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Subsidence Evaluation for the <br /> Exhibit 60E Southern 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 1.0d 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 11). 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 at <br /> West Elk Mine, and is projected to range from 1.0d 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 11, 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 /> 831-032.912 Wright Water Engineers, Inc. <br /> December 2020 <br />