Study on multi-field evolution of overlying strata and surface subsidence laws in backfill mining of shallow-buried medium-thick coal seam

To scientifically guide the mining of shallow-buried medium-thick coal seam resources under buildings, railways,and water bodies and to protect the surface ecological environment,it is necessary to clarify the multi-field evolution of overlying strata and the surface subsidence laws under backfill mining. Taking the No. 3306 working face of Chaili Coal Mine as the engineering background,this study systematically investigates the response characteristics of the displacement field,stress field,and fracture field of the overlying strata as well as surface subsidence under backfill mining by integrating theoretical analysis,similar material simulation experiments,and numerical simulation. The results show that the mechanical model of the overlying strata load-bearing structure established based on elastic foundation beam theory indicates that the critical filling ratio required to ensure safe mining of the working face and the safety of surface buildings and structures is 85%. The fractal dimension can quantitatively characterize fracture development in the overlying strata;its evolution undergoes four stages:lowspeed,medium-speed, high-speed, and stable-equilibrium stages, with a final value of 1. 2027. Fractures are mainly concentrated in the immediate roof and main roof, indicating that the backfill body can effectively constrain overlying strata movement and fracture development. The displacement variation of the overlying strata corresponds well to the stages of fractal dimension evolution. As the stratigraphic horizon rises, the width of the displacement subsidence curve gradually decreases, eventually forming a trapezoidal subsidence pattern with larger subsidence in the middle of the goaf and smaller subsidence on both sides. The surface movement basin undergoes three stages as the working face advances:subcritical mining,critical mining, and supercritical mining. The results of similar material simulation experiments and numerical simulation show that an 85% filling ratio can control the deformation of surface buildings and structures within the Grade I damage range. The stress in the overlying strata generally evolves through four stages: stress concentration, rapid unloading, backfill adjustment, and final stabilization. Its influence range,stress concentration factor,and fluctuation amplitude all decrease with increasing stratigraphic horizon. The multi-field evolution of the overlying strata and surface subsidence laws under backfill mining are validated by monitoring results from backfill working faces in multiple mines. The research results can provide a reference for backfill mining parameter design and surface subsidence control under similar geological conditions.