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Subsidence Evaluation For Exhibit 808 South of Divide Mining Area Page 5 • 5.0 SUBSIDENCE PREDICTION BASED ON LONGWALL MINING AT WEST ELK MINE Subsidence, as it relates to mining, is defined herein as the local downwazd 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) neaz 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. Tkris 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 1=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.St for longwall mining of the B Seam and all the mining aeeas at West Ells Mine. This includes the Apache Rocks and Box Canyon mining areas (Koontz, oral communication Mazch 2004). Based on the stratigraphic and lithologic infornration obtained from drill holes in the South of Divide mining azea, the rocks consist of a greater proportion of shales, siltstones, and claystones than aze present in the Apache Rocks and Box Canyon mining aeeas. The height of the caved zone is therefore projected to range from Zt to St, 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 St. 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 lazge displacement or rotation of the fractured rock. Displacements in the fracture zone and severity of fracturing tend to decrease upwazd as lateral and vertical confining stresses increase. Based on width and conductivity of fractures Peng (1992, p. 143) states that the upper one-thud of the fractured zone (in terms of height) has only minor fractures with little potential for water conductivity. In the lower two-thuds of the fractured zone, water conductivity commonly increases progressively downwazd. 831-032.620 Wright Water Engineers, Inc.