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<br />Woodwanl.Clyde Consultants <br /> <br />..~ f""'I F\ .1 <br />~v:Jl <br /> <br />. <br /> <br />VI. FOUNDATION SEEPAGE <br /> <br />1. General <br /> <br />. <br /> <br />We have reviewed the seepage evaluations performed by the <br /> <br /> <br />Bureau for the proposed Narrows Dam fr~e safety standpoint. <br /> <br /> <br />The approach by the Bureau to alleviate potential foundation <br /> <br /> <br />seepage and piping problems, consis~~f~impervious soil <br />filled cutoff trench, a slurry t7ren~and a~~ system is, <br />, , . 1 I " <br />1n our op1n1on, proper y conser,va,~lve. <br /> <br />2. Review of Bureau AnalYSes;l ~ <br />The Bureau has performe~two and three dimensional seepage <br />analyses of the dam COnfigUrati~~the proposed slurry <br />trench (Ref. 21). The~hYdr,Olo~data to the model was <br />limited; however, the dat'a~llab~was generally interpreted <br />[rom a conservat~nd~in~y the Bureau (Ref. 1, 2, 3). <br />, In addition, condi~ons Chos~~to model the system were also <br />- -- -- - - - _cooservati ve,( &2- -- - - '-' - ---- <br />The primar~ objective of the Bureau analyses was to <br />calculate/thelex~~ent of the seepage water near the toe <br />of the~am. The ~deled region of interest was centered where <br />the ~~y trench js proposed to terminate. Seepage through <br />the slurr~e~~as assumed to be zero. The analyses were <br />performed assuming an upstream pool elevation of 4448 feet. <br /> <br />..;..-~- -------- <br /> <br />. <br /> <br />I <br /> <br />Both the two and three dimensional analyses indicate exit <br /> <br />gradients near the toe could approach a value of 1, which would <br /> <br />- 22 - <br /> <br />IIDtr. <br />.. <br />