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West Elk Mine <br />mining both the E and B Seams, compressive strain is estimated to range from -0.025 to -0.062 in <br />thickness for thickest and thinnest overburden, respectively. <br />Box Canyon Mining Area - Maximum projected horizontal tensile strain in the Box Canyon mining <br />area is expected to range from 0.002 at West Flatiron (overburden depth about 2,300 feet) to about <br />0.017 above the northern panel center beneath Box Canyon and the unnamed canyon to the west, <br />where the minimum overburden depth above the centerline of the northern panel is about 750 feet. <br />Maximum tensile strain in both areas of discussion may be considerably greater than the predicted <br />range along slopes steeper than 30 percent and on the cliffs and ridges that flank West Flatiron. <br />Maximum horizontal compressive strain (-ER,) in the current subsidence monitoring network ranges <br />from about -0.2 to -0.8-(Sn,/d) above longwall panel boundaries and about 3 to 4-(S,,,/d) in the <br />centers of the longwall panels between chain pillars. The range in four mining areas of the Western <br />United States by Mr. Dunrud is -1.5 to -4-(S/d) and averages -2.5-(S/d). Using an average value of <br />-2.0-(Sm/d), maximum compressive strain estimates above the centers of the longwall panels range <br />from -0.003 on West Flatiron to -0.026 above the northernmost longwall panel beneath Box <br />Canyon. <br />South of Divide Mininz Area - Maximum tensile and compressive horizontal strains have been <br />calculated for the South of Divide mining area, using the values obtained from the West Elk <br />Mine area (see Exhibit 60B, Figure 5 and Table 2). These values are believed to be conservative, <br />based on Mr. Dunrud's annual observations since 1998 in the Apache Rocks mining area. <br />• Panels El to E8: For these eight panels, projected horizontal tensile strain ranges from 0.004 <br />to 0.022 (0.4 to 2.2 percent) where the planned coal extraction thickness is 8 feet; from 0.007 <br />to 0.027 (0.7 to 2.7 percent) where the extraction thickness equals 12 feet; and from 0.007 to <br />0.039 (0.7 to 3.9 percent) where the extraction thickness equals 14 feet. <br />Horizontal compressive strain ranges from -0.004 to -0.024 (-0.4 to -2.4 percent) where the <br />extraction thickness equals 8 feet; -0.007 to -0.029 (-0.7 to -2.9 percent) where the thickness <br />equals 12 feet; and -0.007 to -0.042 (-0.7 to -4.2 percent) where it equals 14 feet. <br />• Panel E9: Predicted horizontal tensile strain in this single panel ranges from 0.018 to 0.027 <br />(1.8 to 2.7 percent) where the coal extraction thickness equals 12 feet and 0.021 to 0.031 (2.1 <br />to 3.1 percent) where the extraction thickness equals 14 feet. <br />Compressive strain this panel is predicted to range from -0.018 to -0.029 (-1.8 to -2.9 <br />percent) where the extraction thickness equals 12 feet and -0.021 to-0.034 (-2.1 to -3.4 <br />percent) where it equals 14 feet. <br />Maximum Depth of Surface Cracks <br />Curvature, or differential tilt (curvature is the second derivative of vertical displacement with <br />respect to horizontal distance) of subsided rock layers causes horizontal strain. Comparison of <br />calculated curvature values and horizontal tensile strain derived from horizontal displacement <br />measurements, therefore, provides a means of calculating the depth of the neutral surface, and <br />hence the maximum depth of tension cracks from the surface. The neutral surface is the <br />boundary between tensile and compressive strain. <br />2.05-132 Revised June 2005 PRIO. Rev. Alarch 2006; A1ap 2006 PRI G. Nor. 2006TR107.Apri12007TR108; Sep. 2007 PR12; Feb. 2008 PR-12