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West E!k Mine <br />• E~fecfs of ToQpemnhv on Subsidence Processes <br />]n contrast to subsidence of rock units behaving as fixed-end, laterally constrained, 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 <br />and Hsuing (1986) found that horizontal displacement is affected by slopes greater than 20 <br />percent. Displacements on steep slopes and cliffs can cause cracks to open more along <br />faults, fractures, and joints than would occur in subdued topography where the rock units <br />are laterally constrained. Therefore, steep slopes and cliffs, which commonly are <br />susceptible to rock falls and landslides anyway, may become less stable when undermined. <br />• Cracks that aze typically wider, deeper, and which may remain open longer above rigid <br />chain pillars or mine boundaries on steep slopes where there is little or no lateral <br />constraint. In addition, the direction of mining relative to slope direction may control <br />crack width, depth, and abundance. For example, tension cracks were wider, deeper, and <br />more abundant on steep canyon slopes that faced in the direction of mining than they were <br />on slopes facing in directions opposite the mining direction (Dunrud and Osterwald 1980, <br />p. 26-29; Gentry and Abel 1978, p. 203-204). Cracks aze projected to be locally wider <br />`~ and deeper on the steep slopes and cliffs flanking West Flatiron. In the Apache Rocks <br />mining azea, maximum crack depth on steep slopes and cliffs (in isolated locations) is <br />conservatively estimated to reach a maximum depth of 150 feet deep, and as much as 200 <br />feet deep in the Box Canyon muting area. These cracks may remain open until they are <br />filled by processes of mass wasting and sedimentation. However, their location on steep <br />slopes and cliffs relative to hydrologic resources is such, that these cracks will cause <br />minimal impacts. <br />• Stresses aze 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. <br />2 Mine in October 1953, after methane gas and water were encountered in quantifies too <br />costly to control at that time. Overburden thicknesses in the area of the Oliver No. 2 <br />Mine increase from about 325 to 1,250 feet within a distance of about 1,500 feet beneath <br />the ridge north of the first east-trending side canyon off Sylvester Gulch (Dunrud 1976). <br />Large volumes of methane and water appazently flowed from cracks in the mine floor in <br />the top entry of 6 East after only limited mining. Water flow in the east side canyon was <br />reduced shortly after the mine was closed (Bear 1972). <br />2.05-124 RevisedJwt. 1995 PR06; IN6 RN03; RevisedJm,. /998 PR08 <br />