2013-09-18_PERMIT FILE - M2013064 (10)

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Slope Stability Analysis Varra Gravel Operation — Coulson Pit Greeley, Colorado Page 2 Overview of Stability Analyses The assumptions used in the bank stability analysis include the following: • The static depth to groundwater at the distance to no pumping influence is 6 feet below ground surface and the water table will intersect the pit bank approximately ten feet above the ultimate pit depth (seepage face) during steady state dewatering. • 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 sand and gravel deposits vary between 28 and 38 degrees. • The analysis does not account for load surcharges. • During extraction activities the pit bank slope ratio is 1.25H:1 V. The software package PC -STABL was used to evaluate slope stability. Simulations using Spencer, Modified Bishop and Modified Janbu methods were run to determine the most conservative safety factor. The soil strength properties used in the analysis are presented on computer generated data sheets which are presented in Attachment C. A review of Attachment C shows proposed bank profiles are stable with a calculated safety factor 1.27 during dry mine conditions. Discussion Slope failure analyses for proposed open cut gravel operations at the Varra Coulson site indicate proposed slope cuts will be stable for dewatered or static water table conditions. The high permeability of the bank material reduces the potential for excessive pore pressures within the bank during drawdown events such as dewatering. If any significant modifications to proposed pit wall slopes occur this analysis should be considered invalid.