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~~ tX~; Consulting Engineers and Geologists <br />of 620 gallons (2350 liters) per day, per foot of embankment. For the <br />10 year, 24 hour maximum flood elevation, the seepage at the maximum <br />section is on the order of 320 gallons (1200 liters) per day per linear <br />foot of the embankment. At this time, it is our opinion that these <br />seepage rates do not pose a problem to stability. <br />Slope Stability Analysis. The embankment consists of wide varieties of <br />soils and corresponding engineering properties. A modified Bishop <br />limiting equilibrium slice and slip circle computer analysis was <br />selected to analyze the embankment due to the probability that the <br />failure of the embankment would resemble a circular arc and be a toe <br />type of failure not extending into the foundation materials. Due to the <br />siesmicity of the region, a pseudo-static earthquake coefficient of 5% <br />of gravity was incorporated into the stability analysis. Using the <br />statistical results from the previous studies, the cohesion value equal <br />to 1750 pounds per square foot (854.4 grams per square centimeter) <br />and the angle of internal friction value equal to 34 degrees, the <br />total shear strength along the failure surface was derived using weighted <br />shear strength parameter values according to their frequency of <br />occurrence. Results from the computer analysis show a factor of safety <br />equal to 1.52 for the high water level condition. Refer to <br />Appendix B for computer printout of stability analysis. The dry con- <br />dition of the embankment was also analyzed. The factor of safety <br />under dry conditions is equal to 1.53. Refer to Appendix C for computer <br />printout of stability analysis. The influence of the water level shifts <br />the failure surface slightly. The high water level steady state seepage <br />condition on the downstream face is the most critical state of the analyses. <br />The stability of the upstream face will be greater than 1.53 due to the <br />reduced seepage force acting on the upstream face. From the statistical <br />approach used in determining the confidence interval for shear strength <br />parameters used, we are 95$ confident that the factors of safety for the <br />embankment obtained for these analyses are greater than 1.52. It should <br />be noted that the strength values used in the shear strength determinations <br />were conservative values based on the low end of the confidence level <br />selected. <br />-10- <br />