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Subsidence Evaluation for the <br />Exhibit 60E South of Divide and Dry Fork Mining Areas Page 19 <br />i <br />6.0 RATE AND DURATION OF SUBSIDENCE <br />A point on the surface begins to be affected when the longwall mining face is within 0.1d to 0.6d <br />(d = overburden depth) of the point and is near maximum downward velocity. Subsidence is 50 <br />percent complete when the face is 0.2d to 0.5d beyond the point, and is more than 90 percent <br />complete when the face is 1.0d to 1.4d (average about 1.2d) beyond the point if longwall mining <br />is done. Data obtained above the 5th NW longwall panel at the West Elk Mine plot between the <br />National Coal Board (NCB) and Somerset curves (Figure 9). The data also show that subsidence <br />is more than 95 percent complete when the longwall face has moved 1.0d beyond the points of <br />measurement. Critical extraction width, therefore, is approximately 1.0d for the B =seam panels <br />at West Elk Mine, and is projected to range from 1.0d to 1.2d for the South of Divide and Dry <br />Fork mining areas. <br />Rate and duration of subsidence above longwall mining panels, therefore, are a function of <br />mining rate. The faster and more uniformly the longwall face moves, the less time any surface <br />cracks present will be open to potentially impact surface or ground water. Therefore, rapid, <br />uniform mining beneath streams and other sensitive features causes minimum mining impact. <br />The duration of subsidence above room - and - pillar mines is less predictable, however, because <br />not all pillars are removed. In Figure 9, subsidence at a given point (p) was only about 60 <br />percent complete after room- and -pillar mining was completed within the area of influence of the <br />point. <br />r� <br />U <br />0 <br />831 - 032.810 Wright Water Engineers, Inc. <br />M <br />