Bearing rule and instability criterion of pier column roadside supports of GER in fully mechanized mining face

The stability of the rock mass at the interface between roadside supports and the roof is critical for gob-side entry retaining. To address the issue of 'roof piercing and column failure' caused by instability at the pier-column and roof contact interface in thick coal seam fully mechanized caving faces, analyzed the key loading stages on the underlying coal-rock roof during the main roof fracture and movement. A mechanical model was developed to simulate multi-span discontinuous loading, and an additional stress equation induced by the pier-column was derived. The stress transfer mechanism and instability modes at the pillar-roof interface were investigated, a slip model based on the Mohr-Coulomb criterion was constructed to identify the factors affecting the stability of the top coal. Experimental results indicated that energy concentration is highest near the pier-column top and decreases with depth, while the mid-span roof between adjacent columns exhibits lower shallow energy concentration, which increases with depth. The greatest energy gradient occurs at the pillar-roof interface edge, leading to coal deformation and slip. When top coal thickness, residual boundary distance, and loading width remain constant, the ultimate load is determined by the cohesion and friction angle of the top coal, both of which positively correlate with strain rate. Shorter dynamic pressure durations during overlying strata movement increase the ultimate bearing capacity at the pier top. Numerical simulations revealed that strengthening the coal at the roof improves the ultimate bearing capacity at the pillar-roof interface, creating a more stable force chain between columns and reducing the risk of “roof piercing and column failure”. Based on the 91-101 roadway at Wangzhuang Coal Mine, reasonable support parameters and a top coal reinforcement plan were proposed. Field implementation confirmed that this approach effectively prevented “roof piercing and column failure”, while keeping surrounding rock deformation within acceptable limits.