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<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 south <br /> <br />bank in the reservoir area and will be open to reservoir water when <br /> <br />the dam is constructed. Clay blankets to minL'I\ize seepage lllto these <br /> <br />sediments were considered in earlier plans but were thought to be only <br /> <br />partially effective. <br />If the borrow area on this south bank is stripped of its cover, <br /> <br />then access from the reservoir to the aquifer in the buried channel <br /> <br />would be increased. Present plans call for a cutoff wall in the form <br /> <br />of a slurry trench on line with the dam axis. This slurrJ trench would <br /> <br />extend into the south bank approximately to S.tation 232 (near drill <br /> <br />hole DO 32). This will provide a positive cutoff at least as far as <br /> <br />the end of the slurry trench. The detour of groundwater around the end <br /> <br />of the trench must not be excessive in seepage 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 between the movement of groundwater and <br /> <br />water pressure. The latter can be controlled by pressure relief wells <br /> <br />and drains. <br /> <br />In a MemorandUIl\, dated November 8, 1977, to the Chief of the <br /> <br />Dam Branch from the Head of the Technical Analysis Section on "Seepage <br /> <br />Analysis for the Stability of Narrows DalII," several conclusions were <br /> <br />dra<om 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 />