To investigate the mechanical bearing characteristics of the “excavation-backfill” composite after the excavation of coal pillars and backfill replacement with gangue-based cemented paste backfill, mechanical bearing characteristic experiments are conducted on a series of coal samples with rectangular “excavation-backfill” roadways under uniaxial loading, covering the full deformation and failure process. The MTS universal testing machine and DS5-type acoustic emission signal acquisition system are employed, and a high-speed camera is adopted to monitor and record the full failure process. The mechanical bearing characteristics and crack evolution mechanisms of unfilled coal pillar (U-C) and backfill coal pillar (B-C) samples are explored. The results show that with the increase in “excavation-backfill” width, the uniaxial compressive strength and elastic modulus of U-C samples decrease significantly, and the samples exhibit brittle–ductile failure. When the “excavation-backfill” width is 60 mm, the backfill can distinctly improve the strength and elastic modulus of B-C samples, showing a strong strength recovery effect. The temporal characteristics of AE signals indicate that both U-C and B-C samples experience four stages subjected to uniaxial compression: quiet period, rising period, active period, and post-peak rising period. In the quiet period and rising period, the b-value fluctuates upward with energy release; in the active period, the b-value decreases significantly with large energy release; in the post-peak rising period, crack propagation and frictional slip increase, leading to an enlarged fluctuation amplitude of the b-value. Based on the location of AE sources, the three-dimensional crack chain evolution is inverted. The crack chain evolution of the U-C is mainly distributed along the dip direction (75°~90°, 255°~270°) and vertical direction (165°~180°) of the coal bedding plane, while the B-C is more uniform, indicating that the backfill evidently affects the crack distribution. This study provides new insights for predicting the crack evolution and failure mode of coal–rock composites.

Coal and gas outburst disasters severely restrict the safe and sustainable development of deep coal resources. Outburst fluid migration is violent and highly nonlinear, posing significant challenges to its mathematical modeling. This paper presents a high-Mach-number compressible flow model for outburst fluid migration. By combining physical and numerical simulation methods, the spatiotemporal evolution characteristics of the outburst two-phase flow were revealed, and the influence of the turning angle on the migration law of the outburst fluid was analyzed. The results show that the impact pressure presents a strong–weak alternating distribution characteristic in the roadway. In the straight roadway, the impact pressure under the non-arc condition is generally higher than that under the arc condition. In the turning roadway, the impact pressure of arc and non-arc roadways increases sharply, especially in the non-arc roadway. The arc turning structures are helpful in promoting the passing ability of pulverized coal when compared with the non-arc turning structures. An arrangement of acute and right angles effectively reduces the disaster intensity of the dynamic pressure of airflow in the straight roadway but increases the disaster intensity after the turning roadway. An obtuse angle arrangement increases the disaster intensity of static pressure in the whole roadway.