Synergistic Bearing Effect and Failure Mechanism of Backfill–Coal Pillar Composite Structure

Constructing backfill walls adjacent to the yield coal pillar can effectively enhance its stability. The backfill and coal pillar form a synergistic bearing structure that jointly maintains the stability of the overlying strata. To reveal the synergistic bearing effect and failure mechanism of the backfill–coal pillar structure, this study first investigated the AE characteristics and energy evolution of the composite structure through uniaxial compression tests. An energy dissipation damage constitutive model was established. Subsequently, a particle flow code (PFC) model was established to study the crack propagation of the backfill–coal pillar structure. Finally, a mechanical model was developed to analyze the equivalent mechanical parameters of the backfill–coal pillar structure and elucidate its failure mechanism. The results indicate that: (1) Equal-width backfill and coal pillar exhibit optimal synergistic load-bearing effects. As the width of the coal pillar increases, the mechanical properties of the composite structure degrade nonlinearly. (2) The failure mode of the composite structure is predominantly tensile, supplemented by shear failure, with the coal pillar failing before the backfill. (3) Narrower backfills become more prone to failure under coal pillar lateral pressure. This study provides valuable insights for engineering applications involving coal pillar reinforcement using backfill walls.