C153705 Feasibility Study - Laserfiche WebLink

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I I I I I I I I I I I I I I I I I I I -,'. /" Beaver Reservoir Company Attention: Mr. Neil Thompson February 4, 1993 Page 2 The site was initially investigated in 1940 by the U.S. Bureau of Reclamation (Reclamation). The proposed project was an approximate 150 foot high dam with a crest elevation of 8160 feet and a crest length of 11 00 feet. Exploration during this investigation consisted of eight drill holes and eight test pits in the general dam foundation footprint. Additional test pits were excavated in the proposed reservoir borrow area. Based on this investigation, it was concluded that the site had foundation problems and that foundation preparation would likely be costly (Merriman 1940). The exploration program encountered evidence of coal beds in the left abutment area which had burned and resulted in loss of volume and disturbance to the overlying bedrock layers. Considering the foundation problems and associated costs of remediation, Reclamation elected not to construct this project. In the late 1950s, BRC constructed a scaled down versionofthe Reclamation project. This dam has a height of approximately 113 feet.wjth a crest elevation of 8111 feet and a crest length of about 870 feet. A spillway 'is,located in the right abutment area. It is our understanding that the design of this dam was based on the exploration completed by the Reclamation in 1940 and that no additional exploration was performed. It is also our understanding that reservoir seepage has been experienced since the completion of the project and several repairs have been completed, including partial blanketing of the reservoir. The seepage has, at times, been accompanied by the formation of sinkholes in the,reservoir. The causes ofthe'historic'and existing sinkholes are not known with reasonable engineering certainty at this time. Based on discussions with representatives of the BRC and a review of the Colorado State Engineer's ftles, we understand that sinkholes started to appear approximately twenty-eight years ago and have reappeared fairly regularly since. The sinkholes have been backfilled with sandy materials and capped with clayey materials but continue'to reappear. Embankment and foundation sinkholes are commonly the result of seepage and piping of soil particles into and through coarse embankment zones, outlet conduits and/or coarse or fractured foundation materials. Based on review of the historical record, there is no indication of sinkhole development associated with flow into the outlet conduit from the surrounding embankment materials. The historic sinkholes have appeared in the natural foundation materials just upstream of the embankment on both abutments. These sinkholes were probably created in response to soil particles being transported by seepage of reservoir water into the natural foundation materials. 23067/LEIO 02-04-93(4:24pm)/RPT