Research on Strata Movement Control in Deep Composite Lithologic Gangue Backfill Mining of Super-Thick and Weak Cementation Overburden Based on PFC–FLAC Coupling Simulation

In order to alleviate geological disasters such as rock burst, mine earthquake, and surface subsidence and realize the coordinated development of coal resources safe mining and environmental protection in Ordos coalfield, this article proposes a local filling and multiface coordinated mining rock movement control method. The influence of movement deformation of complex lithologic gangue fills and overburden structure characteristics on the synergistic effect of complex carrier-overburden structure is discussed by PFC–FLAC coupling simulation. The results show that the synergistic effect between the overburden structure and the composite carrier is greatly affected by the spatial location of the master-subkey layer. The synergistic effect between the overburden structure and the composite carrier of the upper main key layer and the lower subkey layer forms a complete mushroom arch or half mushroom arch in the overburden rock, while the synergistic effect between the overburden structure and the composite carrier of the lower main key layer and the upper subkey layer does not form a strong synergistic effect. The composite carrier mainly acts as an isolation pillar to isolate the gob into several non-critical gob. In contrast, the secondary compression deformation of the composite lithologic gangue has a great influence on the mining behavior of the upper main key strata and the lower sub-key strata, and the bearing range of the composite carrier is reduced by 4.6% to 5% at most, and the synergistic interaction between the composite bearing coal pillar and backfill body evolves in a pattern of initial enhancement followed by gradual weakening. In addition, after calculation, the secondary compression rate of the composite rock gangue reaches 7% to 8%, ultimately leading to a secondary surface subsidence rate of 19%. Therefore, considering the influence of the composite rock gangue filling and its secondary compression deformation is the main link to achieve precise control of surface subsidence.