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. might be present in this zone has a higher probability of being impacted than that occurring in the <br />~ centers of the panels. <br />Critical Extraction Wulth olMininp Panels <br />Critical extraction width (W) is the width of mining panels necessary for maximum subsidence to <br />occur at a given overburden depth (d). Values for critical W/d typically range from about 1.0 to 1.4, <br />with an average of about 1.2. Based on the subsidence development data for the SNW longwall <br />panel (Figure 8, Exhibit 60), the critical extraction width-to-depth iatio may be closer to 1.0 in the <br />Apache Rocks and Box Canyon permit revision areas. <br />Zones of Tensile Strain in Relation to Mine Geometry <br />Tensile strain caused by subsidence commonly reaches a maximum value in lineaz 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 zone 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 pillazs. <br />Any surface water or neaz-surface water that might be present inlthese 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 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 otSubsidence <br />A point on the surface begins to be affected when the longwall mining face is within O.ld to 0.6d <br />(d~nining depth). This point attains maximum downward velocity when the face is 0.2 to O.Sd <br />beyond the point. This corresponds with a time when subsidence is approximately 50 percent <br />• complete. Subsidence is more than 90 percent complete when the longwall mining. face is 1.0 to <br />1.4d (average about 1.2d) beyond the point Figure 8, Exhibit 60). <br />2.05-122 March 1005PR11 Ig~l <br />