Structural stability of overlying rock and mechanism of water inrush and support crushing in extra-thick coal seam mining within syncline structural area

With the aim of addressing the frequent occurrence of water inrush and support crushing (WISC) accidents in the fold structure belt of the Yonglong mining area, this study is conducted against the engineering background of the Guojiahe Coal Mine. The research methods encompassing field investigation, physical experimentation, and theoretical analysis are adopted. Accordingly, the main aquifer leading to WISC in the working face is determined, obtaining the failure and instability characteristics of overlying rock during coal seam mining within the syncline structural area. A mechanical model considering the influence of confined water on the overlying rock structure is established, deducing the critical water pressure and revealing the mechanism for WISC at the working face within the syncline structural area. The study reveals that: ① The WISC accidents occur at the mining faces 1303, 1304, and 1308 within the syncline axis region. The Yijun formation serves as the principal aquifer responsible for these incidents. The key waterproofing layer composed of medium-grained sandstone with a thickness of 28.53 m is located at 102.5 m above the coal seam. Upon the rupture of the key waterproofing layer, the upper load layer with a thickness of 100.51 m fractures simultaneously, resulting in the propagation of a water-conducting fracture zone towards the Yijun aquifer. ② During the decline mining stage of the syncline structural area, a multi-layered sub-key layer is observed in the roof overburden, leading to the formation of a multi-layer separation zone. The angle of strata breakage along the syncline axis exhibits vertical development, resulting in the elevated degree of roof failure and posing a significant risk of support crushing. ③ The instability of the overlying rock structure, resulting from water inrush from the roof, constitutes a pivotal factor contributing to the occurrence of the support crushing accident. The impact of confined water on roof strata stability is manifested primarily in the following ways: it enhances the load transfer continuity in overlying strata, diminishes the bearing capacity of combined subkey strata, and constitutes an additional load for combined subkey strata. ④ The required support resistance to prevent water inrush and support crushing increases with the rise of confined water pressure, and the critical confined water pressure for a WISC in the working face is 0.98 MPa. Therefore, controlling the development height of the water diversion fracture zone and reducing the confined water pressure of the Yijun formation are crucial for preventing pressure frame accidents in the working face.