Optimization and Research on Coal Pillar Width in Roadway Excavation Along Goaf Based on the Second Invariant of Deviatoric Stress

In order to determine the reasonable coal pillar width between the 4220 roadway and the 4308 goaf in Chengzhuang Mine, a combination method of theoretical calculation, numerical simulation, and on-site testing is adopted to carry out research. Based on the theory of limit equilibrium zone, the reasonable range of coal pillar width is calculated to be 7.41-8.55 m. By establishing FLAC^3D numerical model, and taking the second invariant of deviatoric stress as the key criterion, the stress distribution of coal pillars and plastic zone evolution laws under different coal pillar widths are analyzed. The results show that when the coal pillar width is 8 m, the coal pillar transitions from a plastic state to forming a stable elastic core, its bearing capacity is significantly improved, the concentration degree of shear stress in the core area is relatively controllable, and the plastic zone has a small range and weak connectivity. The optimal coal pillar width is comprehensively determined to be 8 m. The on-site industrial test adopts a coal pillar width of 8 m to excavate the 4220 auxiliary roadway. The monitoring indicates that the final displacement amount of the roadway two sides is 200 mm, the final subsidence amount of the roof is 100 mm, and the surrounding rock deformation control effect is good.