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percentages of shale to sandstone aze similaz in the B-Seam and E-Seam roof rocks, a much higher <br />• degree of local variability occurs above the E-Seam. <br />The B and E-Seam roof rocks above the first 20 feet consist of shale, siltstone, lenticular sandstones, <br />and thin coal beds. A marine sandstone, locally consisting of a lower and upper tongue and ranging <br />from about 30 to 125 feet thick, underlies the D and E-Seams; the D-Seam occurs a foot, to as much <br />as 50 feet below the E-Seam. <br />Mr. Dunrud estimates that the caved zone in the Apache Rocks and Box Canyon mining areas <br />will range from 2 to 4 extraction thicknesses. Caved zone heights closer to 2 times the mining <br />thickness (t) aze 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 area <br />and Box Canyon lease tract is 2.St per Mr. Dunrud. <br />Fractured Zone <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 vertical <br />constraints in the adjacent overburden strata and the caved rocks below minimize further <br />displacement or rotation of the fractured rock. Displacements in the fractured zone and severity of <br />fracturing tends to decrease upward as lateral and vertical confming~ stresses increase. <br />Peng (1992) reports that the combined height of the zone of caving and fracturing ranges from 20 to <br />• 30 extraction thicknesses (20 to 30t), and that the height of the fractured zone is greater for hard, <br />strong rocks than it is for soft, weak rocks. As discussed in mote detail in Exhibit 60, Richard <br />Dunrud has concluded that within the Apache Rocks and Boxy Canyon mining areas, a more <br />appropriate range is 15 to 20t. <br />Compression arches (arcuate zones of compressive stress) commorly develop, or partially develop; <br />above the mining panels. These azches temporarily transfer overburden stresses to the panel barrier <br />or chain pillazs and also to the caved zone and the mining face ,(Dunrud 1976). The azches in a <br />given azea commonly move upward and disperse as longwall iriiniug is completed in the area. <br />Compression arches may not disperse where the room-and-pillaz mining method is used, because <br />pillars and stumps left after mining may prevent dissipation of the arches. The rocks affected by the <br />azches temporarily are subjected to increased stress and strain as the arches move upwazd. However, <br />in the longwall mining azea, this increased stress and strain commonly is less than it is in room-and- <br />pillaz mining azeas because stresses are relieved as the azches move iupwazd and disperse. <br />The height of the zone of fracturing is a function of lithology and layer thickness, according to Peng <br />(1992). For example, the zone of fracturing commonly is higher for strong, thickly-bedded, brittle <br />sandstones than it is for thinly layered, soft, plastic shales and claystones. Liu (1981), reports <br />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 80t) aze reported in strong brittle <br />rocks, such as siliceous sandstones and limestones; a; value of 28t was reported for <br />• <br />2.05-I l3 March 2005PR!! f <br />07 <br />~ I~ <br />