Utilization of broken rock in shallow gobs for mitigating mining-induced water inrush disaster risks and environmental damage: Experimental study and permeability model.

Coal mining-induced groundwater losses may trigger water inrush disasters and surface ecological degradation. The compaction and seepage characteristics of broken rock in gobs can be used to find the balance point of water inrush prevention and water resource protection in shallow coal seam groups. These characteristics, as well as geological and engineering parameters of shallow coal seam mining, are experimentally determined in this study. The performed permeability tests revealed that the percentage of voids in broken rock exponentially decreased with the axial stress. The water seepage of broken rock in the compaction process conformed to the Forchheimer theory, with the permeability ranging from 10 to 10 D. The initial value and reduction range of mudstone permeability in the three lithologic samples were the smallest. The uniaxial compression strength reduction caused by the increase in unit mass water due to water saturation of natural rock samples were 5.8 and 3.2 % for coal and sandstone, respectively. Based on the experimental results on compaction and seepage of broken rock, the axial stress-percentage of voids-permeability model considering compaction and re-crushing was established. The mudstone roof was found to be the key rock stratum during re-mining for ecological protection and hydraulic connection evaluation of the overburden.