Rock destruction under high‐temperature conditions is a key issue for nuclear waste treatment projects, underground coal gasification, and improvement of the use of geothermal energy for heating. Therefore, in this study, various methods and techniques were integrated to study the changes in mechanical properties, mineral composition, and microscopic fracture characteristics of Sichuan sandstone treated at 600°C. First, the fracture toughness and indirect tensile strength of untreated sandstones and high‐temperature treated sandstones were tested by the MTS testing machine, and the double‐K model (DKFM) was used to estimate the unstable fracture toughness. Then, the diffraction spectra of sandstone were analyzed with an X‐ray diffractometer to determine the mineral composition change after heat treatment. Finally, the microscopic features of sandstone were observed through a scanning electron microscope (SEM) and optical microscope. The results show the following. (1) There is no significant change in the tensile strength and fracture toughness of Sichuan sandstone treated at 600°C. (2) The brittleness of Sichuan sandstone decreases and the ductility increases after high‐temperature treatment. (3) The unstable fracture toughness value K_un obtained by the double‐K model (DKFM) is significantly larger than the apparent fracture toughness value K_if. (4) After treatment at 600°C, the clay minerals in the sandstone changed significantly. Kaolinite is dehydroxylated to metakaolinite, which may increase the ductility of the rock.

High temperature can change the mechanical properties of granite, with significant nonlinear characteristics, and at the same time change its microstructure. Therefore, two kinds of granites are used in this paper: one is normal temperature granite and the other is granite treated at 600°C, and a detailed comparative study is made. The fracture toughness of two kinds of rocks was tested by fracture tests, and the results were analyzed by a nonlinear fracture mechanics model (SEL). At the same time, in order to understand the influence of high temperature on the mineral composition and microstructure of granite, XRD, optical microscope, and SEM were used to observe the mineral composition, microcracks, and fracture morphology of granite. The results show the following: (1) high temperature significantly changes the fracture mechanics parameters of granite. The fracture toughness of granite treated at 600°C is significantly lower than that of untreated granite, which is reduced by more than 60%. (2) No obvious size effect was found in the untreated granite, while the size effect of the granite after treatment at 600°C was significant. (3) The granite after high‐temperature treatment showed strong nonlinear characteristics, and the SEL can reasonably describe and explain its nonlinear fracture characteristics. (4) The brittleness of the granite treated at 600°C decreased and the ductility increased. The microscopic morphology of the fracture was rough, with obvious steps and rivers. The microcracks and porosity had increased significantly, but the main components did not change significantly.

It is of great significance to study the reasonable position of mining roadway under strip coal pillar for increasing the stability of mining roadway, reducing the waste of resources, and realizing the safety production of working face. Based on the research background of the working face under the strip coal pillar in Jinqiao Coal Mine of Jining, Shandong Province, through theoretical analysis, similar material simulation experiment, and numerical simulation experiment, the stress distribution law, plastic failure range, and rationality of coal pillar setting in different width sections are systematically studied. Finally, the tailentry of working face is determined at the position of 5m from the bottom of strip coal pillar to 1308 goaf. During the mining period of 1310 working face, the peak value of side abutment pressure is at the position of 34m; beyond 25m in front of the coal wall, the deformation of the surrounding rock on the tailentry surface is small. After entering the advanced support section, the deformation of the two sides is mainly longitudinal crack expansion and local shallow small flakes; however, the roof is complete and stable. Therefore, the selection of tailentry location and coal pillar width has played a good role. The research results of this study can provide some reference for similar mine with similar geological and production technical conditions.

Aiming at the backfill with prefabricated fracture under seepage‐stress coupling, the concepts of fracture macrodamage, loaded mesodamage, seepage mesodamage, and total damage of backfill were proposed. Based on the macroscopic statistical damage model, the coupling effect of seepage, stress, and initial fracture was considered comprehensively and the damage model of backfill with prefabricated fracture under seepage‐stress coupling was established. The mechanical properties of backfill with prefabricated fracture under different seepage water pressures and confining pressures were tested and the rationality of the model was verified. The research shows that the mechanical properties of backfill with prefabricated fracture under the seepage‐stress coupling are determined by the seepage water pressure, the load, the initial fracture, and the coupling effect. Fracture and seepage have significant effects on the damage of the backfill. When the seepage water pressure is low, the fracture damage dominates; however, when the seepage water pressure is high, the seepage damage dominates; the total damage under the coupling action is more serious than the single factor. The development laws of the total damage evolution curves under different seepage water pressures and confining pressures are basically the same, and they show the S‐shaped distribution law with the increase of the axial strain. With the increase of confining pressure, the damage effect of fracture and seepage on the backfill is weakened, indicating that the confining pressure has a certain inhibitory effect on the damage evolution of the backfill. The research results can provide a theoretical basis for the study of the stability of backfill with geological defects such as joints and fractures in deep high‐stress and high‐seepage water pressure coal mines.