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SubTerra, Inc. Twentymile Coal Company 02/I I/99 <br />Northern Mining District Subsidence Evaluation -Final Report <br />• 2. What remedial activities are likely to be required to mitigate impacts to the Energy <br />Spur? <br />3.2 Predicted Subsidence <br />Predicted track subsidence, corresponding to the end of the five year mining cycle (i.e., <br />completion of Panel 16), is shown in Figure 4 and Table 1. The track will be subject to five mining <br />cycles resulting in up to 67 inches of subsidence and gradient changes on the order of 1%. <br />3.31m~act Mitigation <br />Partial re-leveling will be used to mitigate the effects of subsidence during mining each longwall <br />panel. The sequence of events will mirror practice established for 8-Right in the Eastern Mining <br />District and include: <br />• Survey and mark the track on 100-ff centers to establish actual line and grade. <br />• Predict daily subsidence and required ballast quantities for partial re-leveling. <br />• Establishing ground stations behind the track stations that will be used to monitor daily <br />subsidence rates. <br />• Determine culvert extension requirements. <br />• • Prepare monitoring plan. <br />• Prepare detailed mitigation plan and schedule. <br />• Partial re-level during undermining and as-built final line and grade. <br />3.4 Monitoring Reouirements <br />Initial track surveys should be completed well in advance of mining so that detailed planning can <br />be completed, culverts extended, and ballast can be delivered to the site. Daily rail and ground <br />station monitoring should begin when each longwall panel is within approximately 1,000 feet of the <br />first station to be affected and may end once the longwall panel has passed approximately 1,000 <br />feet past the last station to be affected. <br />4. Evaluation of Imaacts to Fish Creek and Its Alluvial Vallev Floor (AVF) <br />Fish Creek meanders eastwards across its alluvial valley floor and passes over the NMD starting <br />from a location above the north mains (Drawings 1-7). From the north mains, it flows along the <br />centerline of Panel 13-Right turning north-east at the mid-point (between panel start and recovery <br />rooms) of Panel 13-Right and flows north-eastwards over Panels 14, 15, and 16-Right. <br />l J <br />PN: 98-12 <br />