Study on the Migration Laws of Overlying Strata in Backfill Mining of Close-Distance Coal Seams

To clarify the migration characteristics of overlying strata during backfill mining of close-distance coal seams, the 3306 working face of Chaili Coal Mine was taken as the engineering background, and similar-material simulation, fracture-fractal analysis, and FLAC3D numerical simulation were carried out under an 85% backfill ratio. The study reveals the coordinated inherited and reactivated evolution of fractures, displacement, and stress in the overlying strata during successive extraction of the upper and lower seams. The results indicate that the movement of the overlying strata shows pronounced stage dependence and inheritance. After extraction of the upper No. 3 coal seam, the response of the overlying strata evolves from local disturbance to overall structural readjustment, with continuous bending subsidence and progressive fracture propagation, and ultimately forms a two-belt structure. During extraction of the lower No. 3 coal seam, the response develops on the basis of the structural state formed after upper-seam mining and is manifested mainly by the reactivation and readjustment of the pre-existing fracture network and displacement field. The fractures undergo a dynamic process of generation, development, closure, redevelopment, and reclosure. Compared with upper-seam mining, lower-seam mining produces a larger vertical displacement and a weaker stress response. The maximum vertical displacement in-creases from 478.85 mm to 1019.76 mm, whereas the stress concentration coefficient of the immediate roof decreases from 2.01–2.03 to 1.93–1.99. Under the geological and mining conditions considered in this study, the 85% backfill ratio maintains overall bending subsidence of the overlying strata and alleviates strata pressure manifestations during lower-seam extraction. These findings provide a reference for strata control under similar backfill mining conditions.