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• 7.0 EFFECTS OF TOPOGRAPHY AND STRUCTURE ON SUBSIDENCE <br />PROCESSES <br />In contrast to subsidence of rock units as fixed-end, laterally constrained, multiple plates, <br />subsidence in steep topography may occur as non-fixed end, laterally unconstrained multiple plates <br />(rock units). This lack of lateral confinement may cause reversals of horizontal displacement and <br />excessive tensile strain may occur on steep slopes. Peng and Hsuing (1986) found that horizontal <br />displacement is affected by slopes greater than 20 percent. Displacements on steep slopes and <br />cliffs can cause cracks to open more along faults, fractures, and joints than would occur in subdued <br />topography where the rock units are laterally constrained. Therefore, steep slopes and cliffs, which <br />commonly are susceptible to rockfalls and landslides anyway, may become less stable when <br />undermined. <br />The topography is less rugged in the South of Divide and Dry Fork mining areas than in the Box <br />Canyon mining area. However, there are steep slopes and local cliffs and ledges. Therefore, these <br />steeper slopes and cliffs may become less stable when they are undermined. <br />7.1 Effects of Topography on Subsidence Cracks <br />Cracks commonly are 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 observed to be wider, deeper, and more abundant on steep canyon slopes that <br />faced in the direction of mining than they were on slopes facing in directions opposite the mining <br />direction (Dunrud and Osterwald 1980, p. 26-29; Gentry and Abel 1978, p. 203-204). <br />Cracks are projected to be the 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 and Deep Creek. <br />Maximum crack depth on these steep slopes and cliffs is estimated to locally be from 15 to as much <br />as 35 feet deep. Due to the lack of lateral constraint, these cracks may remain open until they are <br />filled by processes such as sheet wash and sedimentation. <br />7.2 Effects of Rugged Topography on Subsidence and Mine Stresses <br />The subsidence factor (a) reportedly can vary significantly in draws and on ridges in rugged <br />topography. Gentry and Abel (1978, p. 203-204) report that vertical displacement was 25 to <br />30 percent greater on a ridge than it was in an adjacent draw in the York Canyon (Raton, New <br />Mexico) longwall mining area (Figure 4). Based on this information, the subsidence factor is <br />projected to be closer to 0.6 in deep draws and closer to 0.8 on isolated points and ridges in the <br />South of Divide and Dry Fork mining areas. No significant similar influence is expected in this <br />mining area because there are few, if any, isolated ridges. <br />Based on observations by Dunrud (2006, p. 16) in the Somerset Mine in the mid-1970s, stresses <br />tended to be significantly higher beneath isolated ridges than they were beneath more uniform <br />overburden of similar thickness. For a similar mine geometry, roof falls, bumps (rock bursts), and <br />floor heaving were noticeably greater beneath the ridges than they were beneath more uniform <br />overburden of similar thickness, because there is little or no lateral constraint to distribute the <br />weight of the isolated load of the ridge. <br />• <br />Tetra Tech - 0907171P 16