Study on rib spalling characteristics and influencing factors of coal wall in 10 m ultra-large mining height working face with thick and hard roof

To address the issue of coal wall spalling in ultra-high mining faces, the 122104 ultra-high mining face at Caojiatan Coal Mine was taken as the background, and field monitoring, numerical simulation, and theoretical analysis were used to investigate the distribution characteristics of the plastic zone, stress, and displacement of the coal wall under different roof pressure conditions. A mechanical model of coal wall spalling was established, the location of the maximum deflection point of coal wall was determined, and the mechanism on the stability of coal wall affected by the roof pressure was revealed. The results show that coal wall failure was primarily due to tensile cracking, often occurring in the middle or upper-middle areas, typically manifesting as a crescent-shaped concave or stepped form. The average depth of coal wall spalling was 0.41 m, with an average maximum spalling depth of 1.53 m. The spalling layer mainly occured at a height range of 4.4–8.0 m above the floor. The degree of coal wall spalling was directly correlated with roof pressure, with spalling exacerbated during periodic weighting. After the initial weighting, cantilever and combined cantilever structures were formed. As the mining face advances, roof collapse and coal wall damage intensify, the peak of the advanced abutment pressure and the stress concentration factor increase significantly. The degree of coal wall spalling was positively correlated with the mine pressure. Numerical simulation results show that roof pressure increased and coal wall stability decreased with increasing mining depth. The coal wall pressure under different depths exhibited an asymmetric characteristic with respect to coal wall deflection, which increases with depth. The location of the maximum coal wall deflection moved upward along the coal wall with the increase of depth, and the maximum deformation area occurred at the location of 0.59−0.94 times the mining height.