2011-09-23_REPORT - C1980007 (8)

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• • • Subsidence Prediction Based on Measurements Appendix A at West Elk Mine Page 6 8 feet; from 0.007 to 0.027 (0.7 to 2.7 percent) where the extraction thickness equals 12 feet; and from 0.007 to 0.039 (0.7 to 3.9 percent) where the extraction thickness equals 14 feet. Horizontal compressive strain ranges from -0.004 to -0.024 ( -0.4 to -2.4 percent) where the extraction thickness equals 8 feet; -0.007 to -0.029 ( -0.7 to -2.9 percent) where the thickness equals 12 feet; and -0.007 to -0.042 ( -0.7 to -4.2 percent) where it equals 14 feet (Table 2). • Panel E9: Predicted horizontal tensile strain in this single panel ranges from 0.018 to 0.027 (1.8 to 2.7 percent) where the coal extraction thickness equals 12 feet and 0.021 to 0.031 (2.1 to 3.1 percent) where the extraction thickness equals 14 feet. Compressive strain this panel is predicted to range from -0.018 to -0.029 ( -1.8 to -2.9 percent) where the extraction thickness equals 12 feet and -0.021 to -0.034 ( -2.1 to -3.4 percent) where it equals 14 feet (Table 2). Maximum Projected Depths of Surface Cracks Curvature, or differential tilt (curvature is the second derivative of vertical displacement with respect to horizontal distance) of subsided rock layers causes horizontal strain. Comparison of calculated curvature values and horizontal tensile strain derived from horizontal displacement measurements, therefore, provides a means of calculating the depth of the neutral surface, and hence the maximum depth of tension cracks from the surface. The neutral surface is the boundary between tensile and compressive strain In terrains with slopes less than about 30 percent, the depth of the neutral surface can be estimated by dividing the maximum horizontal strain values by those of maximum curvature at a given location. The calculated depth of the tension zone to the neutral surface —the boundary between tension above and compression below— ranges from 50 to 100 feet in the subsidence monitoring network at West Elk Mine. Crack depth may be much less than this projected 50 to 100 foot range of maximum values because most of the monitoring network was located on slopes exceeding 30 percent. An unpublished study for the U. S. Bureau of Mines (Engineers International) indicated that surface crack depth rarely is greater than 50 feet. Cracks will also be less extensive or terminate where shale and claystone layers occur. Based on annual field subsidence observations, maximum crack depth in bedrock in the South of Divide mining area is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent, 2) 10 to 35 feet in terrain sloping more than 30 percent, and 3) 40 to 50 feet in thick, brittle sandstones in ridges. Crack depth will likely be at a maximum value above massive coal barriers. Crack depth may therefore be greatest above the 700 - foot -wide protective barrier system projected between longwall panels E4 and E5. The crack depth is projected to be less (probably 10 to 20 percent less) above the panel chain pillars, where even the rigid pillars are predicted to yield 10 to 30 percent of the coal extraction thickness. 831 - 032.690 Wright Water Engineers, Inc.