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West Elk Mine <br />• Zones oi'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 I50 feet wide above mine boundaries and <br />from 100 to 250 above the chain pillazs. This zone is located approximately duectly 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 zone of maximum tensile strain above the chain pillars between the longwall panels is <br />approximately twice the strain values measured above mine boundazies. Cracks tend to be more <br />common and more permanent in zones above mine boundaries, bamer pillars, or rigid chain pillazs. <br />Any surface water or neaz-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 - Tcp Aquifers in Northern <br />West Virginia (1992), "Analysis of water level and spring flow records indicates that the effects aze <br />greatest at the edges of the longwall panels, in the tensional regime," and by Leavitt and Gibbers <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 O.ld to <br />0.6d (d =overburden depth) of the point and is near maximum downward velocity. <br />Subsidence is 50 percent complete when the face is 0.2d to O.Sd beyond the point, and is <br />more than 90 percent complete when the face is 1.Od to 1.4d (average about 1.2d) beyond <br />the point if longwall mining is done. Data obtained above the 5th NW longwall panel at <br />West Elk Mine plot between the National Coal Board (NCB) and Somerset curves <br />(Figure 9, Exhibit 60B). The data also show that subsidence is more than 95 percent <br />complete when the longwall face has moved 1.Od beyond the points of measurement. <br />Critical extraction width, therefore, is approximately 1.Od for the B Seam panels at West <br />Elk Mine, and is projected to range from 1.Od to 1.2d for the South of Divide mining azea. <br />Rate and duration of subsidence above longwall mining panels, therefore, aze a function of <br />mining rate. The faster and more uniformly the longwall mining occurs, the less time any <br />surface cracks present will be open to potentially impact surface or ground water. Therefore, <br />rapid, uniform mining beneath streams and other sensitive features causes minimum mining <br />impact. <br />i <br />2.05-l36 RevrsedJw~e 2005 PRIG, Rev. March 2006; May 2006 PRIO <br />