Study on the rational position of externally offset mining roadway in the lower split during layered mining of a thick coal seam

To improve coal recovery rates and address the challenge of determining optimal locations for the external offset of lower split mining roadways in thick coal seams, a targeted study was conducted using the 52,202 working face mining roadway at the Wulansetai Coal Mine in Shenmu City, Shaanxi Province, China, as the research context. First, a systematic computational analysis was conducted on the strength of the wide coal pillar in the 60-meter section of the upper stratum and the load it bears. The results show that the strength of this coal pillar is 15.7 MPa, which is significantly greater than the 6.7 MPa load it bears under the influence of the goaf; Based on the theory of limit equilibrium, further calculations show that the width of the limit equilibrium zone for the wide coal pillar in the upper stratum section is 13.08 m, with its elastic zone extending from 13.08 m to 46.92 m from the right edge of the coal pillar. Second, using the finite difference numerical simulation software FLAC3D, we simulated and analyzed the evolution of the stress field and displacement field in the lower-level mining roadway under different external offset distances. Simulation results indicate that when the external offset distance for a stratified mining roadway beneath a thick coal seam is set to 24 m, the stress distribution in the surrounding rock is relatively uniform, deformation is minimized, and the difficulty of roadway maintenance is lowest, making this the most feasible option. Finally, this optimized design was implemented in the 52,202 working face mining roadway at the Wulansetai Coal Mine. Field results confirmed that the 24 m external offset distance for the lower-level mining roadway fully meets the mine's requirements for safe and efficient production. The findings of this study provide scientific guidance and engineering insights for the rational layout of stratified mining roadways beneath thick coal seams under similar geological conditions.