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Exhibit 60C <br />Subsidence Evaluation and 2004 Geologic Hazard Field Observations <br />West Flatiron Lease Area <br />• <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 thicknesses (2-St) above the mine floor; 2) <br />fracture for the next 10-20t; and 3) deform downward (downwarp) as continuous, multiple 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 (Sh), tilt (M), 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 />i <br />Vertical displacement, tilt, and horizontal strain predictions for the West Flatiron Lease area aze <br />based on the baseline data obtained from subsidence measurements above the 1NW, 2IVW, and <br />3NW mining panels at the West Elk Mine (see Table 1 in Exhibit 60 for detailed information). <br />The maximum subsidence parameters for the adjacent 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 from 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 ('t'able 1). <br />i <br />831-032.650 Wright Water Engineers, Inc. Page 10 <br />November 2004 <br />