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West Elk Mine <br />. calculations (0.8 to 4.2 feet). Maximum vertical displacement above the longwall panel <br />centers, however, is about equal to the median values projected in the conceptual model <br />calculations (4.8 to 11.2 feet). <br />2. The ranges calculated for vertical displacement in the conceptual model are conservative. <br />The ranges account for changing rapidly changing overburden thickness in the local rugged <br />terrain of the South of Divide mining area and for changing lithology such as lenticular <br />sandstones, coal seams, and shales in the overburden rocks. <br />Effects of Topography and Structure on Subsidence Processes <br />In contrast to subsidence of rock units behaving as fixed-end, laterally con trained, multiple <br />plates, subsidence in steep topography will typically occur as non-fixed end, laterally <br />unconstrained multiple plates (rock units). This lack of lateral confinement may locally cause <br />reversals of horizontal displacement and excessive tensile strain on steep slopes. Peng and <br />Hsuing (1986) found that horizontal displacement is affected by slopes greater than 20 percent. <br />Displacements on steep slopes and cliffs can cause cracks to open more along faults, fractures, <br />and joints than would occur in subdued topography where the rock units are laterally <br />constrained. Therefore, steep slopes and cliffs, which commonly are susceptible to rockfalls and <br />landslides anyway, may become less stable when undermined. <br />Stresses are concentrated within the overburden and coal beds beneath ridges and peaks. <br />Abnormally high stresses may have led to the closure and abandonment of the Oliver No. 2 Mine in <br />• October 1953, after methane gas and water were encountered in quantities too costly to control at <br />that time. Overburden thicknesses in the area of the Oliver No. 2 Mine increase from about 325 to <br />1,250 feet within a distance of about 1,500 feet beneath the ridge north of the first east-trending side <br />canyon off Sylvester Gulch (Dunrud 1976). Large volumes of methane and water apparently flowed <br />from cracks in the mine floor in the top entry of 6 East after only limited mining. Water flow in the <br />east side Carl- was reduced shortly after the mine was closed (Bear 1972). <br />The topography is less rugged in the SOD mining area than in the Box Canyon mining area. <br />However, there are steep slopes and local cliffs and ledges. Therefore, these steeper slopes and <br />cliffs may become less stable when they are undermined. <br />Effects of Topography on Subsidence Cracks <br />Cracks are commonly wider, deeper, and may remain open longer above rigid chain pillars or mine <br />boundaries on steep slopes where there is little or no lateral constraint. In addition, the direction of <br />mining relative to slope direction may control crack width, depth, and abundance. For example, <br />tension cracks were wider, deeper, and more abundant on steep canyon slopes that faced in the <br />direction of mining than they were on slopes facing in directions opposite the mining direction <br />(Dui d and Osterwald 1980, p. 26-29; Gentry and Abel 1978, p. 203-204). <br />Cracks are projected to be locally wider and deeper on the steep slopes and cliffs tanking West <br />Flatiron. In the Apache Rocks mining area, maximum crack depth on steep slopes and cliffs (in <br />isolated locations) is conservatively estimated to reach a maximum depth of 150 feet deep, and as <br />much as 200 feet deep in the Box Canyon miring area. These cracks may remain open until they <br />2.05-138 Revised June 2005 PRIG, Rev. March 2006.- May 2006 PRIO. Nov. 2006TRI07,April 2007TR108;Sep. 2007 PR12; Feb. 2008 PR-12