Development and application of overburden structure composed of caving arch and towering roof beam in deep longwall panel with thin bedrock

Deep coal seam with an overburden of thin bedrock and thick alluvium is affluent in Jiaozuo coalfield. Due to the special composition of overburden, roof movement is active and surface subsidence coefficient is large. Mining activity is threatened by strong mining pressure and water-sand inrush accident. The fracture and water are observed at the surface of mined-out area, which destroys buildings and agricultural land. In order to improve ground control in such longwall panels, overburden fracture development, structure formation and its bearing mechanism are analyzed by taking No. 14030 longwall panel in the second coal mine of the Zhaogu mining area as the background. Based on the results from laboratory test, theoretical analysis and field measurement, the overburden deformation is divided into four stages, including initial static, slow increase, quick increase and catastrophe increase stages. The overburden is stable in the first two stages. It becomes instable due to deformation localization in the third stage and a fracture development appears due to discontinuous deformation in the last stage. Mining induced fracture initiates in the thick alluvium, propagating downward into the bedrock. Full-thick rupture of the bedrock leads to the formation of overburden structure composed of caving arch and towering roof beam. Overburden movement is dominated by the key stratum in the continuous stage, which becomes dominated by the fracture propagation of the alluvium in the discontinuous stage. Fracture initiation is explained based on the deformation characteristics of the key stratum. The energy principle is used for explaining the fracture penetration phenomenon at the bedding plane. Full-thick shear rupture condition for the bedrock is deduced from fracture mechanics. After that, the mechanical model is established for the overburden structure composed of caving arch and towering roof beam. Load-bearing capacity and actual load transmitted onto the caving arch are determined, which reveals the structural instability condition of the arch. Mining induced stress rotation makes overburden load be transmitted toward the skew back of the arch, which greatly improves the arch stability. Load-bearing capacity of the towering roof beam is calculated by assigning constant normal stuffiness condition to the rupture fracture of the bedrock and moreover, the support capacity necessary to keep the balance of towering roof beam is determined. Based on the bearing mechanisms of the combined overburden structure, a double parameter method, including strength and stiffness, is proposed for support design and a flowing path identification method for water-sand inrush is also put forward. Field measurements indicate that the support type is basically reasonable in No. 14030 longwall panel. Accurate identification of water and sand flowing path provides an effective guidance for the parameter determination of the grouting borehole in roof strata.