Influence mechanism of pillar-side backfilling on the overburden-coal pillar system's stability under upward mining

The stability of the residual coal pillar is the key to the re-mining above mined-out area. To improve the stability of overburden-coal pillar group, the method of pillar-side backfilling is proposed and the collaborative bearing mechanism for pillar-side backfilling is constructed. The compressive strength of the pillar-side backfilling and lateral restraint force of pillar-side backfilling on the coal pillar are derived. Moreover, this study takes the mining above mined-out area of 6# coal seam in Yuanbaowan coal mine as the actual engineering background. By using the PFC2D numerical simulation software, the stability of the residual coal pillars during mining above mined-out area was studied. The stress distribution of the overlying strata structure and the deformation characteristics of surrounding rock were analyzed under three different modes of pillar-side backfilling: without backfilling, bilateral pillar-side backfilling, and unilateral pillar-side backfilling. The results showed that without pillar-side backfilling, as the mining face advances, the pillar-type residual mining area will experience chain instability under the influence of advancing support pressure. After pillar-side backfilling, as the width of the filling body increases, the concentration stress in the overlying strata of the goaf gradually decreases. The movement and deformation of interlayer rock strata reduce, and the overall stability of the "residual coal pillar group-interlayer rock strata" system continuously strengthens, ensuring the safe re-mining of the 6# coal seam. When the width of the residual coal pillar is constant, the surrounding rock control effect of bilateral pillar-side backfilling is significantly superior to that of unilateral pillar-side backfilling. The reasonable selection of the width and backfilling method of pillar-side backfilling can effectively improve the overall stability of the residual coal pillars, achieving the purpose of preventing and controlling chain instability.