Optimization of paste backfill material proportion and roof control in strip mining of a fully mechanized face

To address the environmental pollution caused by long-term coal gangue discharge in western mining areas, as well as surface subsidence and associated geological hazards induced by coal mining, this study takes the Shendong Mining Area in western China as the research subject. Based on the geological conditions of the mining area, a fully mechanized longwall strip paste backfill mining process is adopted. The movement behavior of surrounding rock in the backfill working face is systematically analyzed, and the roof control strategy and load-bearing characteristics of the backfill body are determined, including the ultimate caving step distance L of the working face roof along the strike, the backfill body width S, and the paste strength P_p. Through physicochemical analysis of coal gangue, multiple paste backfill material formulations using coal gangue as the primary raw material were developed. Using spreadability, slump, and compressive strength as evaluation indicators, the optimal mix proportion of the backfill material was determined. Meanwhile, FLAC^3D numerical simulation software was employed to analyze the influence of paste backfill bodies with different strengths on roof control and surface subsidence, verifying the rationality of the designed strip backfill strength, backfill material mix proportion, and roof control scheme for fully mechanized mining. The results show that the optimal mix proportion of the coal gangue-based paste backfill material is a fly ash-to-coal gangue mass ratio of 3∶7, with cement accounting for 14% of the total mass. The compressive strength of the backfill material reaches 5.4 MPa after 28 days of curing. Numerical simulation results indicate that the strength of the paste backfill body with this mix proportion meets the roof control requirements of the working face. The research findings provide valuable guidance for the comprehensive utilization of coal-based solid waste and the advancement of green mining in western coal mining regions.