The behavior of fluid flow through rock fractures is a critical issue in many subsurface rock engineering projects. Previous studies focused more on the permeability evolution of rock fractures under loading stage, while the change of fracture permeability under unloading stage is more consistent with the actual engineering excavation. To examine the unloading-induced changes in fracture permeability under different temperatures, we performed flow through tests on fractured core samples subjected to decreasing confining pressures and constant temperatures. The results show that the permeabilities of fractures under unloading confining pressure increase but decrease with increasing temperature because of fracture closure caused by asperity damage and thermal expansion. A coupled thermal-mechanical model considering asperity damage is used to explain the unloading-induced changes in fracture permeability. The coupled thermal-mechanical model properly predicts the experimental results.
In the underground space creation or energy extraction, the continuous excavating inevitably breaks the original mechanical equilibrium state of rock masses, and the stress redistribution occurs in the surrounding (Florence et al. 2010). Fracture permeability is strongly dependent on stress paths, i.e., fracture permeability exhibits hysteretic behavior (Barton et al. 1985; Selvadurai 2015). Therefore, it is of practical significance to consider the effects of cyclic unloading on the seepage properties of fractured rocks.
Formation temperature increases with depth, and the increase of temperature will lead to the reduction of elastic modulus and compressive strength, as well as the increase of ductility in the post-peak zone (Huston et al. 2009), and the thermal expansion will cause the change of rock microstructure (Chaki et al. 2008). A number of tunnels are being or will be constructed in extremely warm or cold regions, e.g., the Blunkou-Gongger Hydropower Station in Xinjiang, China, is 82 °C, and the tunnel of Qirehataer Hydropower Station in Xinjiang, China, is up to 98 °C (Zeng et al., 2020; Zhao et al. 2021). The influence of temperature on permeability should be paid attention in the study.
