Multi-Factor Coupled Stability Mechanism and Safety Boundaries of Upward Re-Mining Above Goafs Supported by Remnant Coal Pillar Groups

Upward re-mining of coal seams overlying goafs with remnant coal pillar groups is jointly affected by pillar loading, interburden damage, repeated mining disturbance, and goaf water, making the stability evolution and safety boundaries difficult to characterize using existing single-factor approaches. To address this issue, the progressive failure behavior of remnant coal pillar groups and the multistage damage evolution of the interburden strata were first analyzed. The results indicate that individual pillars undergo progressive failure from their margins toward the elastic core, while boundary and key pillars are more susceptible to instability and may trigger cascading failure of the pillar group through load redistribution. Meanwhile, the interburden strata experience sequential damage induced by initial mining, long-term water-induced weakening and creep, and renewed disturbance during upper-seam extraction. On this basis, a theoretical mechanical model incorporating overburden load, horizontal in situ stress, pillar-group support, and goaf water pressure was established. The stress field and maximum yield-failure height of the interburden strata were derived, and the stability of upward re-mining was evaluated by integrating pillar bearing capacity, interburden integrity, and water-inrush criteria. The results reveal pronounced competitive and synergistic effects among horizontal stress, mining depth, pillar width, and goaf water pressure. Increasing the lateral pressure coefficient and pillar width is beneficial for suppressing roof failure, whereas increasing mining depth markedly raises pillar loading, making pillar bearing capacity a critical constraint on safe upward re-mining. Goaf water pressure can partially share the overburden load and thereby reduce pillar stress and roof yielding; however, excessive water pressure increases the risk of floor water inrush, indicating the existence of a safe pressure window that balances structural stability and hydraulic risk. Under the investigated conditions, the reasonable goaf water pressure ranges from 0.72 to 0.96 MPa. A minimum pillar width of approximately 11 m is required under the investigated geometric and mechanical conditions. These findings demonstrate that the safety of upward re-mining is governed by the coupled interaction among pillar bearing capacity, interburden integrity, and goaf water pressure, providing a theoretical basis for safe recovery of residual coal resources above pillar-supported goafs and for water-inrush risk control.