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<br />. <br /> <br />Woodward.Clyde Consultants <br /> <br />- 1')." ,_. <br />~ \J' t) ~) <br /> <br />. <br /> <br />Colorado Water Conservation Board <br />February 16, 1978 <br />Page 2 <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;f~n be developed <br />for an earth dam at this location. T~e~e are some foundation <br />and embankment material situations, Is~iscussed in our <br />report, which should be given caref,lfll'9onsideration during <br />final design of the dam. For us t6/comme~t~n the safety <br />aspects of the design finally adop.ted, i~wou~d be necessary <br />,"P :--.._ <br />for us to review the data and studies develop.ed)by the <br />Bureau in their final deSign~;> ~ <br /> <br />Based on available geologic /.in.formation, our brief field <br />observations and our knowleag~ of~he area, we have <br />concluded that there are do ~nd~cations of geological <br />structural features or landslfaeJPotentials that present <br />a risk to the safety of the Ptqposed dam. Deposits of <br />gypsum within the pierre formatton~should be investigated <br />with respect to leaching. V <br /> <br />We have concluded th~t t~~ream maximum section of the <br />dam as proposed by th~B~re~ulSho~ld be stable under steady <br />state reservoir condition~( ;'This conclusion is based on <br />our independent_~nalyses~utilizing the feasibility grade <br />data now availabl~ BeCaU~~\of the low permeability and <br />. low cohesive;fsyrengtp of th~proposed Zone 2 material we <br />__ _ __ _ _ _ _ ___ __haYe_SOme__cpngernithat~.,.upstr.e.am_slop'e_may_no_t_ b.e_stabl.e__ _ _ , . <br />under rapid drawdown cond~€ions and some slumping may occur. <br />This cond~tJ~n~shqald be-carefully studied during the final <br />design and €he seiection of materials for the upstream <br /> <br />Zone ~ ~~ <br /> <br />In 9Ul;;/6'Pin1'o;;:\ t~risk of earthquake induced ground <br />motions adver~ely affecting the safety of the dam is low, <br /> <br />W~~~ieve a 7lJsonablY effective cutoff can be achieved <br />at t:he "r.ight ,abJtment by a combination soil backfilled <br />trench'ana,s1urry trench carried to competent Pierre <br />, ~ / <br />shale. we,recommend extending the slurry trench to about <br />Station 224,6r providing an impervious soil blanket between <br />about Stations 235 and 195 as an alternate solution. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br /> <br />. <br />