2008-03-21_PERMIT FILE - C1980007A (8)

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Dry Fork Mining Area: Maximum tensile and compressive horizontal strains are calculated, using the values obtained from the West Elk Mine area, and as projected in Figure 5 (Table 2). These values are believed to be conservative, based on Dunrud's annual observations over the last seven years in the Apache Rocks mining area. • Panels E2 to E6: For the five panels that extend into the Dry Fork mining area, projected horizontal tensile strain ranges from 0.0059 to 0.0085 (0.6 to 0.9 percent) where the planned coal extraction thickness is 9 feet and from 0.0062 to 0.010 (0.6 to 1.0 percent) where the extraction thickness equals 10 feet. Horizontal compressive strain ranges from -0.0059 to -0.0092 (-0.6 to -0.9 percent) where the extraction thickness equals 9 feet and -0.0066 to -0.0106 (-0.7 to -1.1 percent) where the thickness equals 10 feet (Table 3). 5.3.2 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 An unpublished study for the U. S. Bureau of Mines (Engineers International) indicated that surface crack depth rarely is greater than 50 feet.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 and Dry Fork mining areas 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 (Tables 2 and 3). 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 (Figure 1). 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 (Tables 2 and 3). Cracks that occur above the mine panel area also tend to close, once mining faces move out of the surface area of influence (DeGraff and Romesburg 1981). Any local bed separations during active subsidence between rocks of different strengths (Figure 2) will likely close once equilibrium conditions occur. However, any cracks present above rigid chain pillars, barrier pillars, or mine boundaries may remain open where permanent tensile stresses remain after mining is completed due to the convex curvature of the subsidence profile. During the past ten years of annual observations in the West Elk mining area by Dunrud (2006, p. 11), particularly the Apache Rocks mining area, no cracks were observed above mined-out longwall panels in colluvium more than an estimated ten feet thick. No cracks have been observed in alluvium above mined-out longwall panels. No cracks were observed in the alluvium and 18 colluvium of Sylvester Gulch and Deep Creek (estimated thickness range is 25 to 150 feet) during periodic field observations in the Apache Rocks and Box Canyon mining areas. The near-surface alluvial material consisted of primarily sand, silt, clay, and soil in the two areas mentioned, and was located above rigid pillars and panel boundaries where the overburden depth ranges from 800 to Tetra Tech - 050717/P 11