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Client Memorandum <br />Geotechnical and Slope Stability Analyses <br />Sylvester Gulch Facilities Area <br />March 20, 2009 <br />Page 6 <br />strength values for similar sandy to gravelly clay materials, internal friction angles are often over <br />30 degrees and cohesions often over 1,000 psf (USBR, 1987; and USDN, 1971). <br />Slope Stability Analyses <br />The computer model "Slide" (RocScience, 2009) was used to perform the slope stability model. <br />Circular failure surfaces were analyzed by the simplified Bishop and Janbu methods. <br />Groundwater was not included in the model because it was not encountered in any of the borings <br />to depths of 35 feet below ground surface. Static conditions were analyzed with a required factor <br />of safety (FS) of at least 1.5. Seismic conditions were analyzed with a pseudo-static analysis for <br />a maximum earthquake with a 10 percent probability of exceedance in 50 years (USGS, 2002). <br />This seismic event would produce a peak horizontal ground acceleration at the site of 9 to 10 <br />percent of the acceleration of gravity (g). Therefore, a horizontal seismic coefficient of 0.1 Og and <br />a vertical seismic coefficient of 0.033g were used in the pseudo-static analyses. The required <br />minimum seismic FS was 1.1 for all cases considered. <br />The maximum excavation slope was modeled at 2(h):1(v) with a height of 24 feet. It was <br />conservatively assumed that the pond is empty to determine the static and seismic FSs. The <br />minimum static FS was determined to be 1.8. The seismic FS for the excavation slope was <br />determined to be 1.5. A sensitivity analysis, performed for the worst case strength parameters, <br />yielded a minimum seismic FS of 1.3, which remains above the minimum FS of 1.1. <br />The maximum embankment section was modeled with a 2:1 interior slope, a 2.5:1 exterior slope, <br />a crest width of 12 feet and a maximum embankment height of 10 feet along the centerline of the <br />crest. Because of the sloping ground surface, the maximum crest-to-toe height is approximately <br />15 feet. A small rock berm is included along the toe approximately 2-feet high. This material <br />would come from the pond excavation. The pond will be lined with geomembrane; however, for <br />this analysis it was conservatively omitted so that a phreatic surface was modeled from the <br />normal pond water surface to the downstream toe. Based upon these conservative assumptions, <br />the downstream static FS was determined to be 2.2 and the seismic FS was determined to be <br />1.65, both well above required minimums. <br />A live surcharge load of 100 psf was applied on the embankment crest and on the upstream slope <br />above the water level along with seismic loading. This loading could result from either <br />maintenance equipment or heavy snow load. This live embankment surcharge load condition <br />yielded a FS of 1.6. The seismic load plus embankment surcharge load was analyzed for the <br />worst-case soil strength parameters for both the embankment and native subsurface materials. <br />This worst-case condition was found to have a FS of 1.36, which is still above the minimum <br />acceptable value of 1.1. <br />The upstream embankment slope was also analyzed for a rapid drawdown condition without a <br />pond liner to simulate conservative conditions. This rapid drawdown analysis yielded a FS of <br />2.3. <br />Rev. 04109-TRI16