Evolution law of bed separation and mining-induced fractures during mining under thick limestone in the Changxing Formation

To address the challenge of roof bed separation water inrush in the Northern Guizhou Coalfield, a study was conducted on the evolution mechanisms of bed separation cavities and mining-induced fractures during the development process of such water disasters. By focusing on the typical working face, we examined the structure and movement processes of the key strata in the overburden roof, along with the expansion of mining-induced fractures and the development of separation spaces, through theoretical analyses and similarity simulation tests. Taking a specific longwall panel in this mining area as the research subject, this study investigates the structure and movement of key strata, as well as the development of bed separation cavities and the propagation of mining-induced fractures, through key strata theoretical analysis and physical similarity modeling tests. The results show that: (1) During the uncoordinated movement of the subkey strata of the Changxing Formation limestone and the underlying strata, bed separation occurs at the bottom boundary of the strata, providing storage space for water enrichment. (2) Before the collapse of the subkey strata of the Changxing Formation limestone, the development of bed separation space and the height of the water-conducting fracture zone are synchronous. The height of the water-conducting fracture zone increases in a step-like manner along with the fracture of the overlying strata, and the maximum value is 51.45 m, which is the distance between the bottom boundary of the Changxing Formation limestone and the coal seam. The mining-induced fractures on the surface of the experimental model exhibit parallel patterns in the transverse direction, with the spacing between parallel lines being the periodic collapse step distance of the roof, with an average value of 21.98 m. The average fracture angle above the mined-out area is 66.2°. The distribution pattern of mining-induced fractures reveals that the two sides of the goaf are dominated by through-type longitudinal fractures, while the central area is dominated by bed separation fractures. Due to the re-compression effect of the overlying strata in the central part of the goaf, some fractures have closed, and the number of fractures in the central part of the mined-out area is less than that on both sides of the mined-out area, resulting in a “saddle” distribution of mining-induced fracture density.(3) The bed separation boundary angle θ is not significantly different between the open-off side and the working face side, and the overall shape of the bed separation is an isosceles regular trapezoid with a base angle of approximately 60°. During the advancement of the working face, the length of the bed separation l s appears to increase linearly, whereas the growth rate of the bed separation height w s tends to first increase but then decrease. When the mined-out area reaches a certain extent, the bed separation height tends to stabilize. The above research results provide important theoretical guidance for the design of bed separation water drainage holes to prevent and control water hazards caused by roof bed separation in coal mines in the region.