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<br />~, ""1 I ~ 1', <br />1..' ,j ... .J <br /> <br />:i' <br /> <br />6 <br /> <br />Narrows Dam site, t.."l.e hills of ....indblown sand cay :be 150 feet high and <br /> <br />10 miles long. The windblown sand contains ~O, sand-sized grainS of <br /> <br />feldspar and 10\ silt-sized particles. <br /> <br />SEEPAGE <br /> <br />In dete:rmining the safety and feasibility of the: Narrows Dam <br /> <br />site, the ability to control seepage, particul~ly around the right <br /> <br />abutment, is of ut:nost im!?ortance. <br /> <br />The north bank is mostly pierre Shale which should prese~t few <br /> <br />seepage problems beneath the ~eathered zone. However, wedges of wind- <br /> <br />blown silt, gravelly sand. and clayey silt and sand do occur on top of <br /> <br />the weathered shale surface and may present sl~ing problems. <br /> <br />On the south bank, in contrast, the Pierre Shale forms a. low <br /> <br />ridge near the river, then drops to the south. forming a c.'Ioe.l11nel much <br /> <br />deeper than the main channel and filled with pervious alluvium. <br /> <br />The deep channel fill contains. from bottom to top; alluvial <br /> <br />sand and gravel, well-qraded sands. silty sands. and windblown silty <br /> <br />sands. The highly permeable sediments are in the lower portion of <br /> <br />this buried channel. <br /> <br />The results of early drilling indicated that a continuous <br /> <br />clay layer was present in the area of the .9lurry trench at depths <br /> <br />:from 30 to SO feet below the surface. LAter drilling .md pump tests <br /> <br />revealed several clay layers, none of which had significant ar~al <br /> <br />extent. They are not thought to be widespread enough to cause a <br /> <br />general perched water table ~ut they ~y, under very ~iqh water <br />