Mechanical and permeability properties of granite under hydro-mechanical coupled triaxial cyclic loading

Water–rock interaction plays a pivotal role in water-related rock engineering applications such as navigation channel excavation, submarine tunnel construction, and harbor development. The mechanical and permeability properties of rocks under cyclic disturbances significantly influence engineering design and construction practices. In this study, hydro-mechanical coupled triaxial monotonic loading and cyclic loading tests were conducted on granite under different confining pressures (σ3). The results reveal that key mechanical parameters—including peak strength, elastic modulus, and crack damage threshold—increase with higher σ3, while these parameters under cyclic loading are consistently lower than those obtained under monotonic loading. The evolution of stress–strain behavior, crack development, and permeability in granite under cyclic loading exhibits distinct stages, with pore pressure significantly influencing crack growth and subsequent permeability changes. Both the number of loading cycles and σ3 markedly affect deformation, damage accumulation, crack propagation, and permeability evolution in granite, ultimately leading to dominant shear failure. Scanning electron microscopy and backscattered electron analyses further reveal that higher σ3 results in smaller fracture angles, narrower crack widths, and denser rock structures. This study elucidates the hydro-mechanical coupling effects under cyclic disturbances, providing valuable insights into the failure mechanisms and permeability evolution of water-saturated granite that are critical for long-term stability assessments in underwater rock engineering.