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Introduction
Water protection is a major issue for large-scale underground mining, especially in the provinces of Inner Mongolia, Xinjiang, Gansu, Shannxi, Ningxia, and Shanxi, in northwest China, where most of the coal seams underlie arid areas and water resources are extremely precious. These six provinces contain more than four fifths of China’s proven coal reserves. If the overlying strata are sufficiently permeable, surface and groundwater can drain into the underground mine, which is hazardous to miners and deteriorates the region’s already fragile ecological environment.
Longwall mining, which revolutionised underground coal mining with its capacity for safe, cost effective and efficient large-scale extraction, has been extensively used around the world. Longwall mining uses mechanical shearers to cut coal while self-advancing hydraulic-powered supports hold up the mine roof. As coal is removed, the supports are moved forward and the roof collapses behind them. The overburden can be disturbed from the seam level all the way to the surface, which creates fractures and changes the permeability of adjacent units by several orders of magnitude (Booth 2002; Peng 2008). The hydraulic properties of the overlying strata are changed and high permeability pathways frequently form (Booth 2002).
Researchers (Kendorski 1993, 2006; Peng 2008; Singh and Atkins 1982; Singh and Kendorski 1981; Tieman and Rauch 1987) have defined zones using various parameters (e.g. Fig. 1) to describe the decreasing order of fracture severity and groundwater effects from the immediate roof toward the surface. Booth (2002) divided the overburden into three zones, the intensely fractured zone, an intermediate zone, and a near-surface fractured zone. Singh and Kendorski (1981) concluded that the intermediate zone in the overburden is crucial for water protection, which has been affirmed by many researchers (Booth 1986; Hutcheson et al. 2000; Singh et al. 2008; Tieman and Rauch 1987). The critical thickness of the intermediate zone is quite different for different sites (Coe and Stowe 1984; Elsworth and Liu 1995; Hutcheson et al. 2000; Van Roosendaal et al. 1995). Elsworth and Liu (1995) found that the value for mining in the Appalachian Plateau was about 90 m under valleys and 150 m under hilltops and plateau. Van Roosendaal et al. (1995) observed that the overburden prevented the loss of water from shallow aquifers into a longwall coal mine only...