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<br />4 <br /> <br />difficult to distinguish weathered from unweathered shale. Loess <br /> <br />can be confused with pierre 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. Ha~ever, it is more difficult to know <br /> <br />when the excavation for the slurrj' 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 Pierre Shale surface. <br /> <br />Beneath the proposed d~ are some highly 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 embankment unless care is <br /> <br />taken to excavate down 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 />The lowest alluvium, resting on top of the Pierre Shale, extends <br /> <br />along the entire length of the South Platte River. It not only covers <br /> <br />the floor of the several-mile-wide buried channel south of the present <br /> <br />river at the Narrows Dam site but it is the thickest, coarsest. and <br /> <br />most permeable of all alluvial deposits in this area. <br /> <br />Analyzed samples of this feldspathic and granitic sand and <br /> <br />gravel contain 45\ pebbles, 30\ sand, and 25\ silt. The maximum <br />