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West Elk Mine <br />Mr. Dunrud estimates that the caved zone in the Apache Rocks and Box Canyon [Wining areas <br />. will range from 2 to 4 extraction thicknesses. Caved zone heights closer to 2 times the mining <br />thickness (t) are expected in dry mining conditions, whereas wetter conditions will produce <br />caved zone heights closer to 4t. An acceptable average value for the Apache Rocks mining <br />area and Box Canyon lease tract is 2.St per Mr. Dunrud. <br />Fractured Zone <br /> <br />A zone of fracturing and local separation along rock bedding planes and joints occurs above the <br />zone of caving. In this zone, which is transitional to the underlying caved zone, lateral and <br />vertical constraints in the adjacent overburden strata and the caved rocks below m;n;m;~e further <br />displacement or rotation of the fractured rock. Displacements in the fractured zone and severity <br />of fracturing tends to decrease upward as lateral and vertical confining stresses increase. <br />Peng (1992) reports that the combined height of the zone of caving and fracturing ranges from 20 <br />to 30 extraction thicknesses (20 to 30t), and that the height of the fractured zone is greater for <br />hard, strong rocks than it is for soft, weak rocks. As discussed in more detail in Exhibit 60, <br />Richard Dunrud has concluded that within the Apache Rocks and Box Canyon mining azeas, a <br />more appropriate range is 15 to 20t. <br />Compression arches (azcuate zones of compressive stress) commonly develop, or partially <br />develop, above the mining panels. These azches temporarily transfer overburden stresses to the <br />panel barrier or chain pillas and also to the caved zone and the mining face (Dunrud 1976). 'The <br />arches in a given area commonly move upwazd and disperse as longwall mining is completed in <br />the area. Compression arches may not disperse where the room-and-pillar mining method is used, <br />because pillars and stumps left after mining may prevent dissipation of the arches. The rocks <br />affected by the arches temporarily are subjected to increased stress and strain as the arches move <br />upward. However, in the longwall mining area, this increased stress and strain commonly is less <br />than it is in room-and-pillar mining azeas because stresses aze relieved as the arches move upward <br />and disperse. <br />The height of the zone of fracturing is a function of lithology and layer thickness, according to <br />Peng (1992). For example, the zone of fracturing commonly is higher for strong, thickly-bedded, <br />brittle sandstones than it is for thinly layered, soft, plastic shales and claystones. Liu (1981), <br />reports ranges of heights of the zone of fracturing for various rock types as follows: <br />• Heights of 20 to 30 times coal extraction thickness (20 to 30t) are reported in strong <br />brittle rocks, such as siliceous sandstones and limestones; a value of 28t was reported <br />for overburden containing 70 percent sandstone. Also, because of hardness, fractures <br />do not close as readily in brittle rocks as they do in soft rocks during recompression. <br />• Heights of 9 to 11 times the coal extraction thickness (9 to l lt) are reported where all the <br />rocks consist of soft, plastic shales and claystones. The fractures also cotmonly close <br /> <br />2.05-113 RevisedJw.1995 PROb: 1Nb RN03: 1198 PA08 <br />