258 Shaanxi Province, western China. The mine exploits the 2-2 coal seam, which is characterized by stable occurrence, simple geological structure, and favorable regional mining conditions. The coal seam thickness along the working face ranges from approximately 7.2 m to 12.3 m, with an average thickness of about 10.5 m, making it representative of extra-thick coal seams that are traditionally difficult to mine using single-pass methods. The seam dip is gentle, generally less than 6°, and the coal strength coefficient ranges from 2 to 3, indicating a hard coal seam. The burial depth of the working face varies from approximately 270 m to 360 m, corresponding to moderate in-situ stress levels. 2.2 Thick and Hard Roof Structure A defining geological feature of the Caojiatan Coal Mine working face is the presence of multiple layers of thick and hard sandstone roof strata above the coal seam. Within approximately 100 m above the seam, several sandstone layers with individual thicknesses of 10–30 m are developed, exhibiting high strength and good integrity. In-situ testing indicates uniaxial compressive strengths commonly exceeding 60 MPa, with some layers approaching 100 MPa. These thick and hard roof strata do not readily cave after coal extraction, resulting in large suspended roof areas and strong periodic weighting behavior. Field investigations reveal a typical multi-layer “thick hard roof assemblage”, forming a complex overburden structure that significantly amplifies mining-induced stresses during large-height extraction. A simplified stratigraphic profile of the roof structure is recommended as Fig.1, highlighting the vertical distribution and thickness of key hard roof layers (green in Fig. 1) that control strata behavior. Fig.1 Schematic profile of multi-layer thick and hard roof strata above the 10-m working face. 2.3 Engineering Challenges Specific to 10-m Mining Height The combination of extra-thick coal seam and multi-layer thick hard roof results in a series of engineering challenges that intensify significantly with increasing mining height. When the mining height reaches 10 m, these challenges become fundamentally different from those encountered in conventional large-height longwall mining. 1100 1080 1060 1040 1020 980 960 Supplementary Borehole No. 58 Supplementary Borehole No. 65 Borehole at 5900 m Borehole at 5600 m Borehole at 5100 m Borehole at 4500 m Borehole at 4100 m Strata Level (m)
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