Unloading Response and Failure Mechanism of Water-Bearing Sandstone in Mining-Induced Stress Paths

Excavation in coal‐bearing strata essentially constitutes an unloading process of rock mass, altering the stress field and hydrogeological conditions around the mining field. As a major lithology and aquifer in the coal bearing strata, sandstone exhibits distinct unloading‐induced mechanical responses and failure mechanisms under mining‐induced stress paths, which represent a key geological factor in engineering hazards such as roof instability. In this paper, the unloading responses of water‐bearing sandstone through triaxial tests under constant and unloading confining pressures are investigated so as to simulate the realistic mining geomechanical conditions. The stress‐strain behavior, deformation and failure modes, fracture fractal characteristics, and microstructural evolution were systematically analyzed. The research findings indicate that the failure mode of sandstone depends strongly on the loading path. Under constant confining pressure, rock failure is predominantly governed by shear failure, where the confining pressure suppresses the development of moisture‐induced pores and fractures. In contrast, during the unloading of confining pressure, the failure mode transitions to tensile‐shear composite failure, where in an increase in moisture content significantly promotes the propagation and coalescence of tensile cracks. The radial deformation markedly exceeds axial deformation and higher moisture content amplifies both damage extent and sensitivity to confining pressure reduction under the unloading of confining pressure. Microstructural analysis reveals that water saturation promotes partial dissolution of kaolinite and dispersion of illite‐smectite mixtures. This process undermines the shear resistance and facilitates the development of tensile micro‐crack, thereby contributing to more pronounced tensile‐shear failure under unloading conditions. When the tensile‐shear ratio of crack strain exceeds 1.41, sandstone exhibits tensile‐shear composite failure, with damage severity escalating with moisture content. These findings provide a scientific basis for understanding the unloading response mechanisms of water‐bearing sandstone under mining‐induced stress paths and support improved control strategies for surrounding rock in coal‐bearing strata.