The physical and mechanical properties of rocks change significantly after being subjected to high temperatures, which poses safety hazards to underground projects such as coal underground gasification. In order to investigate the effect of temperature on the macroscopic and microscopic properties of rocks, this paper has taken sandstone as the research object and conducted uniaxial compression tests on sandstone specimens at different temperatures (20–1000 °C) and different heating rates (5–30 °C/min). At the same time, the acoustic emission (AE) test system was used to observe the acoustic emission characteristics of the rock damage process, and the microstructural changes after high temperature were analyzed with the help of a scanning electron microscope (SEM). The test results show that the effect of temperature on sandstone is mainly divided into three stages: Stage I (20–500 °C) is the strengthening zone, the evaporation of water and the contraction of primary fissures, and sandstone densification is enhanced. In particular, the compressive strength and elastic modulus increase, the macroscopic damage mode is dominated by shear damage, and the fracture micromorphology is mainly brittle fracture. Stage II (500–600 °C) is the transition zone, 500 °C is the threshold temperature for the compressive strength and modulus of elasticity, and the damage mode changes from shear to cleavage damage, and the sandstone undergoes brittle–ductile transition in this temperature interval. Stage III is the physicochemical deterioration stage. The changes in the physical and chemical properties make the sandstone compressive strength and modulus of elasticity continue to decline, the macroscopic damage mode is mainly dominated by cleavage damage, and the fracture microscopic morphology is of a more toughness fracture. The effect of different heating rates on the mechanical properties of sandstone was further studied, and it was found that the mechanical properties of the rock further deteriorated under higher heating rates.

The process of micro crack formation in sandstone subjected to freeze-thaw cycles is pivotal for strip mine in cold regions. A discrete element numerical model considering the variation of frost heave force and water saturation during the freeze-thaw cycles is established by PFC3D in this paper. The results indicate that: (1) During freeze-thaw cycles, the development of cracks can be divided into three stages: stable, growth and explosive stage. The crack quantity in the stable and growth stage increases exponentially, while the quantity of cracks in the explosive stage increases linearly quickly. (2) Repeated freeze-thaws cause progressive damage to rocks from surface to interior. As the times of freeze-thaw increase, cracks gradually develop, and their distribution becomes more uniform on different angles. (3) Sandstone that have undergone freeze-thaw cycles is more prone to tensile failure, and the distribution of cracks becomes even more complex. These research findings provide a more accurate understanding of micro-fracture mechanisms in rocks in the freeze-thaw condition.

The damage and degradation of coal-measure sandstone in cold-region open-pit mines due to freeze–thaw effects has become one of the significant factors inducing instability in the rock mass of open-pit mine slopes. This study conducts experiments on the physical and mechanical properties of saturated coal-measure sandstone under varying freeze–thaw cycle counts and freezing temperatures, revealing the intrinsic mechanisms of damage and degradation in saturated coal-measure sandstone due to freeze–thaw effects. The experimental results indicate that, with an increase in the number of freeze–thaw cycles and a decrease in the freezing temperature, the elastic modulus and peak compressive strength of the specimens exhibit an exponential decrease. In contrast, the peak strain shows an exponential increase. However, compared to the freezing temperature, the increase in the freeze–thaw cycle frequency leads to a more significant change in the mechanical parameters of the specimens, indicating that the frequency of freeze–thaw cycles has a more pronounced effect on the deformation resistance of saturated coal-measure sandstone than the freezing temperature. The failure mode of coal-measure sandstone specimens under uniaxial compressive loading primarily exhibits shear failure; however, as the number of freeze–thaw cycles increases and the freezing temperature decreases, the specimens begin to exhibit tensile failure modes, which gradually develop into a combined tensile and shear failure mode. Based on the experimental data, two sets of surface equations were fitted to characterize the relationship between the mechanical properties (peak compressive strength, elastic modulus) of the specimens and the experimental parameters (number of freeze–thaw cycles, freezing temperature). The research findings can provide references and insights for engineering disasters caused by the degradation of coal-bearing sandstone in cold-region open-pit mines.

