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<br />.- n..... ..., <br />ll" ;, <br />....~4 1. <br /> <br />1 <br /> <br />7 <br /> <br />conditions, locally cause the water table to be near the surface. <br /> <br />Some of the more permeable sediments are exposed on the souc.h <br /> <br />hank in the reservoir area and will be open to rese:voir water when <br /> <br />the dam is constructed. Clay blankets to minimize see~age L1tO these <br /> <br />sedbnents were considered in earlier plans but were ~~ought to he only <br /> <br />partially effective. <br /> <br />It the borrow area on this south. ban.tc;: is stripped of its cover, <br /> <br />then access from the rese~oir to the aquifer in the buried channel <br /> <br />would .be increased. Present plans call for a cutoff wall in the fort:!. <br /> <br />of a slurry trench on line with the dam axis. This slurry trench .....culd <br /> <br />extend into the south bank approximately to Station 232 (near drill <br /> <br />hole DO 32). This will provide a positive cutoff at least 8.S far ~ <br /> <br />the end of the slurry trenc~. The detour of groundwater around t....e end <br /> <br />of the trench must not be excessive in ~eepage loss or the dam will be <br /> <br />ineffective. It must not be too restricted or the hydraulic gradient <br /> <br />and uplift pressures will be too high and may result in piping. Hence <br /> <br />a. balance must be established be~"een the movement of groWldwater and <br /> <br />water pressure. The latter can be controlled by pressure relief wells <br /> <br />and drains. <br /> <br />In a Memorancum, dated November 8, 1977, to the Chief of the <br /> <br />Dam Branch from the Head of the ~echnical Analysis Section on "Seepage <br /> <br />Analysis tor the Stability of Narrows Dam," several conclusions were <br /> <br />dravn regarding exit gradients, piping, and pressure relief wells. My <br /> <br />comments on this study are as follows: <br /> <br />1. The exit gradient-safety factor approach is logical for <br />