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West Elk Mine <br />Apache Rocks Mining Area -The horizontal tensile strain in the eastern panels after B-Seam <br />mining is projected to range from 0.003 on the south flank of West Flatiron (overburden depth is <br />about 2,250 feet) to 0.013 (units are dimensionless [LIL]) near the head of Sylvester Gulch. <br />Projected horizontal tensile strain for the western panels for E-Seam mining are 0.008 above the <br />thickest overburden on the ridges to 0.023 above the thinnest overburden in Horse Gulch. <br />Tensile strain after both E and B-Seam mining is completed may range from about 0.016 to 0.041 <br />above thickest and thinnest overburden, respectively. <br />Projected horizontal compressive strain will be in the panel centers. In the eastern panel azea, <br />using an average value of 2.0(Sm/d) from the West Elk Mine subsidence network, compressive <br />strain is projected to range from -0.004 neaz the south flank of West Flatiron to -0.020 neaz the <br />head of Sylvester Gulch. Compressive strain above the western panels after E-Seam mining is <br />estimated to range from -0.013 in areas of thickest overburden to -0.035 in areas of thinnest <br />overburden. After mining both the E and B-Seams, compressive strain is estimated to range from <br />-0.025 to -0.062 in thickness for thickest and thinnest overburden, respectively. <br />Box Canvon Mining Area -Maximum projected horizontal tensile strain in the Box Canyon <br />mining area is expected to range from 0.002 at West Flatiron (overburden depth about 2,300 feet) <br />to about 0.017 above the northern panel center beneath Box Canyon and the unnamed canyon to <br />the west, where the minimum overburden depth above the centerline of the northern panel is <br />about 750 feet. Maximum tensile strain in both areas of discussion may be considerably greater <br />i • than the predicted range along slopes steeper than 30 percent and on the cliffs and ridges that <br />flank West Flatiron. <br />Maximum horizontal compressive strain (-E,~ in the current subsidence monitoring network <br />ranges from about -0.2 to -0.8 (Smld) above longwall panel boundaries and about 3 to 4 (Sm/d) in <br />the centers of the longwall panels between chain pillars. The range in four mining areas of the <br />Western United States by Mr. Duttrttd is -1.5 to -4 (S/d) and averages -2.5 (S/d). Using an <br />average value of -2.0(Sm/d), maximum compressive strain estimates above the centers of the <br />longwall panels range from -0.003 on West Flatiron to -0.026 above the northernmost longwall <br />panel beneath Box 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 strain. 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 <br />maximum depth of tension cracks from the surface. <br />The depth of the neutral surface can be estimated by dividing the maximum horizontal strain <br />values by the maximum curvature values at a given location The calculated depth of the possible <br />. tensile zone ranges from about 50 to 100 feet. Below the neutral surface is a zone of <br />2.05-119 RevisedJm~vary 1998 PR08 <br />