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Subsidence Evaluation For Exhibit 6013 South of Divide Mining Area Page 15 2. The ranges calculated for vertical displacement in the conceptual model are conservative. The ranges account for changing rapidly changing overburden thickness in the local rugged terrain of the Souih 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 longwlall mining face is within O.ld to 0.6d (d =overburden depth) of the point and is near maximum downward velocity. Subsidence is 50 percent complete when the face is 0.2d to 0_Sd beyond the point, and is more than 90 percent complete when the face is 1.Od w 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 {l~1CB) 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 1.Od for the B Seam panels at West Elk Mine, and is projected [o range from 1.Od to 1.2d for the South of Divide mining area. Rate and duration of subsidence above longwall mining panels, dterefore, are a function of mining rate. The faster and more unifomdy 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-pillar mines is less predictable, however, because not all pillars are removed. In Figure 9, subsidence at a given point (p) was only about 60 percent complete after room-and pillaz mining was completed within the area of influence of the point. 7.0 EFFECTS OF TOPOGRAPHY AND STRUCTURE ON SUBSIDENCE PROCESSES In contrast to subsidence of rock units asfixed-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 (198 found that horizontal displacement is affected by slopes greater titan 20 percent. Displacements on steep slopes and cliffs can cause cracks [o open more along faults, fiactures, 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 rockfalls and landslides anyway, may became less stable when undermined. 831-032.690 WnghtWaterEngineers Inc.