Periodic roof deformation and failure and associated water inflow characteristics during the mining of typical coal seams in the Inner Mongolia-Shaanxi contiguous area

Objective The disturbance of coal mine mining damages the aquifer/aquiclude structures of the overburden, causing concentrated water inflow and further affecting safe production in mines. Methods Using methods of the physical simulation of similar materials, numerical simulation, and on-site in situ monitoring, this study investigated the water inflow during the mining of the dominant 3-1 coal seam, typically influenced by water in sandstones on the coal seam roof, in the Inner Mongolia-Shaanxi contiguous area. By comparing the measured data of mines with similar geological and mining conditions in the surrounding area, this study delved into the periodic evolutionary patterns of both the coal support pressure and the vertical displacement of the overburden in the goaf during the mining of typical mining faces of the 3-1 coal seam. Accordingly, this study quantitatively analyzed the variations of microseismic events at the moment of periodic roof weighting on the laboratory scale. Finally, this study comprehensively determined the development characteristics of the hydraulically conductive fracture zones on the coal seam roof with mining, along with the trend of water inflow from the roof varying with the periodic weighting of the overburden. Results and Conclusions Key findings are as follows: (1) With the mining face advancement, the overburden exhibited a step distance of about 40 m in the initial weighting and of 12-28 m in periodic weighting. At the moment of the overburden fracturing, the periodic weighting of surrounding rocks first increased and then stabilized. As the mining face advanced to the fifth periodic weighting (advancement distance: 140 m), the advanced support pressure peaked, with the peak vertical displacement of the overburden at the moment of both periodic weighting and fracturing showing zigzag-shaped variations with coal mining. Similarly, comprehensive indicators, such as displacement monitoring data, total energy of microseismic events, and event frequency, indicate that at the moment of the fifth periodic weighting, the vertical displacement amplitude of the overburden peaked, while the height of the plastic zone remained roughly stable. (2) The hydraulically conductive fracture zone, with a maximum height of about 120 m, would be directly connected to the sandstone aquifer of the Zhiluo Formation on the coal seam roof. The groundwater inflow with coal mining manifested two wavy changing trends. The groundwater inflow displayed stepped growth with a long cycle of about 800 m on the global scale of the mining face but, locally, exhibited oscillatory changes with short cycles of 16-48 m and a strong correlation with the weighting period of the overburden. The findings of this study provide a reference for both the prediction of the water inflow of mining faces and the arrangement of the water prevention and drainage system.