Overburden movement laws in thin bedrock workfaces under thick backfilled loose bodies

A detailed investigation was conducted on the geological conditions of thin bedrock workfaces under thick backfilled loose bodies in the first panel of the 22-upper coal seam at Shigetai Coal Mine. Utilizing both physical similarity modeling and numerical simulation techniques, the study comprehensively examined the overburden movement patterns and failure characteristics under these conditions. The physical simulation results indicate that as the working face approaches the open-pit slope area, the height of overburden failure gradually increases. When the thin bedrock becomes unstable due to rotation, unconsolidated materials at the slope base flow into the goaf. Subsequently, roof-cutting pressure on the thin bedrock causes simultaneous movement of backfilled loose bodies at different heights, resulting in minor surface settlement. As mining progresses, fracturing alternates periodically between roof-cutting and relief phases, ultimately expanding the surface subsidence zone. The maximum subsidence of the thin bedrock remains less than the mining height of the coal seam. Numerical simulations reveal that when the working face leaves the open-pit slope, the overburden failure height rises rapidly. Upon entering the thin bedrock workface area, advanced fractures develop, and the thin bedrock undergoes full-thickness fracturing in small cycles with intervals of approximately 10 m. The advanced stress fluctuates around the original rock stress, and the movement of backfilled loose bodies quickly reaches the surface. When re-entering the open-pit slope, the development of advanced fractures ceases, and the overburden failure height increases again. The advanced stress gradually rises and stabilizes in the waste dump area. Subsidence in the thin bedrock workface area consistently exceeds that in the open-pit slope area. Maximum subsidence values at different measurement heights are observed within the thin bedrock zone and shift toward the slope base as height increases. When exiting the slope, subsidence in the open-pit area is greater than when entering it. This research elucidates the dynamic fracturing mechanisms and movement patterns of overburden in thin bedrock workfaces beneath thick backfilled loose bodies, providing a theoretical foundation for safe mining in similar geological settings.