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Ms.Amy Eschberger <br /> RE: Irwin/Thomas Mine M-2016-054 <br /> October 16, 2018 <br /> the revised exhibits as a point of reference for measuring setbacks. The pedestrian path is <br /> outside of the creek top of bank and provides a conservative, stable, and identifiable <br /> reference point for setback distances. <br /> 6.4.12 Exhibit L—Reclamation Costs <br /> 3. A revised Exhibit L, Reclamation Costs, is attached (see Attachment B). <br /> a. The topsoil cover thickness has been revised to 12 inches. <br /> b. The reclamation cost estimate currently includes calculations for backfilling highwalls an <br /> average of 20 vertical feet high with 3H:1 V slopes. The maximum mining depth and <br /> depth of the slurry wall is estimated to be 25 feet, however, the average depth of the <br /> mining cell is conservatively estimated to be 20 feet. The 20-foot average depth is <br /> referenced in the Mining Operation Aspects and Disturbance Areas table of Exhibit D, <br /> Mining Plan. Therefore, no revisions were made to the highwall backfilling cost estimate. <br /> c. The slurry wall unit construction cost has been revised to $5.97 per square foot. <br /> Additionally, with the addition of Cell IA and slurry wall setback from St. Vrain Creek, <br /> the length of the slurry wall has decreased from 11,500 feet to 10,200 feet. <br /> d. While incorporating the Flood Impact and Mitigation Evaluation Report recommendations <br /> into the mining and reclamation plans, Mining Cell IA was created to provide a 500-foot <br /> St. Vrain Creek setback for additional mining in MA1. With the mining and backfilling of <br /> Cell 1 A (please see revised Exhibit C-4, Pre-Mining Map) occurring near the beginning of <br /> mining operations in MA1, the settling ponds have been eliminated. Sedimentation sumps <br /> will be used in each individual mining cell prior to discharge of dewatering flows. <br /> Additionally, the mining and backfilling of Cell IA will occur near the beginning of the <br /> mining operation and will be complete at the point of maximum disturbance. Therefore, <br /> costs associated with the reclamation of Cell IA are not included in the reclamation cost <br /> estimate. Since the slurry wall is now setback from St. Vrain Creek, the slurry wall length <br /> has also be revised in the reclamation cost estimate. <br /> e. The scarification area has been revised to 13.5 acres. <br /> f. Equipment used during reclamation will be similar to the equipment used during mining. <br /> Scrapers will be used for overburden and topsoil placement during backfilling and <br /> material spreading. Dozers are expected to be used during rough and final grading and for <br /> scarification. A dozer will also be necessary to mix slurry and for trench backfilling <br /> during slurry wall construction. Excavators will primarily be used for trench excavation <br /> and for slurry mixing during slurry wall construction;. A long arm excavator will be <br /> necessary for the slurry wall trench excavation. Miscellaneous equipment may also <br /> include a water truck, pumps for slurry mixing and dewatering, and farm equipment for <br /> seeding. All equipment should be readily available from the Longmont area. <br /> Page 2 of 3 <br />