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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: 4 <br />Project: 06261003 <br />c: File <br />preparing and testing the sample in this manner was meant to represent a worst -case scenario in which <br />only the fines are present and the material exhibits its "fully softened" shear strength. <br />The drained shear strength of the colluvium was tested in a similar manner except that the material was <br />prepared through the #40 sieve and to a liquidity index of 1.4. The material was sheared at six normal <br />loads in the direct shear apparatus. The resulting non - linear failure envelope was used in the slope <br />stability analysis. This method was also used to simulate the "fully softened" shear strength of the <br />material. The laboratory testing results are attached. <br />The rock fill buttress material has been assumed to have a shear strength of 40 degrees and zero cohesion. <br />Model Results <br />The predicted pore pressures from the seepage analysis were used in the limit equilibrium slope stability <br />analysis. This pore pressure distribution is presented in Figure 1. This figure shows that the predicted <br />phreatic surface is below the bottom of the buttress. The recently installed piezometers within the buttress <br />and directly beneath the buttress all indicate that the material is unsaturated at this time (late summer <br />2012) as indicated by negative pressure heads, so a phreatic surface located below the bottom of the <br />buttress is accurate based on the latest readings. <br />The ESSA was performed for two different scales: the buttress itself and the overall refuse pile. This was <br />done by constraining the entry-exit ranges such that only the areas of interest were considered and a factor <br />of safety could be reported separately for each one. The factors of safety resulting from the analysis for <br />the modified Phase 1B buttress are shown in Table 1, and the analyses are depicted graphically in Figures <br />2 -5. <br />Table 1. ESSA Stability Analysis Results, Factor of Safety, Modified Phase 1 B Buttress <br />Scale <br />Static <br />Seismic <br />Buttress <br />1.73 <br />1.46 <br />Overall <br />1.58 <br />1.31 <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 />