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West Elk Mine <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 aze 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 quanfities too costly to control at <br />that time. Overburden thicknesses in the azea 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 canyon was reduced shortly after the mine was closed (Beaz 1972). <br />The topography is less rugged in the SOD mining azea 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 pillazs 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 />(Dunrud 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 flanking 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 mining azea. These cracks may remain open until they <br />are filled by processes of mass wasting and sedimentation. However, their location on steep slopes <br />and cliffs relative to hydrologic resources is such, that these cracks will cause minitnal impacts. <br />Cracks aze projected to be widest and deepest on the steep slopes, cliffs and ridges adjacent <br />to and on either side of Minnesota Creek and its tributaries, as well as Lick Creek. <br />Maximum crack depth on these steep slopes and cliffs is estimated to locally be from 15 to <br />as much as 35 feet deep. Due to the lack of lateral constraint, these cracks may remain <br />open until they are filled by processes such as sheet wash and sedimentation. <br />Oliver No. 2 Mine Considerations <br />Block Glide <br />This type of movement occurs where the shear strength, or frictional resistance, along a plane or <br />planes of weakness (commonly bedding planes) is less than the sheaz stress generated by the block <br />1.05-124 Revised November 2004 PRIG <br />