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The height of the disturbed area depends on the thickness of the mined coal, geometry of the mined panel, [he rate <br />of the mining face advancement, and on the geological characteristics of the overburden. According to Singh <br />• (1986), the area of disturbance above a longwall panel is generally divided into the following five zones, based <br />upon the extent of the fracturing: <br />ZONE 1: Zone of primary caving where the caved rock is completely disintegrated; <br />ZONE 2: Zone of bed separation, where separation occurs primarily along pre-existing <br /> bedding planes; <br />ZONE 3: Zone of vertical relaxation where, a slight increase of permeability is experienced; <br />ZONES 4 & 5: Zone of horizontal extension. Zone of tensile strain at the surface where shallow <br /> fractures develop. Zone of horizontal compression. <br />According to international experience, the total thickness of the first and second zones, where the changes of <br />permeability aze substantial, typically reaches 3 to 3.5 times (Ropski and Lama, 1973), and rarely more than ]0 <br />times the height of the extracted seam (Wardell, 1976). The height of the third zone, or the total height where <br />changes in permeability due to subsidence can occur, is described by various authors in a range from 30 t to 60 t <br />(where t is the fully extracted seam thickness), 58 t (Gviroman, 1977), 33.7 t (Williamson, 1978), and 30 t <br />(Wardell, J976). <br />A series of studies performed in the Appalachian bituminous coal region confirmed the experience from overseas. <br />A study of subsidence in the Dunkard Basin (Owili-Eger, 1982) concluded that water levels in units, located within <br />330 feet above the mined coal, recovered after mining, and that there was no lasting deterioration of ground water <br />quality. Another study of subsidence in Western Pennsylvania (Hill and Price, 1983) concluded that with an <br />average thickness of the overburden of 550 feet, the shallow unit system was isolated from major impact caused by <br />mining. Authors also observed that after longwall mining underneath wells, the subsidence slows and the strata <br />• settle and ground water levels rebound as flow paths to the mine become less direct. Stoner (1983) performed a <br />study of subsidence due to mining in the Pittsburgh seam in Greene County in southwestern Pennsylvania. During <br />his study, he monitored a well located above a longwall panel. The water level in the monitored well declined <br />during undermining, but returned within 3 feet of the pre-mining level when mining advanced beyond the well. <br />The U.S. Bureau of Mines (Walker, 1988) monitored surface subsidence and water level fluctuations in 10 shallow <br />observation wells above a series of four adjacent longwall panels in southern Pennsylvania for about 4 years. <br />Results of this study indicate that wells are generally unaffected by mining of a preceding panel. The most <br />important conclusion of this study was that nine out of ten wells investigated recovered to their pre-mining water <br />level after mining was completed. <br />Similar observations were made during several studies of longwall mine subsidence impacts on hydrology in Ohio <br />and West Virginia completed by Hydro-Geo Consultants, Inc. (1988 and 1991). In these studies, monitoring of <br />water levels and water quality before, during, and after undermining by longwall operation indicated that impacts <br />due to undermining were noticeable only in wells or surface water bodies within approximately 200 feet of the <br />mined coal. Typically, in wells installed within zones 2 and 3, water levels declined during undermining and <br />returned to levels somewhat lower than pre-mining, An explanation of the slight decline of the water table after <br />longwall mining was presented in studies where the permeability of the water-bearing strata was measured before <br />and after mining. <br />A study of subsidence effects due to longwall mining on hydrology in Marshal County, West Virginia was <br />published by the U.S. Geological Survey (Schultz, 1988). In this study, ten observation wells underlain by <br />longwall panels were monitored before and after mining. Three of the wells were tested for transmissivity before <br />and after mining. It was found that transmissivity increased substantially after mining in two out of the three <br />wells. The study also concluded that changes in water levels prior to and after mining exceeded ten feet in only <br />two of the ten monitored wells. <br />AP~,'~®V~ID EEB 0 8 2000 <br />TR 99-32 2.05-147 11/15/99 <br />