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Subsidence Evaluation for the <br />Exhibit 60E Southern Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 25 <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <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 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 Southern Panels and Sunset Trail mining areas than in the <br />Box Canyon mining area, while the Apache Rocks West mining area is comparable. However, <br />there are steep slopes and local cliffs and ledges. Therefore, these steeper slopes and cliffs may <br />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 <br />of 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 to <br />and on either side of Minnesota Creek and its tributaries, as well as Lick Creek, South Prong, and <br />Deep Creek. Maximum crack depth on these steep slopes and cliffs is estimated to locally be from <br />15 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 Mexico) <br />longwall mining area (Figure 4). Based on this information, the subsidence factor is projected to <br />be closer to 0.6 in deep draws and closer to 0.8 on isolated points and ridges in the Southern Panels, <br />Apache Rocks West, and Sunset Trail mining areas. No significant similar influence is expected <br />in these 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 heaving <br />were noticeably greater beneath the ridges than they were beneath more uniform overburden of