Shear Behavior and Shear‐Flow Coupling Mechanisms of Red‐Clay‐Infilled Joints Under High Water Pressure

Rock joints are considered as a discontinuity between two continuous media. The presence of infilled joints influences the mechanical and hydraulic properties of joint rock. Understanding these properties is critical for assessing underground excavation safety, slope stability, and water inrush risk. However, few studies have analyzed the influence of high‐water pressure on red‐clay‐infilled joints. Most previous shear‐flow coupling studies have been limited to very low water pressures (<0.23 MPa) due to apparatus constraints. In this study, tests were conducted under higher water pressures of 0.4–0.8 MPa, simulating more realistic engineering conditions. In this study, both direct shear tests and shear‐flow coupling tests are conducted to investigate the effects of surface roughness, normal stress, and water pressure on the shear stress, shear deformation, and hydraulic aperture of infilled joint rock. The results show that failure modes transform from frictional sliding failure to asperities cut‐off failure with the increasing of joint roughness coefficient (JRC). Both peak and residual shear strength increase nonlinearly with the increasing of normal stress. The normal displacement shows pure compression or compression–dilatation behavior during the shear process and pure compression is likely to occur on the infilled joints with low JRC or high JRC with high normal stress. Water pressure does not change the overall failure type but reduces asperity engagement due to hydration‐induced softening and mudification of the clay infill. As a result, peak shear stress decreases with increasing water pressure, and the maximum normal deformation exhibits a nonlinear “first‐decrease‐then‐increase” trend. The hydraulic aperture evolves through three distinct stages—initial, rapid‐growth, and stable. Higher water pressure enlarges the aperture and advances the onset of the rapid‐growth stage, whereas greater normal stress suppresses aperture development. Joints with larger roughness tend to maintain relatively larger hydraulic apertures. These findings provide mechanistic insights into the hydromechanical behavior of clay‐infilled joints under elevated water pressure and support the evaluation of stability and water‐inrush risks in red‐clay engineering environments.