PERMFILE52001 - Laserfiche WebLink

Home Browse Search

Subsidence Evaluation For Exhibit 608 South of Divide Mining Ama Page 12 author during geologic mapping activities also varies in thiclmess 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 colltmvium 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 area 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 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 ]ongwall mining in the South of Divide mining area. 5.5 Break Angle The break angle, the angle (B, from a vertical reference) of a straight line projected firm 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 lm7cely maybe impacted. The break angle generally averages 10 degrees less than the corresponding draw angle, according to Peng and l reng (1982)_ The break angle ranges from -9 to 3 degrees in the West Elk Mine subsidence monitoring aetwork 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) (fable 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 suberitical 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 mining area, is located approximately above the edges of the panels or slightly outside the panel boundaries and above the center of the chain pillars, unless adown-slope component of movement occurs on steep slopes in addition to the differential tilt wmponent Cracks tend to be more common and more permanent in zones above mine boundaries, bamer 831-032690 Wright Water Engineers, Inc.