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<br />11 <br /> <br />top of the unweathered Pierre Shale would provide excellent information <br /> <br />regarding the depth of cutoff needed. <br /> <br />5. The permeability of the sands and gravels in the lower <br /> <br />portion of the buried channel seems unusually high. The sediments <br /> <br />immediately above these sands and gravels are apparently less permeable <br /> <br />but have not been tested directly for permeability. These permeabilities <br /> <br />are some of the most critical factors in establishing both the safety <br /> <br />and the feasiJ:lility of the proposed dam. <br /> <br />6. Seismicity is quite low in the region of the dam site but <br /> <br />some data should be provided to establish the degree of intensity that <br /> <br />may be anticipated. <br /> <br />CONCLUSIONS <br />1. Information available at present is sufficient to establish <br /> <br />a preliminary design for the Narrows Dam. However, it should be noted <br /> <br />that the feasibility and safety of the dam are to a great extent <br /> <br />dependent on the ability to control the passage of water around the <br /> <br />right abutment and to regulate the uplift pressures in this general <br /> <br />area. <br /> <br />2. The cutoff trench under the embankment and the slurry <br /> <br />cutoff trench will be effective only if they bottom in unweathered <br /> <br />shale. <br /> <br />. <br /> <br />3.. Pressure relief wells could provide a safe exit gradient for <br /> <br />groundwater. These wells should bottom in unweathered shale. <br />