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Subsidence Evaluation for the <br />Exhibit 60E Southem Panels, Apache Rocks West, & Sunset Trail Mining Areas Page 24 <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.Od 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.Od beyond the points of <br />measurement. Critical extraction width, therefore, is approximately 1.Od for the B -seam panels at <br />West Elk Mine, and is projected to range from 1.Od to 1.2d for the Southern Panels, Apache Rocks <br />West, and Sunset Trail mining areas. <br />Subsidence monitoring data collected over E -seam longwall panel E1 provides additional <br />information on the rate and duration of subsidence at West Elk Mine. Survey measurements taken <br />11 days after the longwall passed beneath the point showed that total subsidence was 93 percent <br />complete. The location of the longwall face was 1.5d beyond the survey point at the time of <br />measurement. <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 <br />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 not <br />all pillars are removed. In Figure 9, subsidence at a given point (p) was only about 60 percent <br />complete after room -and -pillar mining was completed within the area of influence of the point. <br />831-032.816 Wright Water Engineers, Inc. <br />October 2018 <br />