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<br />. <br /> <br />Woodward.Clyde Consultants <br /> <br />l~O' <br />li .L <br /> <br />. <br /> <br />VI. FOUNDATION SEEPAGE <br /> <br />1. General <br /> <br />I We have reviewed the seepage evaluations performed by the <br /> <br /> <br />Bureau for the proposed Narrows Dam from the safety standpoint. <br /> <br />The approach by the Bureau to alleviate potential foundation <br /> <br />I <br /> <br />seepage and piping problems, consisting of an impervious soil <br />filled cutoff trench, a slurry trench and a relief system is, <br /> <br />in our opinion, properly conservative. <br /> <br />, <br /> <br />2. Review of Bureau Analyses <br />The Bureau has performed two and three dimensional seepage <br /> <br />analyses of the dam configuration with the proposed slurry <br /> <br />. <br /> <br />trench (Ref. 21). The input hydrologic data to the model was <br /> <br />limited; however, the data available was generally interpreted <br /> <br />from a conservative standpoint by the Bureau (Ref. I, 2, 3). <br /> <br />. <br /> <br />In addition, conditions chosen to model the system were also <br /> <br />conservative. <br /> <br />The primary objective of the Bureau analyses was to <br /> <br />. <br /> <br />calculate the exit gradient of the seepage water near the toe <br /> <br />of the dam. The modeled region of interest was centered where <br /> <br />the slurry trench is proposed to terminate. Seepage through <br /> <br />t <br /> <br />the slurry trench was assumed to be zero. The analyses were <br /> <br />performed assuming an upstream pool elevation of 4448 feet. <br /> <br /> <br />Both the two and three dimensional analyses indicate exit <br /> <br />. <br /> <br />gradients near the toe could approach a value of 1, which would <br /> <br />- 22 - <br /> <br />t <br /> <br />(I <br />