Overburden movement and surface response during backfill mining of shallow-buried close distance coal seams: a case study from southwestern china

Shallow-buried coal seams in southwestern China are commonly distributed in mountainous and karst regions characterized by strong topographic relief, closely spaced coal seams, and weakened overburden structures. Under repeated mining disturbance, overburden movement and surface deformation often exhibit asymmetric distribution and localized concentration. To investigate the displacement characteristics of overburden during backfill mining, this study takes the C2–C5 close-distance coal seam group as the engineering background. The upper C2 seam was mined by the caving method, whereas the lower C5 seam adopted high-ratio backfill mining with a backfill ratio of 98%. Based on theoretical analysis, FLAC3D numerical simulation, and monitoring-data reconstruction, the evolution characteristics of roof subsidence, surface subsidence, horizontal deformation, crack development, and backfill response were systematically analyzed. The results show that the maximum displacement during caving mining of the C2 seam nonlinearly rises from 2.707 mm to 87.451 mm. The maximum roof subsidence at the end of C5 backfill mining reaches 24.78 mm, accounting for only 28.34% of the final maximum displacement under C2 caving mining. Field monitoring data demonstrate that the maximum surface subsidence directly above the goaf is 18.7 mm, and large horizontal displacement occurs at the right midslope. This indicates that slope horizontal deformation and crack development are critical control factors for backfill mining of shallow-buried coal seams in mountainous areas of southwestern China. When the backfill ratio increases from 70% to 98%, the maximum roof subsidence declines from 58.6 mm to 24.8 mm, the maximum surface subsidence decreases from 43.5 mm to 18.7 mm, and the subsidence reduction ratio rises from 25.9% to 68.1%. High-ratio backfill mining can significantly restrain overburden movement, while key monitoring should be implemented for the central zone of the goaf, midslope areas and weakened zones within key strata.