2008-03-21_PERMIT FILE - C1980007A (8)

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5.0 SUBSIDENCE PREDICTION BASED ON LONGWALL MINING AT WEST ELK MINE Subsidence, as it relates to mining, is defined herein as the local downward displacement of the surface and overburden rock in response to mining under the influence of gravity. For purposes of describing subsidence effects on overburden material and the ground surface, subsidence can be divided into four zones (Figure 2): 1) caved zone, 2) fractured zone, 3) continuous deformation zone, and 4) near surface zone. 5.1 Caved Zone As coal is extracted and a void is produced, the roof rocks break along bedding planes, joints, and fractures and fall to the mine floor (Figure 2). Rotation of the caved debris occurs during the fall so that the caved fragments tend to pile up in a random fashion. This caved zone, according to Peng (1992, p. 1-2) occurs for the first 2 to 8 mining (or coal extraction) thicknesses (2t to 8t) in the roof rocks (for example, if t=12 feet, the caved zone would range from 24 to 96 feet [2t to 8t]). According Wendell Koontz, senior geologist at West Elk Mine, this caved zone averages about 2.5t for longwall mining of the B Seam and all the mining areas at West Elk Mine. This includes the Apache Rocks and Box Canyon mining areas (Koontz, oral communication March 2004). Based on the stratigraphic and lithologic information obtained from drill holes in the South of Divide and Dry Fork mining areas, the rocks consist of a greater proportion of shales, siltstones, and claystones than are present in the Apache Rocks and Box Canyon mining areas. The height of the caved zone is therefore projected to range from 2t to 5t, depending on water conditions encountered and on specific roof lithology. In a dry environment, where lenticular sandstones comprise the E Seam roof, the caved zone will be closer to 2t. In a wet environment where soft shales and claystones occur in the roof, however, the caved zone will likely be closer to 5t. The average height of the caved zone is projected to average 3t. 5.2 Fractured Zone A zone of fracturing and local separation along rock bedding planes and joints occurs above the zone of caving (Figure 2, Enlargement 1). In this zone, which is transitional to the underlying caved zone, lateral and vertical constraints in the adjacent overburden strata and the caved rocks below prevent further large displacement or rotation of the fractured rock. Displacements in the fracture zone and severity of fracturing tend to decrease upward as lateral and vertical confining stresses increase. Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one-third of the fractured zone (in terms of height) has only minor fractures with little potential for water conductivity. In the lower two-thirds of the fractured zone, water conductivity commonly increases progressively downward. Compression arches (arcuate zones of compressive stress) commonly develop, or partially develop, above the mining panels. These arches temporarily transfer overburden stresses to the panel barrier or chain pillars and to the caved gob and the mining face (Dunrud 1976). Stresses temporarily increase in the zones of these compression arches. However, the arches in a given area commonly move upward and dissipate as longwall mining is completed in that area. Arches may not dissipate where the room-and-pillar mining method is used, because pillars and stumps left after mining can prevent dissipation of the compression arches. The overburden rocks affected by the arches are temporarily subjected to increased stress and strain as the arches move upward. In longwall mining areas, this increased stress and strain commonly are less than in room-and-pillar mining areas because stresses are relieved as the arches move upward and dissipate. Tetra Tech - 0907171P