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Ms. Amy Eschberger <br /> RE: Irwin/Thomas Mine M-2016-054 <br /> July 30, 2018 <br /> 6.4.12 Exhibit L—Reclamation Costs <br /> 3. The costs for slurry wall construction were obtained from bids associated with the 2016 <br /> Treiber Lakes slurry wall construction in Fort Collins, Colorado. The Treiber Lakes mining <br /> cell in Fort Collins is about the same depth (25 feet) as anticipated for the Irwin/Thomas Site <br /> MA1 mining cell. The bid sheet from the slurry wall construction is attached. <br /> An updated Exhibit L is attached that includes costs for backfilling of the sediment ponds in <br /> Mining Cell 6. <br /> 6.4.19 Exhibit S—Permanent Man-Made Structures <br /> 4. As stated previously, the applicant has been discussing the proposed mining and reclamation <br /> operation with owners of adjacent man-made structures, including all adjacent utility owners. <br /> Structure agreements have been offered to all man-made structure owners within 200 feet of <br /> the proposed affected area boundary. Proof of offering structure agreements is attached in <br /> the form of certified letter receipts and copies of the structure agreements and transmittal <br /> letters are also attached. Copies of finalized structure agreements with interested owners are <br /> also included. <br /> 5. Since agreements could not be obtained for all man-made structures within 200 feet of the <br /> affected area boundary, the applicant has had a geotechnical evaluation performed that <br /> provides slope stability analyses for site specific conditions. The evaluation, performed by <br /> Earth Engineering Consultants, LLC, included MA1 and MA2 and is summarized in the <br /> report titled, "Subsurface Exploration and Slope Stability Evaluation, Irwin/Thomas <br /> Properties", and dated February 7, 2018. A supplemental investigation that included MA3 <br /> and MA4 was also performed and the results are summarized in the report, "Subsurface <br /> Exploration and Slope Stability Evaluation — Supplemental Report#1, Irwin/Thomas <br /> Properties",and dated June 27, 2018. Both reports are attached. <br /> Results of the initial evaluation indicate that, at a mining depth of 25 feet with near vertical <br /> highwalls, a setback of 50 feet provides a 1.5 factor of safety for adjacent man-made <br /> structures. To maintain the same factor of safety with stockpiles located within the setback, <br /> the recommended setback is 58 feet. Therefore, the applicant will maintain a minimum <br /> mining cell top of bank setback in MA1 and MA2 of 50 feet or two (2)times the adjacent <br /> mine cell depth (whichever is greater) from the permit boundary and permanent man-made <br /> structures. If the minimum setback (50 feet) is used, stockpiles will not be located within the <br /> setback. A setback distance of at least 60 feet will be provided if stockpiles are located <br /> between mining cells and the permit boundary. <br /> The supplemental investigation for MA3 and MA4 was hindered by limited access. <br /> However, two borings indicated that the sand and gravel substrate has similar properties to <br /> the substrate in MA 1 and MA2. In MA3 and MA4, surface observations suggest that <br /> overburden may be comprised of typical silts and clays, unknown backfill material, or highly <br /> organic wetland soils. It is the depth and composition of the overburden material in MA3 <br /> Page 4 of 5 <br />