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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West & Sunset Trail Mining Areas Page 19 <br />Horizontal compressive strain ranges from -0.013 to -0.049 (-1.3 to -4.9 percent) where <br />the extraction thickness equals 10 feet, and -0.016 to -0.061 (-1.6 to -6.1 percent) where <br />it equals 14 feet (Table 3). <br />Sunset Trail Mining Area: Maximum tensile and compressive horizontal strains are calculated, <br />using the values obtained from the West Elk Mine area, and as projected in Figure 5 (Table 4). <br />These values are believed to be conservative, based on Dunrud's annual observations in the Apache <br />Rocks mining area. <br />• Panels SS 1 to SS4: For the four panels that extend into the Sunset Trail mining area, <br />projected horizontal tensile strain ranges from 0.0031 to 0.0252 (0.3 to 2.5 percent) <br />where the planned coal extraction thickness is 9 feet, and 0.0048 to 0.0392 (0.5 to 3.9 <br />percent) where the extraction thickness is 14 feet. <br />Horizontal compressive strain ranges from -0.0031 to -0.0270 (-0.3 to -2.7 percent) <br />where the extraction thickness equals 9 feet (Table 4), and -0.0048 to -0.0420 (-0.5 to <br />-4.2 percent) where the extraction thickness is 14 feet. <br />5.3.2 Maximum Projected Depths 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 boundary <br />between tensile and compressive strain <br />In terrains with slopes less than about 30 percent, the depth of the neutral surface can be estimated <br />by dividing the maximum horizontal strain values by those of maximum curvature at a given <br />location. The calculated depth of the tension zone to the neutral surface the boundary between <br />tension above and compression below—ranges from 50 to 100 feet in the subsidence monitoring <br />network at West Elk Mine. Crack depth may be much less than this projected 50 to 100 -foot range <br />of maximum values because most of the monitoring network was located on slopes exceeding 30 <br />percent. An unpublished study for the U. S. Bureau of Mines (Engineers International) indicated <br />that surface crack depth rarely is greater than 50 feet. Cracks will also be less extensive or <br />terminate where shale and claystone layers occur. Based on annual field subsidence observations, <br />maximum crack depth in bedrock in the Southern Panels, Apache Rocks West, and Sunset Trail <br />mining areas is estimated to be 1) 5 to 15 feet in terrain sloping less than, or equal to, 30 percent, <br />2) 10 to 35 feet in terrain sloping more than 30 percent, and 3) 40 to 50 feet in thick, brittle <br />sandstones in ridges (Tables 2 and 3). <br />Crack depth will likely be at a maximum value above massive coal barriers. The crack depth is <br />projected to be less (probably 10 to 20 percent less) above the panel chain pillars, where even the <br />rigid pillars are predicted to yield 10 to 30 percent of the coal extraction thickness (Tables 2 and <br />3). <br />831-032.816 Wright Water Engineers, Inc. <br />October 2018 <br />