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Subsidence Evaluation For Exhibit 60 The Apache Rocks And The Box Canyon Mining Areas Page 17 • 5.0 RATE AND DURATION OF SUBSIDENCE A point on the surface begins to be affected when the longwall mining face is within O.ld to 0.6d (d =overburden depth) of the point and is near maximum downwazd velocity. Subsidence is 50 percent complete when the face is 0.2d to O.~d beyond the point, and is more than 90 percent complete when the face is l.Od to 1.4d (average about 1.2d) beyond the point if longwall mining is done. Data obtained above the 5th NW longwall panel plot between the National Coal Board (NCB) and Somerset curves (Figure 8). The data also show that subsidence is more than 95 percent complete when the longwall face has moved I.Od beyond the points of measurement. Critical extraction width, therefore, is approximately I.Od for the current West Elk Mine, and perhaps also for the Apache Rocks and the Box Canyon mining areas. Rate and duration of subsidence above longwall mining panels, therefore, are a function of mining rate. The faster and more uniformly the longwall face moves, the less time any surface cracks present will be open to potentially impact surface or ground water. The dwation of subsidence above room-and-pillar mines is less predictable, however, because not all pillars are removed. Note in Figure 8, that subsidence at a given point (p) was only about 60 percent complete after mining was completed within the azea of influence of the point. 6.0 EFFECTS OF TOPOGRAPHY AND STRUCTURE ON SUBSIDENCE PROCESSES In contrast to subsidence of rock units as fixed-end, laterally constrained, multiple plates, subsidence in steep topography may occur as non-fixed end, laterally unconstrained multiple plates (rock units). This lack of lateral confinement may cause reversals of horizontal displacement and excessive tensile strain may occur on steep slopes. Peng and Hsuing (1986) found that horizontal displacement is affected by slopes greater than 20 percent. Displacements on steep slopes and cliffs can cause cracks to open more along faults, fractures, and joints than would occur in subdued topography where the rock units are laterally constrained. Therefore, steep slopes and cliffs, which commonly are susceptible to rock falls and landslides anyway, may become less stable when undermined. 6.1 Effects of Topography on Subsidence Cracks • Cracks aze commonly 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 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). 831-032.181 Wright Water Engineers, Inc.