Evolutionary response of unloading-induced fractures under different stress paths: Insights from Beishan granite

Investigating cyclic unloading-induced damage in Beishan granite is essential for assessing the long-term stability of China's first underground research laboratory for high-level radioactive waste disposal. Systematic tests were performed on granite specimens subjected to initial confining pressures of 5, 15, and 25 MPa, accompanied by real-time acoustic emission (AE) monitoring and post-failure computed tomography (CT) scanning. Relative to conventional triaxial compression, cyclic loading–unloading reduces peak strength by 26.4%–31.3%, with this reduction attenuating as confining pressure increases. Higher confining pressures markedly decrease cumulative AE counts and energy release. CT analysis reveals that crack volume fraction decreases with increasing confining pressure yet remains consistently higher under Path II than Path I. Rise angle-average frequency (RA-AF) analysis indicates a confinement-driven transition from tensile-dominated to shear-dominated failure; however, Path II retains a notable tensile component even at 25 MPa. Macroscopically, Path I yields a localized shear band with a smaller fracture angle, whereas Path II produces a broader, more dispersed fracture network accompanied by greater cumulative axial strain. Mechanistically, increasing confining pressure suppresses crack propagation, thereby driving a transition in failure mode from tensile splitting to a single dominant shear plane. These findings establish a quantitative correlation among stress path, confining pressure, microcracking mechanisms, and macroscopic failure behavior, offering a theoretical basis for assessing the stability of Beishan granite subjected to cyclic excavation disturbances.