Study on optimization of coal pillar size in gob-side entry driving section of inclined coal seam under dynamic and static loads

Under the condition of gob-side entry driving in inclined coal seams, the coupling of static and dynamic loads significantly complicates the stress state and instability mechanism of coal pillars. Conventional design methods for coal pillar width neglect the dynamic load effect and thus cannot guarantee mining safety. Taking the track gateway of the 2303 working face in the Yangcheng Coal Mine as the engineering background, the bearing mechanism and instability characteristics of coal pillars with different widths under dynamic and static loads were studied using the combined static and dynamic loading test of SHPB, theoretical analysis, and FLAC3D numerical simulation method, and a method to determine the width of coal pillars considering the dynamic load effect was proposed, and field measurements verified the rationality of the optimization scheme. The results show that: (1) the dynamic strength of the coal–rock composite specimens decreases with the decrease in the radial size of the coal, and when the radial size is less than 40 mm, the impact resistance of the composite specimens deteriorates seriously. (2) Based on the limit equilibrium theory, the influence coefficient of the dynamic load is introduced, and the theoretical calculation formula of the coal pillar width considering the dynamic load effect is deduced. The critical coal pillar width is calculated to be 3.81 m. (3) When the width is less than 4 m, the whole section of the coal pillar is in a plastic state, and there is no elastic bearing in the coal pillar; when the width is increased to 5 m, the proportion of the elastic zone width increases from 21.5% to 67.6%, and the dynamic load resistance of the coal pillar is significantly enhanced. (4) A 4 m coal pillar shall be set on site, and the stress and displacement of the surrounding rock shall be monitored. The deformation of the surrounding rock of the roadway shall be controlled within the allowable range of the project, and the coal pillar shall maintain a stable bearing capacity. The research conclusion has guiding significance for the accurate design of coal pillars in the gob-side entry driving section of deep-inclined coal seams.