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• laterally constrained. Therefore, steep slopes and cliffs, which commonly aze susceptible to <br />rock falls and landslides anyway, may become less stable when undermined. <br />Cracks that are typically wider, deeper, and which may remain open longer above rigid <br />chain pillazs or mine boundaries on steep slopes where there is little or no lateral constraint. <br />In addition, the direction of mining relative to slope direc$on may control crack width, <br />depth, and abundance. For example, tension cracks were wider, deeper, and more abundant <br />on steep canyon slopes that faced in the direction of mining than they were on slopes facing <br />in directions opposite the mining direction (Dunnrd and Osterwald 1980, p. 26-29; Gentry <br />and Abel 1978, p. 203-204). Cracks are projected to be locally wider and deeper on the <br />steep slopes and cliffs flanking West Flatiron. In the Apache Rocks mining area, maximum <br />crack depth on steep slopes and cliffs (in isolated locations) is conservatively estimated to <br />reach a maximum depth of 150 feet deep, and as much as 200 feet deep in the Box Canyon <br />mining area. These cracks may remain open until they are filled by processes of mass <br />wasting and sedimentation. However, their location on steep slopes and cliffs relative to <br />hydrologic resources is such, that these cracks will causemuumal 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. 2 <br />Mine in October 1953, after methane gas and water were encountered in quantities too <br />costly to control at that time. Overburden thicknesses in the area of the Oliver No. 2 Mine <br />increase from about 325 to 1,250 feet within a distance of about 1,500 feet beneath the ridge <br />• north of the first east-trending side canyon off Sylvester Gulch (Dunrud 1976). Large <br />volumes of methane and water apparently flowed from cracks in the mine floor in the top <br />entry of 6 East after only limited mining. Water flow in the east side canyon was reduced <br />shortly after the mine was closed (Bear 1972). <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 />mass above them. The material above the plane of weakness theh slides as a block (block glide; <br />decollement-type movement). <br />Block Glide Potential in the Oliver No. 2 Mine Area <br />The Oliver No. 2 Mine was an E/D-Seam coal mine operated east of Sylvester Gulch during the <br />1940s and early 1950s (See Map 5). Based on information contained in the Oliver No. 2 Mine <br />closure report dated October 1953, water-and methane-filled fractures within the mine E/DO-Seam <br />were encountered in the floor of the Oliver No. 2 Mine. (Both water and methane were reportedly <br />under high pressure). The fractures (with a reported trend N77°E) were encountered during pillar <br />development before any fractures produced by floor-heave would likely have occurred. For block <br />• glide to occur, the sheaz strength must be less than the sheaz stress generated by overburden load <br />2.05-124 March 2005PR11 <br />~-146h <br />