Backfilling in underground mining is a widely adopted technique, often employing cement-based or geopolymer materials. However, the high cost and logistical challenges associated with these materials in remote mining locations necessitate alternative solutions. Loess materials offer a viable option for reducing production expenses and mitigating environmental impact. Despite this, the mechanical properties of loess-based cemented materials have been insufficiently investigated. This study introduces a novel slag-loess-based cemented backfill material. A three-dimensional model of the backfill material was constructed using Avizo software, and a numerical calculation model, incorporating the actual three-dimensional geometry of aggregate particles, was developed using PFC3D software. The influence of confining pressure σ₃, curing temperature T_C, and age T_A on the crack evolution, force chain evolution, and particle failure characteristics during the loading process of the cemented backfill material was systematically examined. Results demonstrate that increasing confining pressure σ₃ accelerates crack initiation and propagation, concurrently increasing the maximum value of the dominant force chain within the specimen at each stage. An increase in age T_A has a limited effect on the proportion and quantity of final tensile-shear cracks, though it does intensify the specimen’s ultimate shear failure tendency. At lower curing temperatures (5°C), a significant impact on crack formation is observed, with this effect diminishing as curing temperature T_C increases. The macroscopic failure modes of the specimens, across varying temperatures, predominantly exhibit shear failure.

Coal-bearing rocks are inevitably exposed to high temperatures and impacts (rapid dynamic load action) during deep-earth resource extraction, necessitating the study of their mechanical properties under such conditions. This paper reports on dynamic compression tests conducted on coal-bearing mudstone specimens at real-time temperatures (the temperature of the rock remains constant throughout the impact process) ranging from 25 °C to 400 °C using a temperature Hopkinson (T-SHPB) test apparatus developed in-house. The objective is to analyze the relationship between mechanical properties and the fractal dimension of fractured fragments and to explore the mechanical response of coal-bearing mudstone specimens to the combined effects of temperature and impact using macroscopic fracture characteristics. The study found that the peak stress and dynamic elastic modulus initially increased and then decreased with increasing temperature, increasing in the 25–150 °C range and monotonically decreasing in the 150–400 °C range. Based on the distribution coefficients and fractal dimensions of the fractured fragments, it was found that the degree of damage of coal-bearing mudstone shows a trend of an initial decrease and then an increase with increasing temperature. In the temperature range of 25–150 °C, the expansion of clay minerals within the mudstone filled the voids between the skeletal particles, resulting in densification and decreased damage. In the temperature range of 150–400 °C, thermal stresses increased the internal fractures and reduced the overall strength of the mudstone, resulting in increased damage. Negative correlations between fractal dimensions, the modulus of elasticity, and peak stress could be used to predict rock properties in engineering.

Comprehending the effect of freeze-thaw cycles on the damage and degradation of coal-bearing sandstones is crucial for the end-wall slope stability of open-pit mines in cold areas. In this study, freeze-thaw cycle tests on water-saturated coal-bearing sandstone samples under different freezing temperatures and different freeze-thaw cycles were conducted by a fully automatic low-temperature freeze-thaw testing system, and the effects of freeze-thaw cycle parameters on P-wave velocity and porosity of sandstone samples were obtained. With the assistance of CT scanning imaging technology, the microscopic damage and deterioration mechanism of sandstone samples under freeze-thaw cycles was further revealed, and a characterization method for the damage and deterioration of sandstone samples under freeze-thaw cycles was established, and damage and degradation effects of freeze-thaw cycles on the sandstone samples were predicted. The research results suggest that as the freezing temperature decreases and the number of freeze-thaw cycles increases, the P-wave velocity of the sandstone sample decreases, while the volume of the sandstone sample increases. The relative change rate of P-wave velocity and porosity increment of the sample are positively correlated with freezing temperature, and negatively correlated with the number of freeze-thaw cycles. The CT scan results show that with the decrease of the freezing temperature and the increase of the number of freeze-thaw cycles, the number and geometric size of pores on the sample cross section increase significantly. Additionally, the evolution equation of freeze-thaw damage factors was established with freezing temperature and number of freeze-thaw cycles as parameters, and the internal mechanism and physical characterization of freeze-thaw damage degradation of coal measure sandstone were revealed. This research provides a reference for the safety and stability evaluation and technology research and development of related rock engineering in cold areas.