Experimental study on permeability evolution and nonlinear seepage characteristics of fractured rock in coupled thermo-hydraulic-mechanical environment:a case study of the sedimentary rock in Xishan area

An understanding of how the fracture permeability evolves in a coupled thermo-hydraulic-mechanical (THM) environment at real-time ultrahigh temperature has a great significance for many underground excavations. This paper designs a testing apparatus to take the effects of THM processes into account by coupling high temperatures and high triaxial stress. Correspondingly, the permeability evolution of fractured rock in the coupled THM environment under real-time ultrahigh temperature (20–650 °C) and triaxial stress (confining pressure of 5–25 MPa)/seepage pressure (0.5–6 MPa) conditions is studied. Then, using the X-ray micro-computed tomography technique, the evolution of the fracture aperture in the fractured rock subjected to various conditions is analyzed. The results indicate that for the coupled THM environment, the variations of the volumetric flow rate, nonlinear seepage parameters, and the fracture aperture of the fractured rock with the temperature and the triaxial stress are not straightforward, and the inflexion points of the curves happen at a confining pressure of 15 MPa and a temperature of 300 and 500 °C. Second, in the temperature range of 20–300 °C, in all the triaxial stress states, the volumetric flow rate and the fracture aperture decline slightly with an increase in the temperature due to the recoverable elastic compression on the propping asperities. However, in the temperature range of 300–650 °C (at a confining pressure equal to or lower than 15 MPa), the parameters rise slightly with an increase in the temperature due to the relatively large axial and lateral deformation of the semicylinders. Third, in temperature ranges of 500–650 °C (at a confining pressure equal to or lower than 15 MPa) and 300–650 °C (at a confining pressure higher than 15 MPa), the parameters rise sharply owing to the lateral fracturing of the semicylinders, so the variational trends are irregular. The findings of the current work can provide theoretical guidance on the assessment of the stability and compactness of high-temperature underground excavations.