2016-02-25_PERMIT FILE - C1982056A (6)

Home Browse Search

SubTerra, Inc. Peabody Twentymile Coal, LLC 03/16/2015 Foidel Creek Mine Permit Revision for Longwall Mining in the Wolf Creek Seam - Subsidence Evaluation 3. Importing approximately 100,000 data points, representing the ground surface over the mining area from AUTOCAD and adding these data to the AVF data in the spreadsheet. 4. Numerically subsiding this data set in the spreadsheet. 5. Exporting the numerically subsided data setback to AUTOCAD and creating anew subsided topography. 6. Expanding the 3-D plane, representing the un -subsided water surface of each Creek, to meet the subsided ground surface and determining the new area of this surface. The results of the predictions are shown in Tables 3, 4, and 5; Figure 8; and Drawings 10, 11, 12 and 13. Summary data include: • Subsidence ranging from 0 to 70 inches. • Maximum gradient change of 2.4% along the bottom of Fish Creek. The estimated impacts of this level of predicted subsidence are provided in the following section. 5.3. Predicted Impacts to the AVF 5.3.1. Increase in Fish Creek's Surface Area Our evaluation indicates that subsidence would result in over -topping of either the north or south high bank over portions of the affected creek length. Over -topping will cause Fish Creek to spread out over the AVF and link up with oxbow lakes that were previously part of the creek. The elevation of these areas is typically within about one foot of Fish Creek's high bank(s) so that, in some cases, the creek level will rise up against the surrounding ground. As shown on Drawings 10, 11 and 12, there is no indication that the creek will occupy areas outside of its AVF. The estimated increase in Fish Creek's surface area is shown on Drawing 10 through 12. Under normal flow conditions, subsidence of the AVF is predicted to result in an increase of Fish Creek's surface area of under 50 acres. The impacted surface area of Fish Creek may expand during peak flow months and during off -normal years when peak flows are experienced. However, the relative expansion is expected to be less than previously experienced by the un -subsided AVF as stream velocities, and hence gradients, will decrease in flows over the undermined areas. This may in turn result in sediment deposition over the mined out areas and scouring in areas where subsidence has been limited by the presence of the rigid -yield pillar pair. The net increase in Fish Creek surface area during peak flow has been evaluated, assuming an associated, estimated increase in creek elevation of 2 feet, using the 3-D modeling software. The additional flooded area, for high water conditions was estimated to be approximately 20 acres. 5.3.2. Increase in Foidel Creek's Surface Area The estimated increase in Foidel Creek's surface area is shown on Drawing 13. Under normal flow conditions, subsidence of the AVF is predicted to result in an increase of Foidel Creek's surface area of under .li nnrPs 11 PN: 2013-26