Red sandstone damage fracture and permeability characteristics under combined disturbance of cyclic loading and confining pressure unloading

To explore the rules of rock mass damage and permeability evolution induced by deep engineering excavation, cyclic loading and confining pressure unloading combined disturbance triaxial tests with simultaneous measurement of wave velocity and permeability were conducted. The damage and permeability evolution rules of red sandstone under different conditions were studied. The results showed that: (1) The rock exhibits different stress-strain curve shapes at different stages of combined disturbance, and their end shapes depend on which stage of disturbance the rock is failure. Initial stress ratio is the main factor to determine the difficulty of rock failure induced by combined disturbance. (2) The total peak axial strain increases with the increase of initial stress ratio, and the proportion of combined disturbance axial strain is always low. The absolute value of total peak volume strain increases first and then decreases with the increase of initial stress ratio, and the proportion of combined disturbance volume strain is high. (3) In the initial loading stage, the wave velocity increases and the permeability decreases. In the combined disturbance stage, the wave velocity decreases and the permeability increases. The damage and volume strain are approximately linearly related, and both the damage and permeability show a slow and then fast growth trend with the increase of disturbance times, and the inflection point seems to be on a straight line. (4) The rock exhibits a single shear failure mode during the combined disturbance unloading confining pressure stage, and a conjugate shear failure mode during the combined disturbance cyclic loading stage. (5) The double-high condition (high initial stress ratio and high initial confining pressure) will significantly increase the risk of rock excavation instability and failure.