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<br />en <br />0) <br />N <br /> <br />CHAPTER II <br /> <br />DE.3IGNS AND ESTIMATES <br /> <br />Ge.lollY <br /> <br />Rifle Gap Dam and Reservoir <br /> <br />The dam site is located where Rifle Creek cuts directly through <br />the Grand Hogback. The sandstone and shale la.yers at the site dip 80 <br />to 85 degrees downs tream. The sandstones are medium grained and are <br />hard but somewhat friable. These sandstone strata are jointed and <br />thus are permeable, but the intervening shale beds are firm and imper- <br />meable. The shale beds would form natural cut-off's preventing the <br />escape of water in the abutments. A small fault crosses the canyon <br />obliquely immediately below the site. <br /> <br />Alluvial overb~rden material has been deposited on the floor of <br />the dam si te. This mat erial is as deep as 128 feet and thus makes <br />complete excavation to bedrock impractical. The material consists of <br />sandstone blocks and fragments, interlensed gravel and fines. The <br />upper 25 feet of the material is a tight fan wash containing intersti- <br />tial silt and cla.y. This blanket would probably prohibit excessive <br />seepage. At depth, however, the material is interspersed with stream <br />gravels and is relatively permeable. A drawdown pumping test showed <br />average permeability of the material to be 1.75 to 5.10 feet a da.y. <br />Further testing of the foundation materials would be desirable before <br />the final design of the dam is made. <br /> <br />The upper part of the spillwa.v channel would be founded on stale <br />and sandstone. The lower spillway channel and stilling basin would <br />rest on overburden material. About 80 percent of the diversion tunnel <br />would penetrate shale, and the remainder would penetrate sandstone. <br /> <br />The reservoir basin is in a wide erosional section of Rifle Creek <br />Valley above the Grand Hogback. The basin has been formed by erosion <br />of Mancos shales. The shale 1s compact, and no appreciable quantity of <br />water would seep from the basin. <br /> <br />, <br />~. <br />:~ <br /> <br />< <br />., <br /> <br />C.nstruction Materials <br /> <br />Earth materials could be secured from the reservoir basin wi thin <br />1 mile of the dam site. Final selection of the materials, however, <br />has not yet been made. Acceptable gravel is available from the Colorado <br />River bed immediately south of Rifle, less than 10 miles from the dam <br />site. Material for riprap could be obtained from many of the vertical <br />sandstone ribs exposed in the canyon. <br /> <br />11 <br />