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Geotechnlcal Evaluation of Mlne-Induced Selsmlcity on Monument Dam <br />Dorsey 8 Whitney, LLP <br />• underlying foundation soil were not described in Armstrong, 1983, and have been estimated <br />by GEI for this study based on an assumed sandy clay landslide material. Material properties <br />for each of the four layers are presented in Table 5.1. <br />TABLE 5.1 <br />MATERIAL PROPERTIES USED IN THE STATIC ANALYSIS <br />OF THE DAM <br /> Assumed Saturated Drained <br />Material Type Unit Weight Friction Angle Cohesion <br /> c de rees s <br />Layer 1 Lean Clay 125 15 1,000 <br />Layer2 Clayey Sand 130 22 40 <br />Layer 3 Lean Clay 125 15 1,000 <br />Layer4 Clayey Sand 130 22 40 <br />Foundation Gravelly 130 22 200 <br />Sand Cla <br />5.7 Results of the Analysis <br />The calculated factor of safety for the static steady-seepage analysis of the downstream slope <br />is 0.92, which indicates the downstream slope is unstable for these conditions. The <br />recommended minimum factor of safety for this type of analysis is 1.5. These results <br />generally verify the findings of Armstrong, 1983, for the downstream slope of the dam. <br />The calculated factor of safety for the static analysis of the downstream slope using lowered <br />phreatic surface within the dam is 1.09, which indicates the downstream slope is marginally <br />stable for these conditions. The recommended minimum factor of safety for this type of <br />analysis is 1.5. This stability model was subsequently evaluated for yield acceleration using <br />pseudo-static analysis methods. The yield acceleration was 0.03 g. <br />Results of the static analysis are presented in Table 5.2. Computer plots of the stability <br />analysis results are shown on Figures 5.2, 5.3, and 5.4. <br />TABLE 5.2 <br />RESULTS OF 7HE STATIC ANALYSIS ON EARTHFILL DAM <br /> <br /> <br />Case <br />Phreatic Surface <br />Static FOS Recommended <br />Minimum FOS <br />Yield Acceleration <br />1 Stead See a e Hi h level within dam 0.82 1.5 --- <br />2 C cled Reservoir Lower level within dam 1.09 1.5 -- <br />2A Yield Accel. Lower level within dam -- -- 0.03 <br />GEI L <br />19 <br />0fi033006-0i~lfi Monumm[Oem Gm[echniW E4il <br />March 2006 <br />