Characterizing the effect of longwall retreating speeds on overburden and surface synergistic subsidence behaviors with a thick coal seam: intensive field measurements

The 12 401 longwall face in the Shangwan coal mine currently boasts one of the largest mining height (>8.0 m) longwall faces in the world. In-depth field measurements, including overburden movement, surface subsidence and fissure, and shield support loads, were conducted to elucidate the subsidence and damage behaviors induced by high-intensity mining. The findings identified three control rock strata (CRS1, CRS2, and CRS3), which dominate the formation of two subsidence modes ('two zones' and 'three zones') depending on longwall retreating speed. CRS1 and CRS2 collectively affect the variation of shield support loads. CRS2 governs the movement of the entire overburden, and its rupture precipitates swift subsidence. CRS3, characterized by thick soft rock with robust expansion and bending resistances, serves as a buffer, mitigating displacements caused by the breakage of CRS2. It is instrumental in shaping the 'three-zone' overburden structure. At lower retreating speeds, the height of the water-conduction fracture zone (H-wf) is similar to 200 m, contrasting with 120.47-134.46 m for higher retreating speeds. The CRS2 manifests as a 'step rock beam' at lower retreating speeds, causing a maximum surface subsidence of 6315 mm. Conversely, under higher retreating speeds, it adopts a 'masonry beam' structure, resulting in a maximum surface subsidence of 4947 mm. The distribution of ground fissures is significantly affected by retreating speed, aligning closely with the breakage steps of the control rock strata. Our study offers robust scientific insights into preventing mining damage and promoting ecologically sustainable mining practices.