Experimental research on unloading mechanical properties of sandstone under different initial unloading levels and pore pressures

The mechanical properties of deep rocks are obviously affected by the initial unloading level and water pressure environment. By carrying out triaxial compression tests and unloading tests on sandstone under different initial unloading levels and pore pressures, the ultimate strength, deformation characteristics, deformation damage and shear strength under unloading are studied. The test results show that when the initial unloading level n is kept to be constant, with the increase of pore pressure p, the ultimate strength under unloading decreases quickly first then slowly. The rock is more prone to damage, more brittle, and the damage variable increases. When the pore pressure p is kept to be constant, with the increase of the initial unloading level n, the ultimate strength under unloading increases linearly and the uniform decreasing increment of confining pressure decreases linearly too. The damage variable decreases, especially when n is from 0.9 to 1 of the process, and ω reduction is more pronounced. Considering the influences of the initial unloading level n and pore pressure p, the friction angle and cohesion are fitted by the binary function. It has a certain engineering significance that the values of cohesion and friction angle under the condition of different n and p can be predicted by the fitting formula. And based on the fitting of the experimental data, the expression of Mohr-Coulomb yielding criterion for the sandstone under different initial unloading levels n and pore pressures p is modified.