2011-02-04_REVISION - C1980007 - Laserfiche WebLink

Home Browse Search

Subsidence Evaluation for the Exhibit 60E South of Divide and Dry Fork Mining Areas Page 16 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 - pillar mining at West Elk Mine, which has overburden rock lithology 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). MCC collected survey data from the subsidence monuments following mining of E -seam longwall panel El. WWE's analysis of that data indicates that the mean angle of draw is approximately 16 °, with a range of 14° to 19° predicted for the South of Divide (Table 2) and Dry Fork (Table 3) mining areas. 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. Cracks tend to be more common and more permanent in zones above mine boundaries, barrier pillars, and unyielding chain pillars. Any surface or near- surface water that might be present in this zone has a higher probability of being impacted than that occurring in any other areas above the mining panels. 831- 032.810 Wright Water Engineers, Inc.