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

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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. This should also be the case for the stream valleys in the Dry Fork area, where the overburden is 800 to 1,400 feet thick. 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 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. 5.4 Angle of Draw The draw, or limit, angle (y, from a vertical reference) 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 (Table 1). 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 and Dry Fork mining areas. Agapito (Agapito, 2005) reports similar anticipated angle of draw (15 degrees to 17 degrees) but with a maximum of 25 degrees for longwall mining in the Dry Fork mining area. 5.5 Break Angle The break angle, the angle (B, from a vertical reference) of a straight line projected from the zone of maximum horizontal tensile strain at the ground surface to the boundary of the mine workings, is more important than the draw angle for hydrologic analyses (Figure 3). The break angle provides a means of determining zones, in relation to underground mine workings, where near-surface water most likely may be impacted. The break angle generally averages 10 degrees less than the corresponding draw angle, according to Peng and Geng (1982). The break angle ranges from -9 to 3 degrees in the West Elk Mine subsidence monitoring network area (Table 1). Topography appears to control the location of the zone of maximum tensile strain and consequently the break angle. For example, the break angle is 3 degrees where tilt direction (caused by subsidence) is opposite to the direction to the slope of the ground surface (42 percent slope), but is -9 degrees where the tilt direction is in the same direction as the slope of the ground surface (32 percent slope) (Table 1). Tensile strain caused by subsidence commonly reaches a maximum value in linear zones above mining panels. The location of these zones can be determined by the break angle (the angle of the break line from panel boundaries to the zone of high tensile strain. At panel boundaries with solid coal, subsidence data from the West Elk Mine monitoring network shows that the break angle for subcritical mining panels ranges from -9 to 3 degrees with an average expected value of about 0 degrees. Information from the West Elk Mine subsidence monitoring network also indicates that the zone of increased horizontal tensile strain ranges from 100 to 150 feet wide above mine boundaries and from 100 to 250 feet wide above the chain pillars. This zone, which is also predicted for the South of Divide and Dry Fork mining areas, is located approximately above the edges of the panels or slightly outside the panel boundaries and above the center of the chain pillars, unless a down-slope component of movement occurs on steep slopes in addition to the differential tilt component. Tura Tech - 0907171F 12