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To: Mike Peacock, Kathy Welt <br />From: Paul Swenson, Jed Greenwood <br />Subject: Summary of Additional 2D Stability Modeling for RPE East; Modified Phase 1 B Buttress <br />Date: October 12, 2012 <br />Page: 3 <br />Project: 06261003 <br />c: File <br />The mesh for the analysis consisted of triangular /rectangular unstructured finite elements and rectangular <br />structured elements for some zones. The mesh was made fine enough to capture the effects of any areas of <br />high hydraulic gradient and to ensure that the boundary conditions were applied effectively to the interior <br />of the model. Additionally, infinite elements were placed on the left (downslope) boundary to minimize <br />the boundary effects in this area. <br />Stability Modeling <br />Slope stability modeling was conducted using SLOPE/W, a computer - modeling program developed by <br />GEO -SLOPE International, Ltd. SLOPE/W uses limit equilibrium theory to compute a factor of safety for <br />earth and rock slopes. It is capable of using a variety of methods to compute the factor of safety of a slope <br />while analyzing complex geometry, stratigraphy, and loading conditions. As previously mentioned, <br />SLOPE/W allows the user to import the groundwater head file from the seepage analysis to compute <br />effective stresses. <br />Spencer's method was used as the search technique to determine the factor of safety of the dikes in this <br />stability analysis. This method is considered the most appropriate in this case because it satisfies all <br />conditions of static equilibrium and provides a factor of safety based on both force and moment <br />equilibrium. <br />Model Parameters <br />Two materials are of primary importance for this modeling exercise: the refuse and the colluvium which <br />was used to construct a portion of the buttress (compacted) and underlies the refuse (native). <br />In order to characterize the drained shear strength of the refuse material, samples from the new <br />preparation plant constructed by Mountain Coal Company in 2011 were collected from stockpiles <br />deposited on the Lower Refuse Pile. The samples were prepared through the #200 sieve using a ball mill <br />to simulate the particle breakdown which may occur due to material placement and/or freeze -thaw cycles, <br />wetting -drying cycles, or aging. The material was then prepared to a liquidity index of approximately 1.3 <br />after which it was consolidated to various normal loads in a direct shear device. The four -point direct <br />shear results indicated a slightly non - linear shear strength envelope, with a friction angle of 33.8 degrees <br />and an apparent cohesion of 228 psf. The laboratory testing results are attached. It should be noted that <br />P:\Mpls \06 CO\26 \06261003 MCC Refuse Pile Site Review & Permit \WorkFiles\Slope Modeling\9-27-12\RPE_East_Modeling_Summary_10- <br />12-2012.docx <br />