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<br />6 <br /> <br />Narrows DaJl\ site, the hills of '...indblown sand may be 150 feet high a."ld <br /> <br />10 miles long. The windblown sand contains 90% sand-sized grains of <br />feldspar and 10% silt-sized particles. <br /> <br />SEEPAGE <br /> <br />111 determining the safety and feasibility of the Narrows DaJl\ <br /> <br />site, the ability to control seepage, particularly around the right <br /> <br />abutlnent, is of utn10st importance. <br /> <br />The north bank is mostly Pierre Shale which should present few <br /> <br />seepage problems beneath the weathered 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 slumping 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.'1annel much <br /> <br /> <br />deeper than the main channel and filled with pervious alluvium. <br /> <br /> <br />The deep channel fill contains, from bottom to top: alluvial <br /> <br /> <br />sand and gravel, well-graded sands, silty sands, and windblown silty <br /> <br />sands. The highly permeable sediments are in the lo....er portion of <br />this buried channel. <br />The results of early drilling indicated that a continuous <br /> <br />clay layer was present in the area of the slurry trench at depths <br /> <br />from 30 to 50 feet below the surface. Later drilling and pump tests <br /> <br />revealed several clay layers, none of which had significant areal <br /> <br />extent. They are not thought to be widespread enough to cause a <br /> <br />general perched water table but they may, under very high water <br />