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feet and with the downstream and upstream faces of the structures constructed to 2.5 (horizontal) to 1 (vertical) or <br />flatter slope configurations. It also appears that the embankment crest widths will be 10 feet. It also appears that a <br />keyway 10 feet in width and 3 feet deep will be constructed beneath each structure. <br />A phreatic surface has also been added to this model to analyze the embankment under a high runoff condition with <br />steady state seepage, which is considered to be the worst case scenario. Therefore, a failure of the downstream face of <br />the embankment during peak water storage was analyzed. <br />Based on the laboratory test results completed on a sample of the fill materials that was recently obtained from the site <br />and to be used for the embankment fill materials and keyways, a unit weight (soil density) of 120 pcf, a cohesion value <br />of 800 psf and a phi angle of 0 were used for the anticipated compacted clay fill materials. In addition, a unit weight <br />(soil density) of 110 pcf, a cohesion value of 400 psf and an effective phi angle of 0 were assumed and used for the <br />existing overburden clays. The values used for the overburden clays are relatively conservative and have been obtained <br />from previous projects completed at the mine. The laboratory test results completed for this project are shown in <br />Appendix A. <br />Based on our calculations and analysis, the minimum calculated theoretical factor of safety for a failure of the <br />structures will be 4.430 under static conditions. The slope stability analysis data and a cross section showing the slip <br />(failure) surface are presented in Appendix C. <br />Conclusions: Based on the results of this evaluation, we believe that the proposed runoff structures, if properly <br />compacted and constructed to the approximate configurations shown in Appendix A, should be stable and perform <br />adequately. We strongly recommend that a representative of NWCC observe the construction of the structures and to <br />verify that the subsurface conditions exposed in the excavations are similar to those assumed in this report, as well as to <br />be on-site to test the compaction of the fill materials as they are placed in the embankment fill slopes and keyways. <br />We appreciate the opportunity to be of professional Intact ice to you in this matter. If you have any questions regarding <br />this report, or if we can be of further service, please this office. <br />Sincerely, <br />NWCC, INC., <br />0 RFG?S <br />O LPN ?• ??f,F? <br />Josh P. Frappart, E.I.T. <br />Reviewed by: Brian D. Len, P.E. ' <br />At <br />_ <br />• <br />Job Number: 07-7600 NWCC, Inc. Page 2