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Ms. Kerry SinnQtt 3909 Vineyard Avenue 10 November 2005 Page 4 hadwell&RoIlo approximately 24 feet below the ground surface at the northern portion of the site. The groundwater level at the site will likely fluctuate seasonally several feet. DISCUSSION AND CONCLUSIONS On the basis of our evaluation of the subsurface conditions at the site and the results of our studies, we conclude the project is feasible from a geotechnical standpoint. Slope Stability The computer program SLOPEIW, which uses.the Spencer method to search for the most critical failure surface and lowest factor of safetyl , was used in our slope stability analyses. Results of . our slope stability analyses are presented on Figure 2. The most critical creek channel slope, the slope that would have the lowest factor of safety, was identified from our subsurface exploratiOJi, site recolUlaissance and a site topographic map. It is approximately located at the storm water outfall where the creek channel slope is the steepest. Figure 1 shows the location of the storm water outfall.. The geometry of the most critical slope and soil properties used for the siope stability analyses are included on Figure 2. The most critical channel slope was analyzed for stability under static, seismic and rapid drawdown conditions. The results of our analyses indicate the slope is stable under all conditions, with factor of safety of about 2.7, 2.1 and 1.9 for static, seismic and rapid drawdown conditions, respectively. Seismic stability was evaluated using an acceleration of 0.15 times gravity (O,15g). Using the SLOPEIW program, we determined the yield acceleration for the most critical slope used in our analysis. The yield acceleration is defined as the acceleration at which slope movement occurs and the factor of safety is equal to 1.0, The location of the most critical failure surface for this condition is also shown on Figure 2. Because the yield acceleration is twice the peak ground acceleration that may be experienced at the site, slope movement at the site is unlikely even during a very large earthquake. Construction Considerations The soil at the site consists mainly of stiff silt over hard silts and clays containing varying amounts of sand and gravel. Trenches for the storm water outfall pipe may be as deep as 13 feet. The factor of safety is the ratio of the resistance to sliding over the sliding force. The higher the factor of safety, the more resistance the slope has to failure. Typically, a slope with a static factor of safety greater than 1.5 and a seismic factor of safety of 1.1 with a seismic coefficient of 0.1 to 0.15 is considered stable, (CDMG Special Publication 117).