Study on mechanical properties and mesoscopic damage mechanism of composite jointed rock masses

Joints are widely distributed structural defects in rock masses, and their geometric characteristics play a decisive role in the overall stability of rocks under complex stress conditions. To clarify the influence of joint geometry on the mechanical behavior of jointed rock under such conditions, this study investigated the mechanical properties and failure mechanisms of composite jointed rock specimens with varying joint roughness and joint dip angles. Three typical failure modes under triaxial loading were identified, and a mechanical analysis model incorporating joint roughness and dip angle was established. The failure mechanism was revealed, and a discrete element model was developed to analyze the micro-damage evolution process of the specimens. The results show that the mechanical parameters of the specimens exhibit pronounced anisotropy. Both the elastic modulus and peak strength reach their minimum values at a joint dip angle of 60°. Increasing joint roughness significantly reduces the degree of anisotropy and enhances the energy storage capacity of the specimens. A strong linear relationship is observed between the elastic strain energy and the peak deviatoric stress, confirming the applicability of the linear energy storage law in composite jointed rocks. Discrete element simulations revealed the evolution path and dominant types of microcracks between the joint and matrix. The joint dip angle governs the transition of dominant crack types from tensile to shear and then back to tensile. Increased joint roughness significantly suppresses damage localization along the joint and results in an approximately 20% increase in the proportion of shear microcracks within the matrix. These findings clarify the regulatory role of joint geometrical parameters in the damage evolution process.