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West Elk Mine <br />3. Maximum dynamic compressive strain decreased by an average of 48 percent (0.0062 to <br />• 0.0032) as the face velocity increased from 10 ft/day to 40 ft/day; dynamic horizontal <br />compressive strain decreased by 16 percent as the face increased by 30 ft/day. <br />Critical Extraction Width of Mining Panels <br />Cri6ca1 extraction width (W) is the width of mining panels necessary for maximum subsidence to <br />occur at a given overburden depth (d). Values for critical W/d typically range from about 1.0 to 1.4, <br />with an average of about 1.2. Based on the subsidence development data for the Ss' NW longwall <br />panel, the critical extraction width-to-depth ratio is estimated to be 1.0 in the Apache Rocks and <br />Box Canyon musing areas and 1.2 in the South of Divide mining area (see Exhibit 60B, Figure <br />4). <br />Rate and Duration of Subsidence <br />A point on the surface begins to be affected when the longwall mining face is within O.ld to <br />0.6d (d =overburden depth) of the point and is near maximum downward velocity. <br />Subsidence is 50 percent complete when the face is 0.2d to O.Sd beyond the point, and is <br />more than 90 percent complete when the face is 1.Od to 1.4d (average about 1.2d) beyond <br />the point if longwall mining is done. Data obtained above the 5th NW longwall panel at <br />West Elk Mine plot between the National Coal Board (NCB) and Somerset curves (Figure <br />9, Exhibit 60B). The data also show that subsidence is more than 95 percent complete <br />when the longwall face has moved 1.Od beyond the points of measurement. Critical <br />extraction width, therefore, is approximately 1.Od for the B Seam panels at West Elk Mine, <br />• and is projected to range from 1.Od to 1.2d for the South of Divide mining azea. <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 mining occurs, the less time any <br />surface cracks present will be open to potentially impact surface or ground water. Therefore, <br />rapid, uniform mining beneath streams and other sensitive features causes minimum mining <br />impact. <br />The duration of subsidence above room-and-pillar mines; however, is less predictable because <br />not all pillazs aze removed. For example, in Figure 9 of Exhibit 60B, subsidence at a given point <br />(p) was only about 60 percent complete after mining was completed within the azea of influence <br />of the point. <br />Results of Computer Modeling <br />A computer software package was used to model the results of subsidence measurements at West <br />Elk Mine and to project subsidence amounts in the Apache Rocks and Box Canyon mining areas. <br />The package used is entitled "Comprehensive and Integrated Subsidence Prediction Model <br />(CISPM)," Version 2.0, by Syd S. Peng and Yi Luo, Department of Mining Engineering, College <br />of Mineral and Energy Resources, West Virginia University, Morgantown, WV. This program <br />performed an influence function analysis and best fit of the West Elk Mine subsidence data. The <br />fit between the data points and the influence function output from the model aze shown in Figure <br />• 6, Exhibit 60B. Considering that there was some F Seam influence on the B Seam subsidence <br />2.05-/22 Revised November 2004 PRIG <br />