2011-09-23_REPORT - C1980007 (8)

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• • • Subsidence Prediction Based on Measurements Appendix A at West Elk Mine Page 7 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 A, Enlargement 1) 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 the author (from 1996 to 2004), 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 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 1,050 feet. The alluvium and colluvium in the Dry Fork and Lick Creek drainages (estimated thickness range is 25 to 75 feet), on the average, contains more clay than does the Deep Creek alluvium. Therefore, it is very unlikely that cracks will occur in colluvium and alluvium in the stream valleys of the South of Divide mining area, even considering the shallow overburden. The probable reason for the lack of cracking in alluvial and colluvial deposits is that the fine sand- to clay -sized material and overlying soil can yield without cracking or bulging as it deforms as a discrete unit or units during the subsidence process. The alluvium observed by the author during geologic mapping activities also varies in thickness from about 25 feet to 75 feet in the West Elk mining area — including the South of Divide mining area. This same reasoning also applies to the colluvium in the area. Although subsidence cracks were locally observed in colluvium less than foot to a few feet thick, no cracks were observed in colluvium more than about 10 feet thick. Angle of Draw The draw, or limit, angle (4), from a vertical reference; Figures A, B) in the Somerset area ranges from about 8 to 21 degrees. The angle of draw measured for F -Seam room -and- pillar mining at West Elk Mine, which has overburden rock lithogy similar to the E -Seam, ranged from 11.3 to 16.1 degrees and averaged 14.4 degrees. The angle of draw for B -Seam longwall mining at West Elk ranges from about 15 to 17 degrees after accounting for F -Seam mining influence. Based on this information, the anticipated draw angle will likely be closer to the 15 to 17 degree range than the upper limit of 20 degrees for longwall mining in the South of Divide mining area. Break Angle The break angle, the angle (B, from a vertical reference; Figures A, B) of a straight line projected from the zone of maximum horizontal tensile strain at the ground surface to the boundary of the 831 - 032.690 Wright Water Engineers, Inc.