Water inrush mechanism and water-conducting fractured zone’ developmental patterns of a typical ultra-thick coal seam in the Huanglong coalfield during fully mechanized mining

The mining of thick coal seams in the Huanglong Jurassic coalfield has led to the formation of high water-conducting fractured zones, which are prone to connect the extremely thick sandstone aquifers of the overlying Cretaceous Luohe Formation. Qinggangping Coal Mine in the coalfield exhibits the typical characteristics of pulsed water inrushes during coal mining, posing a severe threat to safe mining. This study investigated the fully mechanized mining face 42105 in Qinggangping Coal Mine. Using 3DEC discrete element simulation, flushing fluid leakage in boreholes, integrated geophysical exploration, and borehole TV, this study examined the developmental patterns of the water-conducting fractured zone and detachment layers in the overburden during coal mining. Furthermore, it explored the water inrush mechanism and the developmental characteristics of the water-conducting fractured zone in a mine under the typical geological conditions. Key findings are as follows: (1) The water-conducting fractured zone along mining face 42105 continuously has extended upward under the periodic collapse of the overburden. Consequently, the static water in porous-fractured sandstones in the Luohe Formation is constantly released and intermittently rushes into the mining face, forming low-amplitude pulsed water inrushes. (2) Detachment layers have been continuously formed in the extremely thick sandstone aquifers, and, accordingly, the water-conducting fractured zone is connected to the water accumulation area of detachment layers in the sandstone aquifers. As a result, instantaneous water inflow into the mining face increases sharply, forming high-amplitude pulsed water inrush. (3) In combination with the measured flushing fluid leakage in boreholes, as well as the results from integrated geophysical exploration and borehole TV, this study revealed the pulsed water inrush mechanism subjected to the superimposed influence of both the static water in porous-fractured sandstones in the Luohe Formation and water in the detachment layers in the Qinggangping Coal Mine. The maximum height of the water-conducting fractured zone was determined at 316.83 m to 333.00 m, with ratios of the height of the water-conducting fractured zone to the mining height ranging from 30.17 to 31.71. The findings of this study can serve as a guide for the prevention and control of water inrushes in mines, as well as safe and efficient coal mining, under similar formation conditions.