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West Elk Mine <br />that the caved fragments tend to pile up in a random fashion. This caved zone, according to Peng <br />• (1992), occurs for the first 2 to 8 mining thicknesses (2 to 8t) in the roof rocks. In the current <br />West Elk Mine longwall panels, the caved zone is estimated to be 2.5 mining thicknesses (2.St) <br />based on roof rock observations from directly behind the current longwall equipment. Any water <br />present in this zone will drain into the mine almost immediately after caving occurs. <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) 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 and Box <br />Canyon mining azeas is 2.St. <br />Based on the stratigraphic and lithologic information obtained from drill holes in the South of <br />Divide mining area, the rocks consist of a greater amount of shales, siltstones, and claystones <br />than are present in the Apache Rocks and Box Canyon mining areas. It is therefore estimated <br />that the caved zone will range from 2t to St, depending on water conditions encountered and on <br />specific roof lithology. In a dry environment, where lenticular sandstones comprise the E Seam <br />roof, the caved zone will be closer to 2t. In a wet environment where soft shales and claystones <br />occur in the roof, however, the caved zone will likely be closer to St. <br />Fractured Zone <br />• A zone of fracturing and local separation along rock bedding planes and joints occurs <br />above the zone of caving. In this zone, which is transitional to the underlying caved zone, <br />lateral and vertical constraints in the adjacent overburden strata and the caved rocks <br />below minimize further displacement or rotation of the fractured rock. Displacements in <br />the fractured zone and severity of fracturing tend to decrease upward as lateral and <br />vertical confining stresses increase. <br />Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one- <br />third of the fractured zone (in terms of height) has only minor fractures with little potential <br />for water conductivity. In the lower two-thirds of the fractured zone, water conductivity <br />increases progressively downward. <br />Compression arches (azcuate zones of compressive stress) commonly develop, or partially develop, <br />above the mining panels. These arches temporarily transfer overburden stresses to the panel barrier <br />or chain pillars and also to the caved zone and the mining face (Dunrud 1976). The arches in a <br />given area commonly move upwazd and disperse as longwall mining is completed in the azea. <br />Compression azches 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 azches. The rocks affected by the <br />azches temporarily are subjected to increased stress and strain as the azches move upwazd. <br />However, in the longwall mining azea, this increased stress and strain commonly is less than it is in <br />room-and-pillar mining areas because stresses aze relieved as the azches move upward and dissipate. <br />2.05-!!0 RevrsedNovember 1004 PR10 <br />