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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 21 <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <br />5.5 Break Angle <br />The break angle, the angle (B, from a vertical reference) of a straight line projected from the zone <br />of maximum horizontal tensile strain at the ground surface to the boundary of the mine workings, <br />is more important than the draw angle for hydrologic analyses (Figure 3). The break angle <br />provides a means of determining zones, in relation to underground mine workings, where near- <br />surface water most likely may be impacted. The break angle generally averages 10 degrees less <br />than the corresponding draw angle, according to Peng and Geng (1982). <br />The break angle ranges from -9 to 3 degrees in the West Elk Mine subsidence monitoring network <br />area (Table 1). Topography appears to control the location of the zone of maximum tensile strain <br />and consequently the break angle. For example, the break angle is 3 degrees where tilt direction <br />(caused by subsidence) is opposite to the direction to 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) (Table 1). <br />Tensile strain caused by subsidence commonly reaches a maximum value in linear zones above <br />mining panels. The location of these zones can be determined by the break angle (the angle of the <br />break line from panel boundaries to the zone of high tensile strain. At panel boundaries with solid <br />coal, subsidence data from the West Elk Mine monitoring network shows that the break angle for <br />subcritical mining panels ranges from -9 to 3 degrees with an average expected value of about 0 <br />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 and <br />from 100 to 250 feet wide above the chain pillars. This zone, which is also predicted for the <br />Southern Panels, Apache Rocks West, and Sunset Trail mining areas, is located approximately <br />above the edges of the panels or slightly outside the panel boundaries and above the center of the <br />chain pillars, unless a down-slope component of movement occurs on steep slopes in addition to <br />the differential tilt component. Cracks tend to be more common and more permanent in zones <br />above mine boundaries, barrier pillars, and unyielding chain pillars. Any surface or near-surface <br />water that might be present in this zone has a higher probability of being impacted than that <br />occurring in any other areas above the mining panels. <br />5.6 Angle of Major Influence <br />The angle of major influence, β, (also called angle of influence of the point of evaluation) is defined <br />by Peng (1992, p. 11) “. . . as the angle between the horizontal and the line connecting the inflection <br />point and the edge of the radius of major influence.” The radius of major influence (r) is therefore <br />the horizontal distance from the vertical projection of the inflection point to the point of maximum <br />subsidence and the limit of subsidence (Figure 3). The angle of major influence is used for <br />computer modeling by the influence function method. In the B-seam mining at West Elk Mine, <br />the angle of major influence ranges (from a horizontal reference) from about 70 to 80 degrees. <br />The angle of major influence may also be referenced to the vertical, as has been done for the break <br />angle and angle of draw. The angle of major influence (from a vertical reference) is roughly equal