Effects of infilling materials on shear properties of rock joints: A review and prospects

• Filling type/ratio controls shear strength/failure; soft vs hard materials show distinct responses. • Water content weakens shear via lubrication/softening, modulated by normal stress and filling ratio. • Shear velocity/cycles/boundary conditions cause complex coupling; CNS boosts strength but limits dilatancy. • Multi-factor models and size effect research urgently needed for reliable predictions. Shear strength of infilled rock joints is a critical parameter governing rock mass stability in geotechnical engineering. This review systematically compares the differential influence mechanisms of soft versus hard infill materials on joint shear behavior. For soft infills (clay, sand), increasing infilling ratio monotonically reduces shear strength, stiffness, and residual strength until reaching a critical ratio. Conversely, hard infills (cement) exhibit a non-monotonic response, initially enhancing shear strength through reinforcement before declining at higher ratios due to interface failure. Moisture content consistently degrades shear properties, with the attenuation magnitude dependent on infill type and normal stress. Loading path effects demonstrate that shear velocity and cyclic loading influence joint response in material-specific ways, diverging from unfilled joint behavior. Analysis reveals a systematic bias in existing normalized shear strength models: they consistently underestimate the joint contribution (Component A) while overestimating the infill contribution (Component B), particularly for hard infill materials where reinforcement effects are neglected. These models also oversimplify roughness effects and multi-factor couplings. A critical research gap identified is the absence of size effect studies for filled joints, despite established size dependencies in unfilled joints, joint morphology, and both wall rock and infill material properties. This review provides a comprehensive synthesis of current understanding, exposes fundamental modeling limitations, and establishes the urgent need for scale-dependent investigations to bridge laboratory findings with field applications.