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West Etk Mine <br />. The following paragraphs project these subsidence pazameters for the Apache Rocks and Box <br />Canyon mining azeas. The subsidence projections are based on data obtained from subsidence <br />measurements in the current mining azea and geology, topography and groposed mining <br />configurations (Map 51 and Map 52). <br />Projected Vertical and Horizoraal Displacement <br />~.. <br />Tn flat or gently sloping terrain, vertical displacement typically increases inward from the margin <br />of the subsidence depression, is one-half the maximum value at the point of inflection, and is <br />maximum in the middle of the subsidence depression (also called subsidence basin or subsidence <br />trough). Horizontal displacement and tilt increase inwazd from the margin of the depression to a <br />maximum at the point of inflection and become zero again at the point of maximum vertical <br />displacement. <br />Positive curvature (convex upward) and horizontal tensile strain increase inwazd from the margin <br />of the depression to a maximum about midway between the depression margin and the poim of <br />inflection and decrease to zero again at the point of inflection. Negative curvature (concave <br />upwazd) and compressive horizontal strain increase inwazd from the point of inflection to a <br />maximum about midway between the point of inflection and the point of maximum vertical <br />displacement and decrease to zero again at the point of maximum vertical displacement. <br />Longwall mining panels are defined by their panel width to overburden depth ratio as <br />subcritical, critical, or supercritical. The subsidence literature indicates that a panel width to <br />overburden depth ratio between 1.0 and 1.4 (average of 1.2) generally defines the critical <br />longwall panels. Those panels with panel width to overburden depth ratios exceeding this <br />value are defined as supercritical, and those which are less are defined as subcritical. <br />Apache Rocks Minine Area -Overburden depth above projected eastern longwall panel centers <br />(B-Seam mining only) of the Apache Rocks mining azea ranges from about 950 feet near the head <br />of Sylvester Gulch to about 2,250 feet neaz the south end of West Flatiron (see Map 14 for <br />details). Overburden depth above the western panel centers ranges from about 500 feet to about <br />1,050 feet for the E-Seam and 700 feet to 1,250 feet for the B-Seam. <br />With a projected longwall panel width of 950 feet, and assuming that the chain pillaz dimensions <br />will be similar to the current West Elk caning azea, maximum subsidence (Sm=at) is predicted as <br />follows: <br />• Eastern Panels -The eastern panels, which trend about N80°W, will range from about <br />critical to subcritical in width in the west ends of ganels 1, 2, 3, and 4 SE (numbered from <br />north to south)~to subcritical in the eastern part of the panels. Maximum subsidence (S°,=at) <br />above panel centers is projected to range from 4.8 to 6 feet (0.4t to O.St) beneath areas of <br />greatest overburden depth to 7.2 to 9.6 feet (0.6tto 0.8t) near the head of Sylvester Gulch. <br />The maximum subsidence above the chain pillazs (S~=arot) is projected to range from 2.4 feet <br />(0.2t) in areas of thinnest overburden to 6.0 feet (O.St) feet in azeas of thickest overburden. <br />2.05-115 Revised Jun. 1995 PR06; 1/96 RN03; RevisedJme 1998 PR08 <br />