Deformation and permeability of sandstone at different cycling loading-unloading rates

In order to research the effect of cycling loading and unloading rates on the deformation and permeability of sandstone, cycling loading and unloading tests were performed with a self-made thermal-hydrological-mechanical coupling test system for gas-infiltrated coal and rock. The results show that the axial deformation of sandstone 驻着 1 is larger in the initial cycle and tends to be stable with cycling, but the influence of unloading rate is minimal. The loading deformation modulus and unloading deformation modulus both increase and the rising speed becomes slow with the increase of the number of cycles. In the same cycle, the unloading deformation modulus is bigger than the loading deformation modulus, and their difference gradually decreases with the test proceeding. Meanwhile, the difference of permeability variations in loading and unloading phases decreases with the increasing of unloading rate. Starting from the fifth cycle, the shape of permeability curves is "∞". The evolution rule of permeability can be characterized by using the change regularity of axial strain. The variation of axial strain △ε₁ⁱ is influenced by unloading rate v₂ⁱ and loading stress σ_maxⁱ, which both promote the development of strain. The relationship of them can be characterized with a power function.