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At sites where previous failures have occurred, the residual shear strength of the soils is mobilized <br />along the failure surface. To account for the possibility that the colluvium could mobilize its residual <br />shear strength, a correlation between liquid limit, clay -size fraction, and secant residual friction angle <br />proposed by Stark and Eid (1994) was utilized. The highest liquid limit measured in the colluvium <br />was 49 percent with a clay -size fraction in the 25 percent to 45 percent range. This leads to a <br />residual shear strength envelope of 22.0 degrees and zero cohesion. This envelope was used in the <br />analysis. <br />To account for undrained loading of the colluvium, an isotropically consolidated - undrained triaxial <br />shear test with pore pressure measurement was performed. The resulting undrained shear strength <br />envelope was 14.8 degrees and an apparent cohesion of 487 psf. <br />2.2.3 Model Results <br />The overall slope of the refuse pile includes haul road benches with 2.5H:1 V slopes between the <br />benches, which results in an overall slope of approximately 3.17H:1 V. A buttress with a key trench <br />is included in the design, as is a rock underdrain along the thalweg of No Name Gulch and a rock <br />drain to convey seepage water past the base of the buttress. In -place colluvium in the refuse pile <br />footprint will not be removed prior to placement of refuse. The modeled 2D section is placed <br />approximately in the thalweg of the draw because this will be the deepest zone of refuse and is, <br />therefore, the critical design section. <br />2.2.3.1 Model Results <br />Rule 4.10. l (1)(a) (per 4.09.1(7)) requires a long -term static factor of safety of 1.5 or higher and a <br />seismic factor of safety of 1.0 or higher. Lower factors of safety, on the order of 1.2, are generally <br />considered acceptable in geotechnical engineering practice for end -of- construction (undrained) <br />conditions as opposed to long -term (drained) conditions. The lower factor of safety is justified <br />because it is considered very unlikely that fully - undrained conditions will be present in the refuse <br />pile. It should be noted that the required seismic factor of safety remains 1.0 for long -term (ESSA) <br />and end -of- construction (USSA) conditions. <br />Table 2 summarizes model results. Figures 2 -1 (static) and 2 -2 (seismic) illustrate the design and <br />provide 2D modeling results for the colluvium fully- softened shear strength ESSA, while Figures 2 -3 <br />Revised July 21, 2011 P65 <br />2G��(� it <br />P \Mpls \06 CU26 \06261003 RPE E Permit Mod Support \WorkFiles \Permit Apphcauon \Text \Exhibit 82 RPEE design 2011 -07 -21 TR 127 doc 9 <br />