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Exhibit 60C <br />Subsidence Evaluation antl 2004 Geologic Hazard Field Observations <br />Wesi Flatiron Lease Area <br />Longwall Panels 18 and 19: <br />• Maximum tensile horizontal strain is estimated to range from 0.003 to 0.020 (0.3 to 2.0 <br />percent). <br />• Maximum compressive horizontal is projected to range from -0.003 to -0.031 (-0.3 to -3.1 <br />percent). <br />Lon~wall Panels 20 and 21: <br />• Maximum tensile horizontal strain is estimated to range from 0.003 to 0.014 (0.3 to 1.4 <br />percent). <br />• Maximum compressive horizontal strain is projected to range from -0.003 to -0.022 (-0.3 to - <br />2.2 percent). <br />. 7.2 Maximum Projected Depths of Surface Cracks <br />In areas where coal is extracted by the longwall method, all the coal is extracted within the <br />longwall panel. The surface and rocks above the longwall panels therefore undergo <br />downwarping as multiple plates, or layers, in three dimensions, or beams in two dimensions. <br />Under this conceptual model, no surface water can flow a vertical distance greater than the depth <br />of the neutral surface (the surface where tension above becomes compression below). This <br />vertical distance commonly is one-half the distance of the thickness of the rock unit, or units, <br />behaving as a plate or beam. <br />Using this conceptual model of downwarping as multiple plates, the maximum crack depth-[he <br />depth of the neutral surface~an be estimated by dividing the maximum horizontal strain values <br />by those of maximum curvature at a given location. The calculated depth of the tension zone to <br />the neutral surface-the boundary between tension above and compression below-ranges from <br />50 to 100 feet in the in the subsidence monitoring network at West Elk Mine. Crack depth may <br />be much less than this projected 50-100 foot range of maximum values. An unpublished study <br />• <br />831-032.650 Wright Water Engineers, Inc. Page 15 <br />November 2004 <br />