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<br />OS"; c) <br /> <br />Woodward-Clyde Consultants <br /> <br />Colorado <br />February <br />Page 2 <br /> <br />Water Conservation Board <br />16, 1978 <br /> <br />, <br /> <br />to about Station 230+67 the cutoff is to consist of an <br />impervious soil filled cutoff trench with a slurry trench <br />cutoff into bedrock beneath it. <br /> <br />In our opinion, a safe reliable design can be developed <br />for an earth dam at this location. There are some foundation <br />and embankment material situations, as discussed in our <br />report, which should be given careful consideration during <br />final design of the dam. For us to comment on the safety <br />aspects of the design finally adopted, it would be necessary <br />for us to review the data and studies developed by the <br />Bureau in their final design. <br /> <br />. <br /> <br />Based on available geologic information, our brief field <br />observations and our knowledge of the area, we have <br />concluded that there are no indications of geological <br />structural features or landslide potentials that present <br />a risk to the safety of the proposed dam. Deposits of <br />gypsum within the Pierre formation should be investigated <br />with respect to leaching. <br /> <br />, <br /> <br />We have concluded that the downstream maximum section of the <br />dam as proposed by the Bureau should be stable under steady <br />state reservoir conditions. This conclusion is based on <br />our independent analyses utilizing the feasibility grade <br />data now available. Because of the low permeability and <br />low cohesive strength of the proposed Zone 2 material we <br />have some concern that the upstream slope may not be stable <br />under rapid drawdown conditions and some slumping may occur. <br />This condition should be carefully studied during the final <br />design and the selection of materials for the upstream <br />Zone 2. <br /> <br />In our opinion, the risk of earthquake induced ground <br />motions adversely affecting the safety of the dam is low. <br /> <br />We believe a reasonably effective cutoff can be achieved <br />at the right abutment by a combination soil backfilled <br />trench and slurry trench carried to competent Pierre <br />shale. We recommend extending the slurry trench to about <br />Station 224 or providing an impervious soil blanket between <br />about Stations 235 and 195 as an alternate solution. <br />