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achieved. The resultant back-calculated shear strength was a friction angle of 17.5 degrees, which is <br />reasonably close to the reversal direct shear value and the Stark and Eid correlated value. As a <br />result, the failure surface at Cross-Section C was modeled with a friction angle of 17.5 degrees. <br />The back-calculated residual friction angle at Cross-Section A is described in Section 9.1.3.3. <br />8.2.2.6 Buttress <br />Because the buttress will comprise compacted colluvium just as the embankment material, the same <br />shear strength of 32.2 degrees was used for the buttress in the ESSA. Direct shear testing of <br />compacted colluvium at Lambert and Associates (2006) confirmed this as the appropriate strength <br />value. Two samples of colluvium, collected from test pits, were compacted to 100% standard Proctor <br />density and sheared. The resulting failure envelopes were ~'=30.0 degrees with c'=522 psf and <br />~'=34.0 degrees with c'=255 psf. Neglecting the cohesion intercepts, the friction angle of 32.2 <br />degrees was confirmed. <br />It was assumed. that using better equipment than likely used originally to compact the embankment <br />material and specifying compaction requirements, the undrained strength envelope would be higher <br />than the strength used for the embankment material. Thus, a ~~° of 32.2 degrees was used with no <br />cohesion intercept. Neglecting any cohesion intercept for the USSA case is considered conservative. <br />8.2.2.7 Downstream Drainage Blanket <br />Due to the granular nature of the drainage blanket, the strength envelopes selected for both the USSA <br />and ESSA are the same. A friction angle of 30 degrees with no cohesion was chosen as a typical <br />value. This is likely somewhat conservative, considering that the material will be well-compacted in <br />the field. <br />8.2.2.8 Upstream Clay Blanket <br />The upstream blanket will comprise compacted colluvium just as the material that will be used for <br />construction of the buttress. Therefore, a shear strength of 32.2 degrees was used for both the USSA <br />and ESSA cases. <br />8.2.3 Modeling <br />The slope stability analyses were conducted using SLOPE/W, acomputer-modeling program <br />developed by GEO-SLOPE International, Ltd. SLOPE/W uses limit equilibrium theory to compute a <br />factor of safety for earth and rock slopes. It is capable of using a variety of methods to compute the <br />factor of safety of a slope while analyzing complex geometry, stratigraphy, and loading conditions. <br />As previously mentioned, SLOPE/W allows the user to import the groundwater head file from the <br />seepage analysis to compute effective stresses. <br />P:\Mpls\06 CO\26\0626067\WorkFiles\DesignReport\FINAL\DesignReportFINAL.doc 29 <br />