Fracture structure of the overburden in the layered mining of extremely thick coal seams: Developmental patterns of caving zones and hydraulically conductive fracture zones and fracture characteristics

The Zhunnan Coalfield, a coalfield on the southern margin of the Junggar Basin in Xinjiang, boasts abundant coal resources. However, water hazards in mines severely constrain the safe and efficient coal production in the coalfield. Since hydraulically conductive fractures in roofs serve as primary hydraulically conductive channels formed in the mining of the mining face, it is particularly crucial to investigate the developmental patterns of these fractures during the layered mining of extremely thick coal seams. This study explored the (9-15) 08 mining face of the Liuhuanggou Coal Mine in Xinjiang. Using ground exploration holes and borehole televiewers, this study examined the developmental characteristics of the caving zone and hydraulically conductive fracture zones in the overburden under different mining conditions. Through numerical simulations, this study delved into the developmental patterns of hydraulically conductive fracture zones in the descending mining of multiple coal seams, analyzed the migration and fracture characteristics of the overburden in the layered mining of extremely thick coal seams, and explored the patterns of the fracture structural of the overburden. The results indicate that the layered mining of extremely thick coal seams somewhat inhibits the development of hydraulically conductive fracture zones. An increase in the layer number corresponds to progressively intensifying rock failure of the overburden. As a result, the overburden exhibits the characteristics of weak layers, thereby inhibiting the upward development of hydraulically conductive fracture zones. Under the layered mining of extremely thick coal seams, the fracture structure of the overburden generally presents a macro-large-small pattern. Macroscopically, the fracture structure displays a pressure arch structure within the mining influence range. Additionally, because of the caving and hinge of rock masses in high- and low-lying strata, the high- lying overburden exhibits a large beam structure. In contrast, the low-lying rock masses are subjected to the superimposed effects of layered mining, and the hinge points of the rock masses within the caving zones are connected to form an overall continuous M-shaped pattern. Consequently, the low-lying rock masses emerge as small M-shaped hinge structures. The findings of this study will provide a theoretical basis and guidance for research into the formation mechanisms of accidents, as well as disaster prevention and control, on the mining sites of extremely thick coal seams.