Instability mechanism of deeply buried coal seam floor under mining effects and optimization of extraction roadway layout

Mining-induced stress redistribution leads to rock deformation, fracture propagation, gas outflow, and floor water inrush, which present significant challenges for the layout of extraction roadways. To optimize the layout in deeply buried coal seams, a stress distribution model for the floor under mining effects was developed based on semi-infinite body theory. This model elucidates the characteristics of floor stress distribution. Simulations were conducted to analyze surrounding rock stress in extraction roadways with various layouts, resulting in a proposed optimized configuration through comparative analysis. The principal findings are as follows: (1) The floor stress model indicates that vertical stress decreases with depth, exhibiting a notable reduction after reaching 5 m below the floor. (2) Numerical results align closely with theoretical patterns; specifically, vertical stress forms an “M” shape within the goaf while peak stresses diminish as depth increases. Maximum decay of vertical stress occurs at approximately 10 m beneath the floor. (3) Vertical stress peaks reach their minimum value of 10.55 MPa when the inner offset coal pillar measures 30 m; similarly, horizontal stress peaks are minimized at 19.37 MPa with an outer offset also set at 30 m. Notably, the failure zone is smallest—approximately 2 m—when employing this outer offset arrangement. (4) The optimized layout positions the extraction roadway at a depth of 17 m below the floor while maintaining a coal pillar with an outer offset of 30 m. (5) Monitoring efforts conducted at a Shanxi coal mine revealed that the failure range extended to only 1.9 m, corroborating theoretical predictions and indicating no significant deformations occurred. This study provides a robust theoretical foundation for determining optimal positioning of extraction roadways within deeply buried coal seams and serves as a valuable reference for analogous mining projects.