2014-09-26_REPORT - C1980007 - Laserfiche WebLink

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Subsidence Prediction Based on Measurements AnnPndix A at West Elk Mine Cracks that occur above the mine panel area the surface area of influence (DeGraff and R active subsidence between rocks of differer close once equilibrium conditions occur. Ho 7 so tend to close, once mining faces move out of iesburg 1981). Any local bed separations during strengths (Figure A, Enlargement 1) will likely -ver, any cracks present above rigid chain pillars, barrier pillars, or mine boundaries may rem *in 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 Rooks mining area, no cracks were observed above mined -out longwall panels in colluvium mor 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 a>ad 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 rear- 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 m dr ages (estimated thickness range is 25 to 75 feet), on the average, contains more clay than dons the Deep Creek alluvium. Therefore, it is very unlikely that cracks will occur in colluvium 4nd alluvium in the stream valleys of the South of Divide mining area, even considering the shallow overburden. The probable reason for the lack of crackin� 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 thej subsidence process. The alluvium observed by the author during geologic mapping activities als� 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 (�, from a vertical zeference; Figures A, B) in the Somerset area ranges from about 8 to 21 degrees. The angle of dr4w measured for F -Seam room - and - pillar mining at West Elk Mine, which has overburden rocklithogy similar to the E -Seam, ranged from 11.3 to 16.1 degrees and averaged 14.4 degrees. Th4 angle of draw for B -Seam longwall mining at West Elk ranges from about 15 to 17 degrees afteri 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 vertic 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.