Experimental study on permeability and porosity evolution of host rock with varying damage degrees in excavation damaged area under real-time ultra-high temperature and triaxial stress/seepage pressure condition

In the field of high-temperature underground excavations, previous scholars often focused on the seepage characteristics of intact host rock after high-temperature treatment. However, relatively little is known about the coupled thermo-hydro-mechanical (THM) responses of the damaged host rock in excavation damage zone under real-time ultra-high temperature and triaxial stress/seepage pressure (RTUHTTSSP). Therefore, using a self-designed triaxial testing system coupled with THM response system, the permeability evolution of sandstone with varying damage degrees under RTUHTTSSP was measured in this study. Then, mercury injection measurements were carried out on these specimens subjected to coupled THM conditions. The results show that (1) under RTUHTTSSP, permeability/porosity does not continue to increase with increasing temperature as expected. They show a strong dependence on the interaction of temperature field, stress field, seepage field, and damage regime. At 300 °C, the permeability/porosity of 0–0.8-damaged specimens is expected to decrease due to the closure of internal pores/fissures. However, at 450 and 300 °C, permeability/porosity of the 0.8–0.9-damaged specimens is expected to increase vertically due to the generation of the through-going macrocracks. Subsequently, with the continuous increase in the temperature, the permeability/porosity decreases slightly due to the fracturing/compression of the propping asperities. (2) The relationship between permeability and seepage pressure is closely related to the damage degree. For 0–0.6-damaged specimens, the permeability first decreases and then increases with the increase in seepage pressure. For 0.7–0.8-damaged specimens, the permeability increases with the increase in seepage pressure. The research results of this study can provide theoretical basis for the evaluation of stability of high-temperature underground excavation.