2014-09-26_REPORT - C1980007 - Laserfiche WebLink

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endix A Page 1 SUBSIDENCE PREDICTION BASED ON MEASUREMENTS 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 tol 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.'I 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 (Fiore 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 zo a averages about 2.5t for longwall mining of the B- Seam and all the mining areas at West E Mine. This includes the Apache Rocks and Box Canyon mining areas (Wendell Koontz, oral communication March 2004). Based on the stratigraphic and lithologic is Divide mining area, the rocks consist of a 4 than are present in the Apache Rocks and zone is therefore projected to range from 2t on specific roof lithology. In a dry envirc Seam roof, the caved zone will be closer rmation obtained from drill holes in the South of iter proportion of shales, siltstones, and claystones ox Canyon mining areas. The height of the caved 5t, depending on water conditions encountered and lent, where lenticular sandstones comprise the E- 2t. In a wet environment where soft shales and claystones occur in the roof, however, the jcaved zone will likely be closer to 5t. The average height of the caved zone is projected to avei}age 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),I In this zone, which is transitional to the underlying caved zone, lateral and vertical constraints n the adjacent overburden strata and the caved rocks below prevent further large displacement o rotation of the fractured rock. Displacements in the fracture zone and severity of fracturing teno to decrease upward as lateral and vertical confining stresses increase. Based on the conductivity of fractures Pen (1992, p. 143) states that the upper one -third of the fractured zone (in terms of height) has �nly minor fractures with little potential for water conductivity. In the lower two- thirds of the fractured zone, water conductivity commonly increases progressively downward. j Peng (1992, p.4) reports that the combine height of the zone of caving and fracturing ranges from 20t to 30t, and that the height of the f ctured zone is greater for hard, strong rocks than for soft, weak rocks. 831 - 032.690 1 Wright Water Engineers, Inc.