Study on generation mechanism of overlying strata separation and chain-type catastrophe under extra-thick cretaceous strata

Most deep mines in Shaanxi-Inner Mongolia area are located in the coverage area of extra-thick Cretaceous strata. Under the condition of large-scale mining with multiple working faces, large-range separation spaces are prone to form between high-level Cretaceous strata and the Jurassic Anding Formation. The continuous development of separation spaces can easily give rise to regional stress imbalance, and further trigger multi-source chain disaster risks such as high-energy mine earthquakes, high-potential separation-layer water hazards and surface subsidence. Taking Yingpanhao Coal Mine as the engineering background, a comprehensive mechanical model for extra-thick Cretaceous strata and Jurassic soft-hard interbedded strata is established to analyze the migration characteristics of overlying strata under prominent lithological differences of Jurassic soft-hard interbeds. Based on the probability integral method, the subsidence range and subsidence surface function of the top interface of soft-hard interbedded strata are derived. Combined with the large-area suspended-plate flexural deformation characteristics of extra-thick Cretaceous strata characterized by the thick-plate model, the mechanical mechanism for the generation of separation spaces under the coupled evolution of extra-thick Cretaceous strata and Jurassic soft-hard interbedded strata is revealed. On this basis, the early-warning criterion for separation-layer hazard and the prevention-control technology for multi-disaster chain evolution are constructed.The results show that the maximum subsidence of Jurassic soft-hard interbedded strata is 3.58 m. Mining disturbance induces flexural deformation of extra-thick Cretaceous sandstone, and a separation space with a height of 2.85 m is formed by the subsidence difference. Estimated via double integral, the volume of the separation space is approximately 1.68×10⁶ m³. The morphology of the separation space evolves from an initial trough-shaped basin structure along the dip direction to an inverted-arch long-plate spatial structure, and finally a downward-protruding groove-shaped structure is formed in the central zone. Taking the critical separation-layer height and separation-layer water pressure as key discrimination indices for hazard grading, the separation-layer effective stress and mining disturbance factor are introduced to establish the separation-layer hazard index R. When the separation-layer space develops to the threshold value, the internal stress system of rock mass loses balance and triggers chain disaster responses including high-energy mine earthquake, connection of water-conducting channels in Jurassic aquifuge, high-potential separation-layer water inrush and progressive surface subsidence. Targeting the reduction of separation-layer hazard index R, active prevention-control technologies including isolating grouting for regulating separation-layer spatial structure as well as mining-process optimization of mining height and advancing speed are proposed, which realize accurate early-warning and effective prevention-control of multi-hazards induced by separation-layer development beneath Cretaceous strata.