Experimental study on strain behavior and permeability evolution of sandstone under constant amplitude cyclic loading-unloading

The coupling of in situ stress, seepage, and fracture makes the strata exhibit complex strain behavior and permeability evolution under the stress path of cyclic loading-unloading. In this study, the cyclic loading-unloading experiments of constant amplitude axial stress of sandstone under the combination of different initial confining pressures and loading-unloading rates are conducted. The experimental results show that the heterogeneity of sandstone, Poisson's ratio produced by adjacent loading-unloading, and confining pressure determine the deformation characteristics of sandstone in lateral and axial directions. Sandstone can produce significant shear dilatancy at a low rate; this is because stress perturbation can activate more particle slippage and fracture structure changes. However, the fracture preferentially propagates along the end or edge of the crack with strong stress sensitivity to form a shear failure plane at high rate. The normalized permeability of sandstone decreases with an increase in the loading-unloading rate before failure. The evolution of normalized permeability is closely related to the shear slip of fracture structure and sandstone particles. For the prevention of rock engineering disasters, the high loading-unloading rate results in lower normalized permeability, which favors the prevention of gas-type disasters in rocks with higher gas potential energy; however, it is not conducive to the prevention of rockburst.