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Ms. Amy Eschberger <br /> RE:Irwin/Thomas Mine M-2016-054 <br /> July 30, 2018 <br /> and MA4 that increases the variability and uncertainty of the slope stability. Therefore, the <br /> supplemental investigation recommends a conservative approach for establishing mining <br /> setbacks within these two mining areas. <br /> In MA3, borings from the supplemental investigation and previous subsurface exploration <br /> indicates that the depth of aggregate material is only 13 feet below the existing ground <br /> surface. It would be expected that the depth of material in MA4 is similar to or less than <br /> MA3. Maintaining 50-foot setbacks from the permit boundary and man-made structures <br /> would equal almost 4 times the mining depth. Based on the sand and gravel substrate <br /> analyses that resulted in a 1.5 factor of safety for a setback of twice the pit depth, the <br /> applicant believes that a 3-4 times the pit depth setback is conservative. As recommended in <br /> the supplemental investigation, if materials are encountered that differ significantly from <br /> those observed during the supplemental investigation, the geotechnical engineer will be <br /> notified for additional consultation and analyses. <br /> If you have any questions regarding this application and adequacy comment responses, please <br /> call me directly at(970) 227-2803. <br /> Sincerely, <br /> Blue Earth Solutions, LLC <br /> U41� wIf <br /> 1li4 <br /> William Schenderlein, P.E. <br /> Project Manager <br /> Enclosures <br /> Page 5 of 5 <br />