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West Elk Mine <br />Box Carryon Mining Area -Maximum projected horizontal tensile strain in the Box Canyon mining <br />azea is expected to range liom 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 (-E,,,) in the cunent subsidence monitoring network ranges <br />from about -0.2 to -0.8~(S,,,/d) above longwall panel boundaries and about 3 to 4 (Sm/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 />fmm -0.003 on West Flatimn to -0.026 above the northernmost longwall panel beneath Box <br />Canyon. <br />South of Divide Mining Area -Maximum tensile and compressive horizontal strains have <br />been calculated for the South of Divide mining area, using the values obtained from the <br />West Elk Mine area (see Exhibit 60B, Figure 5 and Table 2). These values are believed to <br />be conservative, based on Mr. Dunrud's annual observations over the last six years in the <br />Apache Rocks mining area. <br />• Panels EI to E8: For these eight panels, projected horizontal tensile strain ranges from <br />0.004 to 0.022 (0.4 to 2.2 percent) where the planned coal extraction thickness is 8 feet; <br />from 0.007 to 0.027 (0.7 to 2.7 percent) where the extraction thickness equals 12 feet; <br />and from 0.007 to 0.039 (0.7 to 3.9 percent) where the extraction thickness equals 14 <br />feet. <br />Horizontal compressive strain ranges from -0.004 to -0.024 (-0.4 to -2.4 percent) where <br />the extraction thickness equals 8 feet; -0.007 to -0.029 (-0.7 to -2.9 percent) where the <br />thickness equals 12 feet; and -0.007 to -0.042 (-0.7 to -4.2 percent) where it equals 14 <br />feet. <br />Panel E9: Predicted horizontal tensile strain in this single panel ranges from 0.018 to <br />0.027 (1.8 to 2.7 percent) where the coal extraction thickness equals 12 feet and 0.021 to <br />0.031(2.1 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 />2.05-1l8 Revised November 2004 PRIO <br />