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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 (d <br />= 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 is <br />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 is <br />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 at <br />West Elk Mine, and is projected to range from 1.0d to 1.2d for the South of Divide and Dry Fork <br />mining areas. <br />Rate and duration of subsidence above longwall mining panels, therefore, are a function of mining <br />rate. The faster and more uniformly the longwall face moves, the less time any surface cracks <br />present will be open to potentially impact surface or ground water. Therefore, rapid, uniform mining <br />beneath streams and other sensitive features causes minimum mining impact. The duration of <br />subsidence above room-and-pillar mines is less predictable, however, because not all pillars are <br />removed. In Figure 9, subsidence at a given point (p) was only about 60 percent complete after <br />room-and-pillar mining was completed within the area of influence of the point. <br />• <br />0- <br />Telra Tech - 0907171'P 15