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Mark Balderston <br />June 27, 2003 <br />Page 2 <br />Based on our review of regional geologic maps and field reconnaissance, it appears that the <br />steep slopes are comprised of glacial drift. A gradation analysis (minus 4 inch fraction) <br />performed on a bulk sample of the glacial drift is presented in Figure 1. The glacial drift <br />appears to be a silty sand and gravel (SM-GM) with cobbles and boulders. <br />We understand that the existing steep slopes will not be mined but will be graded to knock <br />down the bluffs and revegetated. The proposed cut slopes for the mining operation will be <br />reclaimed to a 3 horiwntal to 1 vertical (3H:1V) slope and revegetated. <br />Limit Equilibrium Stability Analysts: Based on our current understanding of the site <br />geology we have evaluated two conceptual cut slope options: 1) a 1H:1V cut slope, and 2) a <br />2H:1 V cut slope. It was assumed that the cut slopes at the base of the steep slopes would be <br />up to about 70 feet and would not extend below the river level. The global stability of the <br />conceptual cut slopes were evaluated by limit equilibrium stability analysis. The computer <br />program XSTABL and the Janbu method was used for the safety factor calculations. Soil <br />parameters used in the analysis assumed that the glacial drift materials have a unit weight of <br />130 pounds per cubic foot (pcf) and an internal friction angle of 32 degrees. [t was assumed <br />that the groundwater level was at the cut slope toe. The factor of safety (FS) for global cut <br />slope stabilii was calculated to be less than 1.0 for a IH:1 V cut slope which is below the <br />typical minimum specified value of 1.5. The FS for global cut slope stability was calculated to <br />be 1.6 fora 2H:1 V cut scope which is above the typical minimum specified value of 1.5. <br />Conclusions and Recommendations: Based on the results of the stability analyses, we <br />recommend that mining operation cut slopes be sloped at 2H:1V or flatter. If seepage is <br />encountered in the excavation, flatter cuts slopes may be necessary and the soil type should be <br />re-evaluated. Both short and long term drainage measures should be implemented in project <br />design and during the mining operation. Surface runoff can be controlled during mining by <br />careful excavation practices. Typically, these include the construction of shallow upgrade <br />perimeter drainage ditches or low earthen berms, and the use of temporary sumps to collect <br />403 193 CaEp'~GTt <br />