Effect of unloading rate and pore water pressure on mechanical properties of sandstone

In order to study the effect of discharge load rate and pore water pressure on the mechanical properties of sandstone, the triaxial unloading tests with different unloading rates (0.005, 0.02, 0.05 and 0.1 MPa/s) and different osmotic pressures (0, 0.3, 0.6, 0.9 and 1.2 MPa) are designed and perfomed. The results show that: (1) At the loading stage, with the increase of the pore water pressure, the gradient of stress-strain curve of rock decreaseds gradually; (2) At the stage of unloading, with the increase of the unloading rate, the soft number of confining pressure is smaller, the confining pressure of rock failure is smaller, the rock strength is relatively high, but the degree of fragmentation is more serious. Moreover, under the same unloading rate, the higher the pore water pressure is, the more obvious the rock lateral expansion phenomenon is, and the rock is much easier to fail; (3) At the stage of unloading, the deformation modulus of rock shows a slow and steep trend, and the smaller the unloading rate is, the higher the pore water pressure is, the larger the deformation modulus deterioration amplitude is; (4) During the process of unloading, with the increase of the unloading rate, the features of rock brittle failure are more obvious. With the increase of pore water pressure, the number of axial tensile cracks and tracking secondary cracks near failure is larger. The stress concentration caused by the pore water pressure in specimen cracks or crack tips is the main reason that leads to the deformation failure. The related research results can provide good references for the analysis of the discharging mechanical properties of the hydrous rock mass.