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<br />. <br /> <br />. <br /> <br />of the utmost importance. Highly permeable sediments <br />in the area of the south end of the dam are in the <br />lower portion of the buried channel. Coombs indicated <br />that the maximum thickness of the oldest part of the <br />alluvium is about 150 feet. (The 150 feet of deepest, <br />channel is the lower half of a total 300' f~ot deep <br />channel. ) <br /> <br />c. The slurry trench would extend to the south <br />bank to about Station 232 (near Drill Hole DD32). <br />The ditch would provide a positive cutoff at least <br />as far as the end of the slurry trench. Detour of <br />qround water around the end ~f the trench must not <br />be excessive in seepaqe loss or the dam will be in- <br />effective. It must not be too restricted or the hy- <br />draulic gradient and uplift pressures will be too high <br />and may result in piping. Bence a balance must be <br />established between the movement of ground water and <br />water pressures. T~e latter can be controlled by <br />pressure relief wells and drains. NOTE: The design <br />and cost of such pressure relief wells and drains were <br />not discussed by Coombs or any of the other independent <br />panelists or the Bureau. (The underlining suggests <br />the dam may not be capable of storing water for later <br />use. ) <br /> <br />d. Coombs pointed out that Pierre Shale has <br />a widespread reputation for swelling under low loading <br />conditions where excess water is available, namely <br />that the shale weathers to a soft clay which is easily <br />removed by running water. Rapid erosion by even low <br />velocity discharges could affect the outlet works if <br />not properly founded on unweathered ,shale. When the <br />excavation cuts into the unweathered shale, then the <br />rock is strong and capable of supporting structures <br />and resisting erosion by running water. NOTE: No <br />cost of excavation of Pierre Shale to get down to the <br />unweathered shale has yet been calculated on the basis <br />of sufficient test drilling. <br /> <br />e. In commenting on the seepage analysis, Coombs <br />made conclusions regarding exit gradients, piping and <br />pressure relief wells: . <br /> <br />(1) Exit gradient--safety factors approach <br />is logical for determining if piping will occur. <br /> <br />-16- <br />