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Subsidence Evaluakion For Exhibit 60B South of Divide Mining Area Page 12 • The probable reason for the lack cracking in alluvium 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 fmm about 25 feet to 150 feet in the West Elk mining azea- including the South of Divide mining azea. 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 (~, from a vertical reference) in the Somerset azea ranges from about 8 to 21 degrees. The angle of draw measured for F Seam room-and-pillaz mining at West Elk Mine, which has overburden rock lithogy similaz 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 ruining 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 mining azea. 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 neaz- surface water most likely maybe 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 azea (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 drrection 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 lineaz 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. hnformation 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 p$lars. This zone, which is also predicted for the • South of Divide mining azea, is located approximately above the edges of the panels or slightly 831-032.620 Wright Water Engineers, Inc.