2011-04-15_PERMIT FILE - M2010049 (4)

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Slope Stability Analysis Varra Gravel Operation - Great Western Sugar Greeley, Colorado Page 2 overview of Stability Analyses The assumptions used in the bank stability analysis include the following: The static depth to groundwater is between 6 and 7 feet below grade and the water table will intersect the pit bank approximately ten feet above the mine floor (seepage face). • The maximum bank height is 45 feet. • A cohesion of 0 is assumed for all unconsolidated deposits. A cohesion of 731 psf is assumed for underlying shale deposits. The internal friction angles of the overlying soil and sand and gravel deposits vary between 26 and 45 degrees. The analysis does not account for load surcharges. • During extraction activities the pit bank is sloped at 1.25:1 to 34 feet below grade and 3:1 to the mine floor. The author used the software package PC-STABL to evaluate slope stability. Simulations using Spencer, Modified Bishop and Modified Janbu methods were run to determine the most conservative safety factor. The soil characteristics as presented earlier were used in the analysis. Computer generated data sheets are presented in Attachment A. Terracon of Greeley, Colorado was contracted to conduct a geotechnical investigation to determine soil strength properties in the area of the mine. There report is provided as Attachment B. A review of Attachment A shows proposed bank profiles are stable with a calculated safety factor of between 1.27 and 1.30 during mine side dewatered conditions. Simulations for pit side flood conditions (static water surface) indicate a safety factor of 1.69. Discussion Slope failure analyses for proposed open cut gravel operations at the Great Western Sugar site indicate that the modeled slope cuts will be stable for dewatered or static water table conditions. Using a consistent slope of 1.25:1 a Safety Factor of