Surface subsidence characteristics of paste backfill mining in high-cutting-height coal seams at kilometer-depths

Deep mining at kilometer depths has become a trend in coal mining development. As a crucial method for green mining, backfill mining holds significant importance for “under-three” coal mining (mining under structures, water bodies, and railways). To investigate the surface strata movement patterns under conditions of large mining height in kilometer-deep mines, this study analyzed two working conditions—conventional mining and paste backfill mining—at a coal mine in China, based on the probability integral method and numerical simulation. The research found significant differences between the two methods calculated using the probability integral method. Under backfill mining conditions, ground deformation in the inclination and horizontal directions was only about 4.1% of that under conventional mining. Numerical simulation results indicated that during conventional mining, surface displacement showed little change when the working face advanced between 50 m and 200 m. However, a large fracture extending to the surface occurred in the roof strata at an advance distance of 200 m. When the advance reached 350 m–450 m, the rate of surface vertical displacement change slowed, with a maximum vertical subsidence of 1.26 m. In contrast, for paste backfill mining, surface horizontal displacement remained almost unchanged compared to conventional mining, but the vertical surface subsidence was reduced to 0.016 m, accounting for only 1.2% of the subsidence under conventional mining. Furthermore, backfill mining eliminated the zones and periods of intense strata subsidence. Comparing the two conditions shows that backfill mining reduces surface subsidence by dozens of times. This keeps building damage within Grade I (extremely slight damage), requiring no repair. The research results provide valuable references for understanding and controlling surface strata movement patterns in this mine and under similar mining conditions.