2011-09-23_REPORT - C1980007 (8)

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• Appendix A Page 1 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 A): 1) caved zone, 2) fractured zone, 3) continuous deformation zone, and 4) near surface zone. Caved Zone SUBSIDENCE PREDICTION BASED ON MEASUREMENTS AT WEST ELK MINE 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 A). Rotation of the caved debris occurs during the fall so that the caved fragments tend to pile up in a random fashion. According to the 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 (Wendell Koontz, oral communication March 2004). Based on the stratigraphic and lithologic information obtained from drill holes in the South of Divide mining area, 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. Fractured Zone A zone of fracturing and local separation along rock bedding planes and joints occurs above the zone of caving (Figure A, 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 the 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. Peng (1992, p.4) reports that the combined height of the zone of caving and fracturing ranges from 20t to 30t, and that the height of the fractured zone is greater for hard, strong rocks than for soft, weak rocks. 831 - 032.690 Wright Water Engineers, Inc.