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<br />.-.. " 0', i <br />\... ~"... <br /> <br />, <br /> <br />4 <br /> <br />difficult to distirl.guish ......eadlered from um..eat.....ered shale. Loess <br /> <br />can be contused with Pie~re Shale as can re-worked shale from in- <br /> <br />place shale. These differences can be seen in the field as excavation <br /> <br />for the embankment progresses. However, it is more difficult to know <br /> <br />""hen the excavation for t.'1e slurry trench encounters sound. rock. This <br /> <br />trench must be founded on impervious rock since the highest velocities <br /> <br />are in the gravels immediately above the Pier=e Shale surface. <br /> <br />Beneath the pr9posed dam are some hig~ly compressible windblown <br /> <br />sands and silt as well as some weathered Pierre Shale. Their physical <br /> <br />properties vary greatly from place to place under this long dam. This <br /> <br />may result in differential settlement of the ~ankment unless care is <br /> <br />taken to excav!3-te do'..,.." to material capable of supporting the load <br /> <br />imposed. <br /> <br />Because of the necessity to reach good, unweathered rock in all <br /> <br />critical excavations, careful and constant field inspection is vital <br /> <br />to this project. <br /> <br />ALLUVIUM <br /> <br />The lowest alluvium, resting on top of the Pierre Shale, extends <br /> <br />along the entire length of the Sout.~ Platte River. It not only covers <br /> <br />the floor of the several-mile-wide buried channel south of t.~e present <br /> <br />river at the Narrows Dam site but it is t.~e thickest, coarsest, and <br /> <br />:DOst permeable of all alluvial deposits in t..'1is area. <br /> <br />Analyzed samples of this feldspathic. and granitic sane and <br /> <br />qravel contain 45' pebbles, 30\ sand, and 25' silt. The max~um <br />