Abstract：Aiming at the coal pillar instability in steep coal seam section under the mountain landform, it is easy to cause the coal pillar group linkage instability, resulting in large cracks in the roof penetrating through to the surface of the landslide and other geological disasters. Based on No. N2821 Face of Fengchun Mine in southwest China, a coal pillar stress model was established to analyze the necessary conditions and reasonable size calculation of the coal pillar instability. The Local Space-Viewer, ArcMap and Rhino3D software were comprehensively used to acquire and process terrain contours and produce 3D geomorphic grid in Fengchun Mine in southwest China, and the 3D mountain geomorphic feature numerical model was established in FLAC3D software, and then the evolution law of coal pillar bearing stress in steep coal seam section under the mountain was studied. The theoretical analysis showed that when the coal seam inclination was greater than the inverse tangent value of the sum of the friction coefficients of the upper and lower surfaces of the coal pillar, the coal pillar had sliding instability. The reasonable coal pillar size of No. N2821 Face was 12 m. The simulation results showed that with the advance of the working face, the coal pillar in the middle of the goaf was most affected by the superposition of stress, and the shear stress on the surrounding rock of the coal pillar gradually increased and concentrated towards the coal pillar. Although the plastic failure of coal pillar under the influence of mining led to the reduction of bearing stress, it still had good supporting capacity. The analysis of field monitoring data was consistent with the simulation conclusion, and the size of coal pillar was reasonable. The research results had certain reference significance for coal pillar under similar conditions.
