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Subsidence Evaluation For Exhibit 606 South of Divide Mining Area Page 15 • 2. The ranges calculated for vertical displacement in the conceptual model aze conservative. The ranges account for changing rapidly changing overburden thickness in the local rugged terrain of the South of Divide mining area and for changing lithology--such as lenticular sandstones, coal seams, and shales-in the overburden rocks. 6.0 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 neaz maximum downwazd velocity. Subsidence is 50 percent complete when the face is 0.2d to O.Sd beyond the point, and is more than 90 percent complete when the face is 1.Od to 1.4d (average about 1.2d) beyond the point if longwall mining is done. Data obtained above the 5th NW longwall panel at the West Elk Mine plot between the National Coal Board (NCB) and Somerset curves (Figure 9). The data also show that subsidence is more than 95 percent complete when the longwall face has moved 1.Od beyond the points of measurement. Critical extraction width, therefore, is approximately l.Od for the B Seam panels at West Elk Mine, and is projected to range from 1.Od to 1.2d for the South of Divide mining area. Rate and duration of subsidence above longwall mining panels, therefore, aze 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. Therefore, rapid, uniform mining beneath streams and other sensitive features causes minimum mining impact. The duration of subsidence above room-and-pillaz mines is less predictable, however, because not all pillars aze removed. In Figure 9, subsidence at a given point (p) was only about 60 percent complete after room-and-pillar mining was completed within the azea of influence of the point. 7.0 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 asnon-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 aze laterally constrained. Therefore, steep slopes and cliffs, which commonly aze susceptible to rockfalls and landslides anyway, may become less stable when undermined. 831-032.620 Wright Water Engineers. Inc.