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Subsidence Evaluation for the <br />Exhibit 60E South of Divide and Dry Fork Mining Areas Page 20 <br />0 <br />7.0 EFFECTS OF TOPOGRAPHY AND STRUCTURE ON <br />SUBSIDENCE 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 <br />plates (rock units). This lack of lateral confinement may cause reversals of horizontal <br />displacement and excessive tensile strain may occur on steep slopes. Peng and Hsuing (1986) <br />found that horizontal displacement is affected by slopes greater than 20 percent. Displacements <br />on steep slopes and cliffs can cause cracks to open more along faults, fractures, and joints than <br />would occur in subdued topography where the rock units are laterally constrained. Therefore, <br />steep slopes and cliffs, which commonly are susceptible to rockfalls and landslides anyway, may <br />become less stable when 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, <br />these 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 <br />mine boundaries on steep slopes where there is little or no lateral constraint. In addition, the <br />direction of mining relative to slope direction may control crack width, depth, and abundance. <br />• For example, tension cracks were observed to be wider, deeper, and more abundant on steep <br />canyon slopes that faced in the direction of mining than they were on slopes facing in directions <br />opposite the mining direction (Dunrud and Osterwald 1980, p. 26 -29; Gentry and Abel 1978, p. <br />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 <br />Creek. Maximum crack depth on these steep slopes and cliffs is estimated to locally be from 15 <br />to as much as 35 feet deep. Due to the lack of lateral constraint, these cracks may remain open <br />until they are 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 30 <br />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 these <br />mining areas because there are few, if any, isolated ridges. <br />Based on observations by Dunrud in the Somerset Mine in the mid- 1970s, stresses tended to be <br />significantly higher beneath isolated ridges than they were beneath more uniform overburden of <br />• similar thickness. For a similar mine geometry, roof falls, bumps (rock bursts), and floor <br />heaving were noticeably greater beneath the ridges than they were beneath more uniform <br />831 - 032.810 Wright Water Engineers, Inc. <br />