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West Elk Mine <br />• Box Canyon mining areas and 1.2 in the South of Divide mining area (see Exhibits 60B 60E, <br />Figure 4). <br />Zones of Tensile Strain in Relation to Mine Geometry <br />Tensile strain caused by subsidence commonly reaches a maximum value in linear zones above <br />mining panels. The locations of these zones can be determined by the break angle. At panel <br />boundaries with solid coal, subsidence data from the West Elk Mine monitoring network shows <br />that the break angle for subcritical mining panels ranges from -8 to 3 degrees with an average <br />value of about 0 degrees, or directly above the panel edges. <br />Information from the West Elk Mine subsidence monitoring network also indicates that the zone <br />of high horizontal tensile strain ranges from 100 to 150 feet wide above mine boundaries and <br />from 100 to 250 above the chain pillars. This zone is located approximately directly above or <br />slightly outside the panel boundaries and above the center of the chain pillars, unless a <br />downslope component of movement occurs on steep slopes in addition to the differential tilt <br />component (see Map 51 and Map 52). <br />The zones of maximum tensile strain above the chain pillars between the longwall panels is <br />approximately twice the strain values measured above mine boundaries. Cracks tend to be more <br />common and more permanent in zones above mine boundaries, barrier pillars, or rigid chain pillars. <br />Any surface water or near-surface water that might be present in these zones is potentially more <br />• subject to impact than in the centers of the panels. This was found by. Werner and Hempel who <br />state in their paper, Effects of Coal Mine Subsidence on Shallow Ridge - Top Aquifers in Northern <br />West Virginia (1992), "Analysis of water level and spring flow records indicates that the effects are <br />greatest at the edges of the longwall panels, in the tensional regime," and by Leavitt and Gibbens <br />(1992) who stated, "Well response was found to be correlated to the location of the well above the <br />mining with greater effects observed in zones of surface tension and compression, and fewer effects <br />in zones which are stress neutral." <br />Rate and Duration of Subsidence <br />A point on the surface begins to be affected when the longwall mining face is within 0.1 d to 0.6d (d <br />= overburden depth) of the point and is near maximum downward velocity. Subsidence is 50 <br />percent complete when the face is 0.2d to 0.5d beyond the point, and is more than 90 percent <br />complete when the face is 1.0d to 1.4d (average about 1.2d) beyond the point if longwall mining <br />is done. Data obtained above the 5th NW longwall panel at West Elk Mine plot between the <br />National Coal Board (NCB) and Somerset curves (Figure 9, Exhibit 60B). The data also show <br />that subsidence is more than 95 percent complete when the Ion-wall face has moved 1.Od beyond <br />the points of measurement. Critical extraction width, therefore, is approximately 1.Od for the B <br />Seam panels at West .Elk Mine, and is projected to range from L.Od to 1.2'd for the South of <br />Divide mining area. <br />Rate and duration of subsidence above longwall mining panels, therefore, are a function of <br />mining rate. The faster and more uniforinly the longwall mining occurs, the less time any <br />surface cracks present will be open to potentially impact surface or ground water. Therefore, <br />2.05-136 Revised June 2005 PRIG, Rev. March 2006; Alay 2006 PRIO, Arov. 2006TRIOTApril 2007TRI08; Sep. 2007 PR12; Feb. 2008 PR-12