Analysis on water loss characteristics of roof aquifer and borehole hydraulic curtain protection in deep coal mining in Inner Mongolia‒Shaanxi mining area

The geological and hydrogeological characteristics of the main mined coal seams in the deep Inner Mongolia‒Shaanxi Mining Area vary greatly from that of the shallow area. Herein, the relevant research results on the geological and hydrogeological characteristics, as well as the mining-induced water-flowing fracture, were analyzed systematically for the main mined coal seams in the deep mining areas, such as Hujierte, Taigemiao and Xinjie. On this basis, it is summarized that a typical indirect water loss (inrush) mode could be formed easily under the drive of coal mining, with the protection of key weak water barrier layer of Jurassic Anding Formation sandy mudstone, taking the thick Cretaceous pore-fissure aquifer as the target water-conserving layer in the mining area. Besides, analysis was performed based on the case mine in the study area, with the change rule of groundwater flow field during the mining of deep coal seams simulated numerically. The simulation results show that: the groundwater flow field under the disturbance of coal seam mining basically forms a falling funnel with the mining space as the center, and the impact area of the falling funnel is expanded with the increase of mining footage, having the total water inflow of working face increasing linearly. Additionally, the increasing amplitude of water loss of the upper Cretaceous aquifer is less than that of the total water inflow. Generally, the simulation results are basically consistent with the measured results in case study. Finally, in view of the contradiction between deep coal resources exploitation and water source protection in the study area, the borehole hydraulic curtain protection of groundwater level in the Cretaceous aquifer was proposed based on the basic principle of taking the Cretaceous system as the reinjection layer, and the top of Anding Formation as the final hole horizon, with the reinjection well as far away from the coal mine area as practical, for the purpose of protecting groundwater level of Cretaceous aquifer. Moreover, the change rule of groundwater flow field in the Cretaceous aquifer under the conditions of normal mining and hydraulic curtain wall construction in the case mine was simulated and analyzed, which shows that the borehole hydraulic curtain has good applicability for the level protection of low permeability aquifer, with reference significance for the water resource protection in mining roof of deep coal mining area.