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<br />. <br /> <br />Woodward.Clyde Consultants <br /> <br />1, "0" r. <br />I.J "1 <br /> <br />. <br /> <br />level will decrease the head difference and reduce the average <br /> <br />gradient. <br /> <br />. <br /> <br />We believe that consideration should be 9iven to the <br /> <br /> <br />following redundant features to minimize risk of unsatisfactory <br /> <br />performance during high reservoir water levels assuming first <br /> <br />that the relief wells do not operate as designed and secondly <br /> <br />. <br /> <br />that there might be "windows" in the slurry cutoff, or gulleys <br /> <br />in the bedrock, not reacheable or recognizable during the <br /> <br />trench excavation. <br /> <br />. <br /> <br />(1) We recommend that the gradient in the area of the end <br />of the slurry cutoff be decreased, by (al continu- <br />ation of the slurry trench to reduce the average <br />gradient to 0.1 (water depth at point of water <br />entrance divided by the distance of travel through <br />pervious soil); or (b) placement of an impervious <br />blanket of compacted earth beneath the upstream <br />pervious zone of the dam between about Stations 235 <br />and 195, and extending the upstream blanket. <br /> <br />. <br /> <br />. <br /> <br />(2) We recommend consideration of an underdrain system <br />downstream of the dam where needed. A practical <br />approach would appear to be placement of observation <br />wells along the toe of the dam at close spacing, with <br />monitoring during reservoir filling, then placement <br />of drain wells or underdrain system at the location <br />where seepage problems become evident, if any. <br /> <br />t <br /> <br />4. Review of Slurry Trench <br />In our opinion, an excellent cutoff can be constructed, as <br /> <br />planned, on the right abutment utilizing a slurry trench cutoff <br /> <br />t <br /> <br />below the normal cutoff trench as shown on the Bureau drawings, <br /> <br />giving consideration to extending the slurry trench or an <br /> <br />t <br /> <br />upstream blanket. Such a cutoff is contingent upon carrying <br /> <br /> <br />the slurry trench into competent pierre formation materials, <br /> <br />- 25 - <br /> <br />t <br /> <br />., <br />