Optimization of section coal pillar size for fully mechanized top-coal caving mining in extra-thick coal seams under soft rock and extremely soft rock conditions

Focusing on the optimization of section coal pillar size for fully mechanized top-coal caving mining in extra-thick coal seams under soft and extremely soft rock geological conditions, this study takes the 1101 working face of the B1 coal seam in Xinjiang Zhundong No. 2 Mine as the engineering background. By comprehensively applying limit equilibrium theoretical analysis, numerical simulation, and energy evolution analysis methods, the stress distribution, plastic zone development, and dissipated energy characteristics under different coal pillar widths are investigated. The results show that the theoretically calculated minimum coal pillar width is 41.46 m. Numerical simulation reveals that when the coal pillar width increases to 45 m, the peak vertical stress stabilizes at 35.0 MPa, the plastic zone depth on the goaf side approaches zero, the dissipated energy decreases significantly, and the deformation of the surrounding rock of the roadway is minimized. The stable boundary of the lateral abutment pressure on the solid coal side is 42.7 m, with an error of 2.9% compared to the theoretical value. It is ultimately determined that the reasonable width of the section coal pillar for a mining height of 16.8 m under soft rock conditions is 45 m, which can effectively bear the overburden load, inhibit plastic failure, and enhance roadway stability.