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• Apache Rocks Mining Area -The horizontal tensile strain in the eastern panels after B-Seam mining <br />is projected to range from 0.003 on the south flank of West Flatiron (overburden depth is about <br />2,250 feet) to 0.013 (units are dimensionless [L/I,]) neaz the head of Sylvester Gulch. <br />Projected horizontal tensile strain for the western panels for E-Seam mining aze 0.008 above the <br />thickest overburden on the ridges to 0.023 above the thinnest overburden in Horse Gulch. Tensile <br />strain after both E and B-Seam mining is completed may range from about 0.016 to 0.041 above <br />thickest and thinnest overburden, respectively. <br />Projected horizontal compressive strain will be in the panel centers: In the eastern panel area, using <br />an average value of 2.0 (Sm/d) from the West Elk Mine subsidence network, compressive strain is <br />projected to range from -0.004 neaz the south flank of West Flatiron to -0.020 neaz the head of <br />Sylvester Gulch. Compressive strain above the western panels after E-Seam mining is estimated to <br />range from -0.013 in azeas of thickest overburden.to -0.035 in areas of thinnest overburden. After <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 (-Em) in the current subsidence monitoring network ranges <br />from about -0.2 to -0.8 (Sa,/d) above longwall panel boundaries and about 3 to 4 (Sa,/d) in the <br />centers of the longwall panels between chain pillazs. The range in four mining azeas 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 />Maximum Projected Depths of Surface Cracks <br />Curvature, or differential tilt (the second derivative of vertical displacement with respect to <br />horizontal distance) of subsided rock layers causes horizontal stcairt. Comparison of calculated <br />curvature values and horizontal tensile strain derived from horizontal displacement measurements, <br />therefore, provides a means of calculating the depth of the neutral surface, and hence the maximum <br />depth of tension cracks from the surface. <br />The depth of the neutral surface can be estimated by dividing the maximum horizontal strain values <br />by the maximum curvature values at a given location The calculated depth of the possible tensile <br />zone ranges from about 50 to 100 feet. Below the neutral surface is a zone of compression. Crack <br />depth may be much less, but not more than these maximum values. An unpublished study for the <br />• U.S. Bureau of Mines (Engineers International) indicated that surface crack depth razely is greater <br />than about 50 feet. Also, cracks will likely ternunate at boundaries'of shales and claystones. <br />2.05-ll9 March 2005PR71 <br />M'y.~~ <br />