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Memo to Travis Marshall 2 August 18, 2009 <br />Gunnison River Pits Setback Review File No. M-2009-048 <br />The DRMS has analyzed the generic slope presented in the geotechnical stability exhibit inputting the <br />standard shear strength parameters and a vertical slope into the cross section. The analyzed cross section <br />incorporates a phreatic surface that is likely to develop in the dewatered pits based on the indication in <br />Exhibit G that the ground water table is as shallow as five feet below ground surface. The analysis was <br />done using Galena slope stability software, version 5.01 and the simplified Bishop method of analysis of <br />potential circular failure surfaces. At the thirty foot mining setback proposed in the permit application, <br />an unacceptably low safety factor of 0.78 resulted. DRMS notes that the weathered shale underlying the <br />sand and gravel deposit is not a factor in this analysis, but is likely to influence analysis of larger setback <br />distances. The printed results of the analysis are enclosed. <br />The Applicant has chosen to provide a generic cross section to justify a general setback to be employed in all <br />of the pits. This is acceptable as-long-as the generic cross section analyzes the maximum pit depth that will <br />be encountered during mining. As discussed in item no. 1 above, the Applicant must provide stratigraphic <br />information so that the critical cross section for stability can be verified or selected. Alternatively, multiple <br />setback requirements for the different pits and pit areas can be established based on the depths to be mined; <br />this might allow for more complete extraction of the resource. The Applicant must provide a proposal for a <br />single general setback or multiple setbacks based on pit depth, and analyses demonstrating the sufficiency of <br />the setback(s) using the DRMS standard strength parameters or strength information measured in samples of <br />onsite deposits. <br />enclosure(s) <br />cAacs files\My Documents 4-19-06 thru\gunnison river pits setback review.docx