Stability in Composite Bearing Structures within Room-and-Pillar Goafs under Repeated Mining

During repeated mining of shallow and closely spaced coal seams, the failure of coal pillars within the upper goaf can induce dynamic hazards—such as shield jamming, support collapse, and roof fall—at the lower fully mechanized working face. To assess the stability of composite bearing structures, this study adopts a comprehensive approach that integrates orthogonal experimental design with single-factor experiments, supported by numerical simulation methods. Firstly, a composite bearing structure model is developed based on the engineering conditions of the Xingelao Coal Mine, followed by a comprehensive mechanical analysis. Secondly, experimental variables such as cement content, fly ash content, river sand content, and solid/slurry concentration are considered to systematically analyze their impact on backfill strength through proportion adjustment experiments. Furthermore, the controlled variable method is applied to adjust the backfill ratio, ultimately determining the optimal backfill mix ratio S8-2/3, which demonstrates a 7-day uniaxial compressive strength of 2.70 MPa with a backfill ratio of 2/3. This ratio satisfies both the mine’s strength requirements and cost-effectiveness criteria. Based on this, a failure model for the “backfill-pillar” composite bearing structure is established by integrating stress-strain curves with observed failure modes during load-bearing processes. Finally, numerical simulation software was utilized to perform a stability analysis on both the composite load-bearing structure formed by post-backfilling in the roomand-pillar goaf and the overlying strata of the mined-out area. Numerical simulation results indicate that, under repeated mining conditions, the use of S8-2/3 backfilling material in room-and-pillar goafs significantly enhances the load-bearing capacity of residual coal pillars. It also effectively controls overburden movement and supports the safe and efficient extraction of coal resources.OPEN ACCESS Received: 23/04/2025 Accepted: 16/06/2025 Published: 22/09/2025