Mechanisms of aquifer protection and surface subsidence control induced by gangue-paste collaborative backfilling in deep coal mining

Mining-induced overburden deformation and surface subsidence pose significant threats to groundwater systems and ecological environments in deep coal mining areas. Developing effective backfilling technologies to control strata movement has therefore become critical for sustainable mining. Based on the deep high-mining-height working face of Pingmei No.11 Mine, this study investigates the influence mechanism of gangue–paste collaborative backfill technology on overlying strata evolution. A collaborative technical system integrating fully mechanized paste backfill behind supports with in-situ gangue backfill is proposed, and a volumetric strain–grading index–confining pressure coupling model and a backfill–surrounding rock–coal pillar collaborative bearing model are established. The evolution characteristics of stress and displacement fields are comparatively analyzed via FLAC3D numerical simulation, and field monitoring verifies the engineering effectiveness. Laboratory results show that continuously graded gangue with a grading index of 0.5 reduces volumetric strain by 38% compared with randomly graded gangue. Numerical simulations reveal that collaborative backfill reduces maximum roof displacement to 139.5 mm and expands the stress plateau zone by 25%. Field monitoring confirms that maximum surface subsidence is only 20 mm, with building crack widths controlled at 1–2 mm, fully meeting Class I protection standards. The proposed system forms a three-in-one control mechanism of mechanical compaction–paste roof contact–spatiotemporal coordination, and reveals the power-law relationship between backfill compression modulus and strata subsidence rate. These findings provide a theoretical basis and engineering paradigm for green mining of deep coal seams.