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<br />9 <br /> <br />and the U.S. Bureau of Reclamation made in 1948. !t is ~~derstocd <br /> <br />that the U.S. Geological Survey is now conducting a regional transient- <br /> <br />state study. 'nhen the results of this analysis are available in t..'\e <br /> <br />fall, they will throw considerable light on the uplift problem at t..~e <br /> <br />N arrows Dam sit e,' <br /> <br />MATERIl\LS FOR CONSTRUcrION <br />An abundance of fine material for the core of t.':le embankment is <br /> <br />available close to the dam site. The preferred location seems to be <br /> <br />on the north bank. A backhoe pit dug in this material showed less than <br /> <br />6 feet of core material resting on a water-bearing sandy stratum. The <br /> <br />movement of water in the sand was noticeable and caused the soil above <br /> <br /> <br />to slough off. The stripping of this thin layer on the north bank <br /> <br /> <br />would cover a broad area and necessitate installing an efficient <br /> <br /> <br />drainage system during construction. <br /> <br />The south bank also contains suitable core material that is <br /> <br />more uniform than that on the north side. However, this relatively <br /> <br />impervious alluvial blanket rorms a partial seal OVer the highly <br /> <br />permeable sands and gravels below. Stripping these fine sediments near <br /> <br />the dam would result in a shorter route for the reservoir water to the <br /> <br />aquifers below and cause a steeper hydraulic gradient to be established <br /> <br />with accompanying higher uplift pressures. <br />Pervious materials are present in the South Platte riverbed <br /> <br />upstream from the proposed dam. They are essentially well-graded sands <br /> <br />with some gravel. !t would be desirable to have a larger. percentage cf <br />