To address the limitations of traditional coal pillar design methods in engineering practice for deep retreat mining roadways, this research focuses on the retreat mining face at the Pinggang Coal Mine in Heilongjiang Province. Combining mechanical experiments, theoretical calculations, numerical simulations, and field tests, this study calculates the ultimate equilibrium size of coal pillars. It also analyses the stress transfer and surrounding rock plastic evolution characteristics under different coal pillar sizes in deep retreat mining faces and carries out a field engineering application. After the factors mentioned above are analysed, the appropriate size for coal pillars that align with the current state of the surrounding rock in deep mines is established. The research findings indicate a correlation between the theoretically calculated ultimate stability range of coal pillars and the numerical simulation results on coal pillar size, suggesting a close consistency. The determined ultimate equilibrium coal pillar size falls within the range of 7.12–8.22 m. When the coal pillar size changes, the stress around the deep retreat mining roadway is transferred, and the plastic state of the surrounding rock also gradually changes. When the coal pillar width is 3–6 m, a stress concentration appears in the coal body, and the coal pillar has a lower bearing capacity and is prone to plastic failure. When the width is 7–8 m, the stress concentration gradually shifts to the sides of the coal pillar, enhancing its control over the roof, and a loosening zone forms in the surrounding rock. When the coal pillar width reaches 9–10 m, the stress concentration shifts completely from the coal body side to the coal pillar side, and the bearing performance of the coal pillar no longer improves. Plastic failure of the coal pillar disappears, but a greater degree of stress concentration occurs on the side of the coal pillar. The optimal size of the coal pillar is determined to be 7–8 m. Verified by the coal pillar retention engineering application at the Pinggang Coal Mine, when an 8 m coal pillar is retained, a peak stress of 27.54 MPa appears 12 m from the roadway, and the maximum deformations of both sides, the roof, and the coal pillar side are within the ideal range, indicating a relatively good overall state of the roadway. These research results provide new theoretical and practical guidance for minimizing coal loss and eliminating the safety hazards posed by unreasonable coal pillar retention at the Pinggang Coal Mine and offer a reference for the reasonable retention of coal pillars under similar conditions in the Northeast China mining area, thereby promoting the sustainable development of the coal industry.

Under the influence of coal mining, the gravel in mining tunnel sections of a fault fracture zone is prone to collapse, and the collapse accumulation body will block the tunnel, which has a very adverse influence on the safety production of coal mining and the evacuation of personnel after underground disasters. The macroscopic and mechanical characteristics of the collapse accumulation body have been studied extensively in previous works. The purpose of this paper is to provide theoretical support and reference for the rapid excavation of the tunnel blocked by the collapse accumulation body in the fault fracture zone. Taking the fault fracture zone in the tunnel as the research background, the physical characteristics and boundary mechanical characteristics of the collapse accumulation body in the fault fracture zone are studied by the method of combining on-site investigation and theoretical analysis. The results show that the force acting on the boundary on both sides of the accumulation body is passive resistance from the side wall, which is derived from the slip effect of the accumulation body slope. Similarly, the unstable boundary of the fault fracture zone caused by tunnel instability is elliptical, and the overlying load of the rescue channel to be excavated in the accumulation body is limited. On the basis of the collapse instability dimensions of the broken zone of the tunnel surrounding the rock, the calculation formulas of the height of the accumulation body and the horizontal force at the boundary were established, respectively, under two conditions of whether the collapse space was filled, and whether the curve relationship between the distribution of the horizontal force at the boundary of the accumulation body and the buried depth in the accumulation body was obtained.