Experimental study on the influence of ground temperature and pore water pressure on the mechanics of excavation unloading sandstone.

As tunnel engineering in western China advances deeper underground, it encounters increasing issues of high ground temperature and high water pressure. To study the mechanical properties of unloading sandstone under the combined effects of temperature and pore water pressure, triaxial unloading and reloading experiments were conducted on sandstone under different temperatures, pore water pressures, and confining pressures. The results showed: (1) The peak strength of unloading sandstone decreases with increasing temperature and pore water pressure. The elastic modulus of unloading sandstone increases with temperature but decreases with increasing pore water pressure. (2) The influence of pore water pressure on the unloading deformation of sandstone has a threshold. At low pore water pressure (1 MPa), temperature has little effect on the deformation of unloading sandstone. At medium to high pore water pressures (2, 3 MPa), temperature-induced unloading softening characteristics are obvious. (3) SEM images show that with increasing temperature, the number and width of microcracks in the rock increase. With increasing pore water pressure, rock cohesion decreases and friction angle increases; under 3 MPa water pore pressure, rock cohesion decreases by 24.8%. (4) The rock's energy storage capacity decreases with increasing temperature and pore water pressure. At high pore water pressure (3 MPa), the effect of temperature on the dissipation energy of sandstone is more significant. In addition, as the temperature rises, the proportion of elastic energy at the rock's peak increases, while the proportion of dissipation energy decreases.