Experimental study on mechanical and permeability properties of subsea granite under cyclic loading with different seepage pressures

Cyclic loading and seepage pressure (P_w) have a significant impact on the mechanical properties, crack evolution, and permeability of rocks, making these factors crucial considerations in rock engineering applications. This study presents the results of triaxial monotonic and cyclic loading tests conducted on subsea granite under varying seepage pressures. The findings indicate that both cyclic loading and P_w weaken the mechanical properties of granite. As the number of cycles increases, granite undergoes greater deformation, damage, and energy dissipation. Initially, the elastic modulus (E) increases before decreasing, while Poisson’s ratio (υ) rises. Under triaxial cyclic loading, granite’s stress-strain behavior, crack development, and permeability evolve through distinct stages, including crack closure, initiation, extension, and connection. Higher P_w accelerates crack evolution and enhances permeability, leading to an earlier transition from compaction to dilation, accompanied by increased deformation, accelerated damage, greater energy dissipation, and reduced strength. At higher P_w, macro-failure characteristics include greater fragmentation and surface cracking. Scanning electron microscopy (SEM) and backscattered electron (BSE) analyses show an increase in micro-scale fracture surfaces and deeper fractures after failure, indicating intensified damage and a looser rock structure.