Stability of coal pillars in room-and-pillar goaf with opposite mining under ultra-close shallow goafs

With longwall working face 2206 under longwall mining goaf of Haiwan Coal Mine and longwall working face 2201 under room-and-pillar goaf of Wangcaihuopan Coal Mine as engineering background, stability of coal pillars in 7 m ultra-close coal seam room-and-pillar mining goaf was researched through FLAC3D numerical calculation, physical simulation, and theoretical analysis. Results have shown that before working face 2201 is mined, with average load of residual coal pillars in room-pillar area of 7.6 MPa, and safety factor of 3.4, coal pillars are overall stable. When two working faces of lower coal seams are mined opposingly at 30 m, coal pillar stress in room-and-pillar goaf enters an influential phase of opposite mining, and a rising inflection point appears in coal pillar stress. When then working faces are mined opposingly at about 10 m, concentrated stress of coal pillars reaches its maximum of 9.1 MPa. In mutual influence section of opposite mining, maximum concentrated stress of simultaneous mining of the working faces is about 4.6% higher than that of single working face mining. In order to avoid large-scale instability above room-and-pillar goaf at working face 2201, the measure of working face 2201 stopping and working face 2206 advancing should be taken at 30 m (mutual influence section), where the safety factor of coal pillars in room-pillar area is 2.96, and the coal pillars are safe and stable. The results were verified by mining practice, which have achieved safe mining. © 2022, Editorial Board of Journal of Mining & Safety Engineering. All right reserved.