Effect of post-peak cyclic load on mechanics and seepage characteristics of sandstone under different confining pressures

The mining disturbance causes the rock mass in a certain range of the coal seam floor to be in a post-peak state. Due to the excavation of adjacent roadways and the mining of coal seams, the post-peak rock mass undergoes a cyclic loading-unloading process. In order to explore the influence of post-peak cyclic loading on rock mass structure under different in-situ stress environments and clarify the gestation process of disasters such as water inrush from coal seam floor, post-peak cyclic loading tests of sandstone under 5,10,15,20,25 MPa confining pressure were carried out based on Rock Top multi-field coupling tester. The results show that: ① Before the post-peak cyclic loading, the unit permeability of rock shows a rapid decline-tend to be stable-sudden rise-sudden decline-tend to be stable. In the post-peak cycle stage, the unit permeability of rock is almost inverted with the axial load. ② Rock elastic modulus, crack closure stress, crack initiation stress, damage stress, peak stress and residual stress are positively correlated with confining pressure, while Poisson 's ratio increases first and then decreases with the increase of confining pressure. ③ Under the confining pressure of 5  and 10 MPa, the intermittent failure of rock occurs, and the brittle failure characteristics are weakened. Under the confining pressure of 15, 20 and 25 MPa, the brittle failure characteristics of rock are obvious, and the failure characteristics of rock are determined by the properties of rock itself. ④ The post-peak axial load mainly promotes the increase of rock permeability, but the promotion effect is weaker than that of confining pressure on rock permeability, and confining pressure is the dominant factor affecting the post-peak permeability change of rock. ⑤ Under different confining pressures, a through-shear crack occurs in all rocks, but with the increase of confining pressure, the degree of rock failure gradually weakens and the failure mode tends to be simple.