In underground coal gasification (UCG) projects, the flow of high-temperature gas leads to the simultaneous exposure of surrounding rock in tunnels to high-temperature and cyclic load, posing risks of rock instability. To investigate the failure modes and precursors of rock instability under high-temperature and cyclic loading, uniaxial compression tests and cyclic loading–unloading tests were conducted, utilizing acoustic emission (AE) technology and discrete element method (DEM). In addition, the microstructural damage of red sandstone after heat treatment was analyzed utilizing scanning electron microscopy (SEM) images. The test results reveal that increasing temperature leads to a delayed appearance of change points in AE counts and b value and more pronounced AE signals. Fatigue loading cannot always cause severe damage to red sandstone at the first cycle of the load level, according to the 25 °C group, 400 °C group, and 600 °C group specimens. RA-AF data distribution reveals that shear crack proportion generated by uniaxial compression in red sandstone subjected to the same temperature treatment is significantly greater than that caused by fatigue loading. Furthermore, the shear crack proportion in red sandstone specimens drops and then climbs with increase in temperatures. Based on the critical slowing down (CSD) theory, this paper analyzes the variance curves and autocorrelation coefficient curves of multiple AE parameters. The sudden drop point of the b value was taken as the starting point of rapid crack propagation, and the mutation points of various AE parameters lagging behind this starting point were searched for. It was found that the change point of AE energy variance is closest to the specimen instability point, which can be employed as a precursor for specimen instability. Heat treatments cause the internal structure of the red sandstone to evolve from a pompon-like structure to a flocculated porous structure. In addition, thermally induced cracks gradually develop and connect.

Jointed composite rock masses are ubiquitous, where the interplay between fractures and bedding planes significantly complicates their failure mechanisms. This study conducted uniaxial compression tests on a composite rock material featuring soft and hard components with unparallel double joints. Leveraging the digital image correlation (DIC) method and acoustic emission (AE) technology, the investigation analyzed the influence of joint angles on the mechanical properties, failure modes, and crack propagation behaviors. The results indicate that the peak strength exhibits an ascending–descending–ascending trend with the variation of upper joint angle, peaking at 90°. The failure modes can be systematically categorized into three types. The failure of the specimen is predominantly influenced by the upper joint. However, when the lower joint angle is relatively flat, it becomes more prone to contributing to the failure of the specimen. The lower joint-dominated failure mode only occurs when the upper joint angle is 90°. As the lower joint angle remains at 90°, an augmentation in the upper joint angle initially leads to a decrease and subsequently to an increase in the proportion of shear cracks. In contrast, when the lower joint angle deviates from 90°, the increase of the upper joint angle initially boosts and then diminishes the proportion of shear cracks.

The stability of bottom structures in block caving mines is significantly challenged by impact loads generated from large rockfalls and ore collapses on slopes. This study aims to investigate the mechanical response and failure characteristics of bottom structures under such dynamic and cyclic loading conditions. Discrete element methods (DEMs) were employed to simulate the impact load amplitudes caused by large rockfalls on bottom structures. Specimens with identical mechanical properties to the bottom structure were fabricated at a 1:100 scale, based on the principle of similarity ratio tests. Three distinct types of impact loads were identified and analyzed: overall impact from large-scale slope collapses, localized impact from partial rock and soil mass collapses, and continuous multiple impacts from progressive slope failures. True triaxial tests were conducted to evaluate the mechanical response of the bottom structure under these loading scenarios. The results indicate that while overall and multiple impact loads from slope collapses do not lead to catastrophic failure of the bottom structure, severe damage occurs under a 100 m thickness of ore and large block impacts. Specifically, the inner walls of ore accumulation troughs peel off, and ore pillars between troughs fracture and fail. This study highlights the need for advanced experimental and numerical approaches to accurately predict the stability and failure modes of bottom structures under complex loading conditions.