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Subsidence Evaluation For Exhibd 608 South of Divide Mining An;a Page 16 The topography is less rugged in the South of Divide mining area than in the Box Canyon mining area. However, there are steep slopes and local cliffs and ledges. Therefore, these steeper slopes and cliffs may become less stable when they are undermined. 7.1 Effects of Topography on Subsidence Crecks Cracks commonly are wider, deeper, and may remain open longer above rigid chain pillars or mine boundaries on steep slopes where there is little or no lateral constraint. In addition, the direction of mining relative to slope direction may control crack width, depth, and abundance. For example, tension cracks were observed to be wider, deeper, and more abundant on steep canyon slopes that faced in the direction of mining than they were on slopes facing in directions opposite the mining direction (Dunrud and Osterwald 1980, p. 26-29; Gentry and Abel 1978, p. 203-204). Cracks are projected to be the widest and deepest on the steep slopes, cliffs, and ridges adjacent to and on either side of Minnesota Creek and its tributaries, as well as Lick Creelt. Maximum crack depth on these steep slopes and cliffs is estimated to locally be from 15 to as much as 35 feet deep. Due to the lack of lateral constraint, these cracks may remain open until they are filled by processes such as sheet wash and sedimentation. 7.2 Effects of Rugged Topography on Subsidence and Mine Stresses The subsidence factor (a) reportedly can vary significantly in draws and on ridges in rugged topography. Gentry and Abel (1978, p. 203-204) report that vertical displacement was 25 to 30 percent greater on a ridge than it was in an adjacent draw in the York Canyon (Raton, l+lew Mexico) longwall mining area (Figure 4). Based on this information, the subsidence factor is projected to be closer to 0.6 in deep draws and closer to 0.8 on isolated points and ridges in the South of Divide mining area. No significant similaz influence is expected in this mining area because there are few, if any, isolated ridges. Based on observations by the author in the Somerset Mine in the mid-1970s, stresses tended to be significantly higher beneath isolated ridges than they were beneath more unifomt overburden of similar thickness. For a similaz mine geometry, roof falls, bumps (rock bursts), and floor heaving were noticeably greater beneath the ridges than they were beneath more uniform overburden of similar thickness, because there is little or no lateral constraint to distribute the weight of the isolated load of the ridge. 8.0 FRACTURE-CONTROLLED DRAINAGES Based on mapping by the author in the Somerset area and the South of Divide mining area, the author believes that there is reasonably good, but certainly not conclusive, evidence that some drainages are controlled by fractures and/or joints. The Dry Fork of Minnesota Creek and some of its tributaries exhibit lineaz trends on satellite images and on high-altitude photographs that indicate, or at least suggest, fracture control (Dunrud 1976, p. 14-15). These fractures may have 831-032.690 Wright Water Engineers, Inc.