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<br />I <br />i I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />'I <br /> <br />The bore holes revealed that the left abutment is covered with from 10 to 21 <br />feet of transported material, consisting of a tan, silty to sandy clay of low <br />plasticity overlying 2 to 5 feet of alluvial sand and gravel. The transported <br />material overl ies bedrock, with the thickness decreasi ng up the abutment. <br />Bedrock is composed of a moderately weathered to fresh sequence of the Fort <br />Union Formation consisting of brownish-gray claystone and weakly-cemented buff <br />colored sandstone. Weathering decreases with depth. The claystone is <br />compact, but parts easily along undulating, waxy bedding planes. The <br />sandstone can be crumbl ed wi th fi nger pressure when wet, but is fai rly hard <br />when dry. Jointing of the rock mass is light. <br /> <br />All of the excavation for the auxiliary spillway channel is expected to be <br />common, that is blasting will not be necessary. The material will consist of <br />sandy cl ay and all uvi al gravel overburden, weathered cl aystone, and sandstone <br />bedrock. This same material will be utilized to raise the embankment from the <br />downstream side. <br /> <br />Although the rock mass at the auxiliary spillway site is generally of a weak <br />nature, it is strong enough to resist erosion during the 26-hour period of <br />flow calculated for the probable maximum flood. <br /> <br />Figure V.2 shows the location of all pertinent boreholes and test pits, as <br />well as a typical gradation of the soils overlying the left abutment. <br /> <br />- 37 - <br />