A new criterion for peak shear strength of rock joints based on shear-direction-related morphology description

Accurate characterization of joint roughness relative to shear direction is crucial for predicting the peak shear strength ( τ p ) of rough rock joints. This study proposes a modified peak shear strength criterion based on shear-direction-dependent morphological parameters. Artificial fractal joints and natural sandstone tension joints with varying roughness were replicated using 3D printing and cast in cement-based materials. Direct shear tests under constant normal load (CNL) conditions were conducted to quantify the mechanical behaviors of rough joints. Joint morphology was characterized using statistical values (mean μ , standard deviation σ ) of the apparent dip angle ( θ * ) distribution along the shear direction. A parameter E ( θ * | θ * >0 ° )- μ , representing the conditional expectation of positive apparent dip angles relative to the shear direction, was proposed to effectively capture shear-direction-dependent roughness. A new τ p criterion incorporating this parameter was established within a Mohr-Coulomb framework. Validation against experimental data and published studies demonstrates its improved accuracy, particularly for low-roughness joints, compared to existing criteria. This approach provides a physically meaningful and efficient method for estimating τ p , enhancing stability assessments in rock mass engineering.