2006-05-31_REVISION - M1978056 (12)

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Slope StaWft Analysis Vana Gavel operation - Durham Pit Gestey, Colorado Page 3 >dry density; >direct shear strength; >grain size distribution; ? liquid Omit; and ? plasticity index. Field observations and gradation test results indicate the majority of the bank material is sand with gravel with a dry bulk density of 115 pounds per cubic foot. The intemal angel of friction was calculated to be 49 degrees. The residual and peak cohesion was reported at 0 and 450 pounds per square foot. The moisture content varied between 7.5 and 11.5%. A cohesion of 0 was used in slope failure calculations. The software program SLOPE/W and SEEP/W was used to estimate static bank failure safety factors. Mine pit and river flood conditions with rapid drawdown scenarios were simulated as well. These simulations were performed by CTL Thompson. The profile of section 43+40 as shown on Plate C15 (Attachment A) was used to predict safety factors. This section was chosen as it identifies the presence of a production gas transmission line and has substantial pit side bank height. The soil conditions were replicated to match the soil profile of soil boring B-6. Slope failure analyses were ran for the river and mine pit sides of the bank. The modified Janbu method was used as it provided more conservative safety factors. The river side stability results indicate factors of safety between 4.05 and 5.66. Simulations for static, pseudo static, mine pit flood and river rapid drawdown were performed. The same simulations were run for the pit side of the bank with calculated factors of safety varying between 1.99 and 2.35. The author used PCSTABL to evaluate slope stability at section 36+40. This section appeared to have the steepest slope between the embankment and river of any cross sections presented by Anderson Consulting. Static conditions and river side and pit side