengths (DDJS) is vital for evaluating the stability of interbedded rock slopes in the Three Gorges region， China. Laboratory direct shear tests were conducted on discontinuity replicas while introducing a joint wall strength combination coefficient (λ) to quantify the combined effects of compressive strength and basic friction angle on DDJS shear behavior. Experimental results revealed that λ significantly affects surface failure characteristics. Peak shear strength and shear stiffness decreased nonlinearly with increasing λ， while peak shear displacement showed an increasing trend accompanied by declining peak dilation angle. Subsequently， PFC-based numerical direct shear tests were performed， with the developed models validated by experimental results. Analysis of micro-crack distributions， asperity cracking processes， and contact force chain evolutions provided microscale insights into DDJS failure mechanisms. With increasing λ， asperity failure transitioned from tensile fractures (induced by inward propagation of tensile micro-cracks) to contact shear failure (characterized by mixed tensile-shear micro-cracks along surfaces); damage initiated earlier and intensified on the weaker wall before peak stress. A comparison of natural irregular and regular sawtooth-shaped DDJS revealed distinct asperity failure behaviors. Irregular DDJS exhibited progressive failure near peak stress， while sawtooth-shaped DDJS experienced synchronous failure in the post-peak stage， with less damage to the stronger wall. These findings offer important implications for geological disaster prevention in the Three Gorges region.