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West Elk Mine <br />• The probable reason for the lack cracking in alluvium is that the fine sand- to clay-sized <br />material and overlying soil can yield without cracking or bulging as it deforms as a discrete <br />unit, or units during in the subsidence process. The alluvium observed by Mr. Dunrud <br />during geologic mapping activities also varies in thickness from more than ten feet to many <br />tens of feet in the West Elk mining area, including the South of Divide mining area. This <br />same reasoning also applies to the colluvium in the area. A-though subsidence cracks were <br />locally observed in colluvium less than one foot to a few feet thick, no cracks were observed <br />in colluvium more than about ten feet thick. <br />Angle of Draw <br />The draw, or limit angle (rb, from a vertical reference) in the Somerset azea ranges from about 8 <br />to 21 degrees. The angle of draw measured for F Seam room-and-pillar mining at West Elk <br />Mine, which has overburden rock lithogy similar to the E Seam, ranged from 11.3 to 16.1 <br />degrees and averaged 14.4 degrees. The angle of draw for B Seam longwall mining at West Elk <br />Mine ranges from about 15 to 17 degrees after accounting for F Seam mining influence. Based <br />on this information, the anticipated draw angle will likely be closer to the 15 to 17 degree range <br />than the upper limit of 20 degrees for longwall mining in the cun•ent and South of Divide mining <br />azeas. <br />Break Angle <br />The break angle, the angle (B, from a vertical reference) of a straight line projected from the <br />zone of maximum horizontal tensile strain at the ground surface to the boundary of the mine <br />workings, is more important than the draw angle for hydrologic analyses. The break angle <br />provides a means of determining zones, in relation to underground mine workings, where surface <br />water most likely may be impacted. The break angle generally averages 10 degrees less than the <br />corresponding draw angle (Peng and Geng 1982). <br />The break angle ranges from 9 to 3 degrees in the West Elk Mine subsidence monitoring network <br />area. Topography appeazs to control the location of the zone of maximum tensile strain and <br />consequently the break angle. For example, the break angle is 3 degrees where tilt direction <br />(caused by subsidence) is opposite to the direction of the slope of the ground surface (42 percent <br />slope), but is 9 degrees where the tilt direction is in the same direction as the slope of the ground <br />surface (32 percent slope). <br />Tensile strain caused by subsidence commonly reaches a maximum value in lineaz zones above <br />mining panels. The location of these zones can be determined by the break angle (the angle of <br />the break line from panel boundaries to the zone of high tensile strain}. At panel boundaries with <br />solid coal, subsidence data from the West Elk Mine monitoring network shows that the break <br />angle for subcritical mining panels ranges from 9 to 3 degrees with an average expected value of <br />about 0 degrees. <br />_ Information from the West Elk Mine subsidence monitoring network also indicates that the zone <br />• of increased horizontal tensile strain ranges from 100 to 150 feet wide above mine boundaries <br />2.05-720 Revised November 2004 PR[0 <br />