With the global push towards energy transition, understanding the mechanical behavior of rocks under high-temperature conditions is becoming increasingly significant, such as nuclear waste disposal. Previous studies have demonstrated that the high-temperature heating has a great impact on the physical and mechanical properties of sandstone, exhibiting both thermal strengthening and deterioration phenomena. However, research on the characteristic stress and microcrack evolution of sandstone under thermal strengthening and deterioration conditions remains limited. The objective of this study is to examine the characteristics of stress, microcrack evolution, and failure mechanisms in sandstone under a range of high temperatures. Sandstone samples were heated respectively to 200°C, 400°C, 600°C and 800°C. The deformation and failure processes were monitored using acoustic emission (AE) and digital image correlation (DIC) techniques. The evolution of characteristic stress and microcrack development under both thermal strengthening and deterioration conditions was analyzed. The results show that the crack closure stress and its threshold ratio are proportional to temperature. When sandstone exhibits strength deterioration, the crack initiation stress and its threshold ratio decrease with increasing temperature. Conversely, when strength enhancement is observed, the crack initiation stress increases, while the threshold ratio initially rises and then declines. Moreover, the effect of thermal treatment on axial deformation is more pronounced than on radial deformation, with the elastic constant and Poisson’s ratio gradually decreasing as temperature rises. Microcrack evolution was predominantly tensile across temperatures, but the macroscopic failure modes varied: monocline shear failure at 25°C, tensile failure at 200°C, 400°C, and 600°C, and a mixed shear-tensile failure at 800°C, where shear cracks became most prominent. This study enhances the understanding of sandstone’s mechanical behavior under high-temperature conditions and provides valuable insights for engineering applications involving thermal treatment.

The utilization of artificial ground-freezing techniques is increasingly prevalent in the construction of water-diversion tunnels. The inadequate mechanical properties of weakly consolidated mudstone (WCM) pose significant challenges for tunneling construction. In this study, a series of triaxial shear tests were conducted on frozen specimens of WCM to elucidate its shear strength characteristics. The experiment involved four freezing temperatures (0, −5, −10, and −20 °C) and four confining pressures (1, 2, 3, and 4 MPa). The results indicate that the shear failure mode of the WCM exhibits distinct shear zone failure characteristics under artificial freezing conditions, particularly prominent in the lower temperature environments. As the freezing temperature gradually decreases, there is a substantial increase of over 200% in the shear strength of frozen specimens, accompanied by a corresponding decrease in yield strain. Furthermore, the cohesion and internal friction angle of frozen WCM increase as the freezing temperature decreases, following a complex exponential function relationship rather than a linear one. As the freezing temperature decreases from 0 °C to −20 °C, there is an increase in cohesion and internal friction angle from 1012 kPa to 1425 kPa, accompanied by a rise in the internal friction angle from 43.2° to 58.1°. Notably, the application of confining pressure exerts a pronounced influence on the shear strength of frozen WCM, with elevated levels of confining pressure resulting in a substantial augmentation of the shear strength. The failure mode of frozen WCM is significantly influenced by freezing temperatures. At low temperatures, the specimen of mudstone exhibits a shear failure behavior, while at high temperatures, it predominantly demonstrates expansion failure. This phenomenon can be attributed to the increased brittleness of specimens caused by ice crystals, rendering it more susceptible to brittle failure under shearing forces. These findings signify an enhancement in the mechanical behavior of WCM within the tunnel sidewall under artificial freezing conditions.