Instability Mechanism and Stability Analysis of the Backfill–Pillar Synergistic Bearing System: A Study on Backfill Ratio and Strength

Due to the constraints of early mining conditions in some coal mines in China, a large number of pillar-type coal pillars remain in the mined-out areas. During the upward mining above the underlying pillar-type goaf, it is usually necessary to backfill the underlying goaf to form a backfill–coal pillar synergistic bearing structure, which jointly bears the load during the upward mining process. In this paper, a combination of laboratory mechanical tests and numerical simulations is used to study the failure characteristics of coal pillars, stress–strain curve characteristics, force chain transmission characteristics, and the number and distribution of fractures under the influence of backfill strength and filling ratio. The critical strength and critical filling ratio of coal pillars with different widths under the coordinated action of different backfill strengths and filling ratios are analyzed. The results show that the composite with a backfill filling ratio of 90% exhibits a stepwise change after coal pillar failure, while the composites with filling ratios of 70% and 50% show a cliff-like drop after coal pillar failure. The composite with a filling ratio of 50% completely loses its bearing capacity after coal pillar failure; the backfill is limited by its height and cannot bear the load repeatedly with the failed coal pillar, and the bearing stage lacks the common bearing stage in which the backfill wraps the failed coal pillar. The number of fractures in the coal pillar decreases with the increase in backfill strength. High-strength backfill can provide higher lateral restraint for the coal pillar through its own anti-deformation capacity. Increasing the backfill filling ratio can reduce the propagation rate of internal fractures in the coal pillar, slow down the deformation time of the coal pillar, and prevent the coal pillar from impact failure. When the coal pillar width is 8 m, the critical filling ratio of the backfill decreases from 84% to 70% as the backfill strength increases from 2 MPa to 6 MPa; when the coal pillar width is 11 m, the critical filling ratio decreases from 69% to 62%; when the coal pillar width is 14 m, the critical filling ratio decreases from 58% to 55%. The research results provide important on-site guiding significance for the safe implementation of upward mining.