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<br />6dribi[ fiOC <br />Subsidence Evaluation and 2004 Geologic Hazard Feld Observations <br />West Flatiron Lease Area <br />7.0 SUBSIDENCE PREDICTION BASED ON PAST LONGWALL MINING <br />Subsidence, as defined in this report, is the local downward displacement (downwarping) of the <br />ground surface and underlying rocks above the mined coal under the influence of gravity (see <br />Exhibit 60). Subsidence causes the overburden material above longwall mining panels to: 1) <br />cave into rubble for the first 2 to 5 coal extraction thiclmesses (2-St) above the mine floor, 2) <br />fracture for the next 10.201; and 3) deform downward (downwarp) as continuous, muhiple layers <br />from the limit of the fracture zone to the ground surface. <br />7.1 Vertical and Horizontal Displacement, Tilt, Curvature, and Horizontal Strain. <br />Differential vertical lowering of the ground surface causes vertical displacement (S), horizontal <br />displacement (S~, tilt (AT), curvature (C}, and horizontal strain (E}. Vertical displacement (also <br />called subsidence), tilt, and strain are the main subsidence parameters used in most subsidence <br />studies, and therefore, these will be the ones discussed in this report. <br />• Vertical displacement, tilt, and horizontal strain predictions for the West Flatiron Lease area are <br />\J <br />based on the baseline data obtained from subsidence measurements above the 11VW, 2AlW, and <br />31dW mining panels at the West Elk Mine (see Table 1 in Exhibit 60 for detailed information). <br />The maximum subsidence parameters for the adacent Box Canyon mining area are determined <br />as well, because longwall panels 18 through 21 are planned to be mined from the West Flatiron <br />Lease area eastward (in retreat) into the Box Canyon mining area. Therefore, in order to project <br />continuity for mining in this rugged country (the overburden depth ranges tiom about 750 feet <br />near Raven Gulch to as much as 2,250 feet beneath West Flatiron), maximum vertical <br />displacement, tilt, and horizontal strain for the total length of the longwall panels are presented <br />herein (fable 1). <br />831-O;i2.650 Wright Water Engineers, Inc. Page 10 <br />November 2004 <br />