Failure and development height of overlying rock of a water flowing fracture in goaf under a karst aquifer.

The factors leading to mine water inrush accidents are mainly sources of water, water channels, and intensity of water inrush. Mine water rush depends mostly on whether damage leads to the overlying strata of the working face penetrating the overlying aquifer. There is therefore a need to characterize how the overlying strata of the coal seam roof fails and the development height of the water-conducting fracture zone during a roof water inrush incident. This study focuses on the Longfeng Coal Mine, northern Guizhou coalfield. Samples of rock from the roof coal seam were obtained by drilling and coring and examined under a scanning electron microscope. The results showed an undeveloped state of primary cracks in the overlying strata of the coal seam roof, with the water channel occurring mainly along mining-induced cracks. Four key strata were identified, and their fracture steps were calculated through integrating the results of the physical test of the overlying rock of the coal seam. The key strata break when the advancing distance of the working face exceeds their fracture steps. At this point, water-conducting cracks develop upwards. Simulation by FLAC3D allowed the analysis of vertical displacement, vertical stress, and evolution of the plastic zone of the roof overburden rock under different working face advancing distances. The results show that the changes in vertical stress and displacement in overburden rock initially increased rapidly, with this rate of increase subsequently slowing. The shape and maximum height of the plastic failure zone was typical trapezoid and 46.12 m, respectively. Finally, the height of the water-conducting fracture zone determined on site was 42-51.8 m. The good agreement between numerical simulation and measurements confirms that the results of this study can guide practical engineering.