Strong ground pressure mechanism and arched face control technology during slicing longwall top coal caving in ultra thick coal seam

Weakly cemented overlying strata could induce intense ground pressure in working faces, potentially leading to support crushing, which severely threaten safety and impede production. This study focused on the top coal caving mining of a 55 m ultra-thick coal seam in the top slice of Zhundong No.2 Coal Mine. Field measurements, theoretical analysis, laboratory experiments, and numerical simulations were employed to investigate the ground pressure behavior and control strategies under weakly cemented overlying strata. The results indicate that the overlying strata in the first mining area of Zhundong No.2 Coal Mine are characterized by thick bedrock layers with weak overall cementation, where soft or extremely soft rock strata account for 87%. During the advancement of the 1101 top coal caving face, the main roof masonry beam experienced secondary fracture instability under dynamic loads from high-position rock block rotation, disrupting the original masonry beam bearing structure. Consequently, the supports transitioned from bearing predetermined loads to jointly resisting deformation with the immediate roof, triggering intense ground pressure. Based on this, a stability equation for the arched structural rock blocks above the supports was established, revealing stress distribution patterns at different positions. A novel “arch-shaped” layout of the working face and corresponding mining technology were proposed to form a dip bearing structure in the roof, thereby controlling strong ground pressure. This structure converted partial overburden loads into internal forces between arched blocks. Numerical simulations further demonstrated that the new technology reduced the development of plastic zones in the roof, decreased roof subsidence by 33.1%, and elevated vertical stress by 18%, as the arched structure transferred part of the overburden load into internal stresses, effectively lowering the overall load on supports. Field implementation confirmed the efficacy of this technology, the peak load on supports decreased by 12%, the range of intense pressure manifestation narrowed from supports No. 20−No. 120 to No. 40−No. 100, and maximum compressive stress at the arch foot facilitated timely fracturing and caving of triangular coal and roof at both ends of the face.