2011-09-23_REPORT - C1980007 (8)

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Subsidence Prediction Based on Measurements Appendix A at West Elk Mine Page 8 mine workings, is more important than the draw angle for hydrologic analyses. 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, Exhibit 60B). 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). 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 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 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 affected than water occurring in any other areas above the mining panels. Angle of Major Influence The angle of major influence, f3, (also called angle of influence of the point of evaluation; Figure B) is defined by Peng (1992, p. 11) ". . . as the angle between the horizontal and the line connecting the inflection point and the edge of the radius of major influence." The radius of major influence (r) is therefore the horizontal distance from the vertical projection of the inflection point to the point of maximum subsidence and the limit of subsidence. The angle of major influence is used for computer modeling by the influence function method. In the B -Seam mining at West Elk Mine, the angle of major influence ranges (from a horizontal reference) from about 70 to 80 degrees. The angle of major influence may also be referenced to the vertical, as has been done for the break angle and angle of draw. The angle of major influence (from a vertical reference) is roughly equal to the angle of draw, and is therefore also predicted to range from 10 to 20 degrees. 831 - 032.690 Wright Water Engineers, Inc.