Abstract: Cyclic disturbances during the excavation of underground powerhouses in large hydropower stations can damage the surrounding rock mass, compromising its mechanical properties. To reveal the failure characteristics and damage process of deep-buried granite in the Yebatan underground powerhouse after cyclic disturbance, this study conducted pre-peak constant amplitude cyclic disturbance tests on granite samples with a confining pressure of 30 MPa. The tests used cyclic stresses of 12.5%–17.5% (low stress), 10.0%–37.5% (medium stress), and 62.5%–72.5% (high stress) of their peak strengths. Subsequently, three types of graded unloading confining pressure tests were conducted, followed by an analysis of the evolution of characteristic parameters and the construction of damage variables for deep-buried granite under pre-peak cyclic disturbances. Under different graded unloading confining pressure paths, the deformation of rock specimens is primarily concentrated during the reduction of confining pressure in the final three stages, and their macroscopic failure mode is compression-shear failure. The elastic modulus shows a downward trend, while the Poisson’s ratio exhibits an upward trend; the maximum dilatancy angle remains largely consistent. After constant amplitude cyclic disturbance under high axial pressure, the rock specimen shows the highest cumulative strain energy in the graded unloading confining pressure test, indicating an increased tendency towards brittle failure. Furthermore, the strain energy of rock specimens under graded unloading confining pressure exceeds that under conventional triaxial compression. This serves as a threshold to identify if rock specimens under graded unloading confining pressure transition into the accelerated damage rupture stage. Using the lateral strain response method and crack volumetric strain method, the closure stress, crack initiation stress, and damage stress of deep-buried granite are found to be 17.9%, 42.7%, and 73.8% of the peak strength, respectively. Based on these insights, a damage variable derived from crack volumetric strain is formulated, allowing the delineation of four distinct stages in the damage process of deep hard rock. The research results provide experimental and theoretical support for analyzing the failure process of high-stress granite.

Key words: pre-peak cyclic disturbance, graded unloading, characteristic threshold, lateral strain response method, crack volumetric strain