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<br />L"" <br />00 <br />Ct) <br />C'.J <br /> <br />YELLOW JACKET PROJECT STUDY <br />AVERY DAM SEISMIC ANALYIS <br />SUMMARY <br /> <br />A seismic refraction survey was conducted on the left abutment of the pro- <br />posed Avery Dam enlargment as part of the Yellow Jacket study for the Colo- <br />rado Water Conservation Board. It was anticipated that bedrock depths over <br />the area might not be as deep as the ~ 100 ft indicated by drill holes done <br />by the Bureau of Reclamation in 1976, therefore allowing a positive cutoff <br />to be constructed of economical length and depth. <br /> <br />The seismic study showed that bedrock depth exceeds 100 ft. over nearly the <br />entire abutment area and in some areas is more than 185 ft. deep. Therefore, <br />it is concluded that a positive cutoff is not feasible. However, the ex- <br /> <br />_istence of a sandy silt layer from 5-20 ft deep covering the abutment area <br />was detected. If this material were connected to bedrock by blanketing the <br />gravel as it is exposed along contours in the upstream part of the foundation, <br />it might provide a more economical design alternative than the positive cutoff <br />originally proposed. <br /> <br />I NTRODUCTI ON <br /> <br />The initial phase of the Yellow Jacket Study concluded that the most economi- <br /> <br />cally attractive storage alternative in the White River Basin was to remove <br /> <br />the present Avery dam and construct a new embankment with a crest elevation of <br /> <br />7115, thus providing a storage capacity of 50,000 acre-feet. The Bureau of <br /> <br />Reclamation carried out prefeasibility level geologic investigations at the <br /> <br />site in 1976 consisting of surface mapping and four drill holes. Two of the <br /> <br />drill holes on the left abutment of the proposed embankment showed bedrock to <br /> <br />be covered by approximately 100 ft of gravel. As construction of a positive <br />