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West Elk Mlne <br />edges of the panels or slightly outside the panel boundaries and above the center of the chain <br />pillars, unless adown-slope component of movement occurs on steep slopes in addition to the <br />differential tilt component. Cracks tend to be more common and more permanent in zones above <br />mitre boundaries, barrier pillars, and unyielding chain pillars. Any surface or near-surface water <br />that might be present in this zone has a higher probability of being impacted than that occurring in <br />the centers of the panels. <br />Critical Extraction Width of Mining Panels <br />Critical extraction width (VJ) 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 <br />1.4, with an average of about 1.2. Based on the subsidence development data for the SNW <br />longwall panel (Figure 8, Exhibit 60), the critical extraction width-to-depth ratio may be closer to <br />1.0 in the Apache Rocks and Box Canyon germit revisian azeas. <br />Zones of Tensile Strain in Relation to Mine Geometry <br />~~ <br /> <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 <br />zone of high horizontal tensile strain ranges from 100 to 150 feet wide above mine boundaries <br />and from 100 to 250 above the chain pillars. This zone is located approximately directly above <br />or slightly outside the panel boundazies 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 maxi_m„m 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 <br />pillars. Any surface water or near-surface water that might be present in these zones is gotentially <br />more subject to impact than in the centers of the panels. This was found by Werner and Hempel <br />who state in their paper, E,lfects of Coal Mine Subsidence on Shallow Ridge -Top Aquifers in <br />Northern West Virginia (1992), "Analysis of water level and spring flow records indicates that the <br />effects are greatest at the edges of the longwall panels, in the tensional regime," and by Leavitt <br />and Gibbers (1992) who stated, "Well response was found to be correlated to the location of the <br />well above the mining with greater effects observed in zones of surface tension and compression, <br />and fewer effects in zones which aze stress neutral. " <br />2.05-122 Revisedhm. /995 PR06; //96RN03; RevisedJme. 1998 PR08 <br />