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<br />~-,-. <br />10'\ \.:. <br /> <br />CHAPTER II--INVESTIGATION OF REVIEW ISSUES <br /> <br />SAFETY OF NARROWS' DAMANDRESERVOI R SEEPAGE <br /> <br />" <br /> <br />Dam Sa f ety <br /> <br />Backqround <br /> <br />The dam safety issue is concerned with the stability of the earthfill <br />embankment considering the lack of a positive seepage cutoff on the <br />south abutment. <br /> <br />The feasibility design presented in the Final Environmental Statement <br />includes a 20-foot-deep cutoff trench and an 8-foot-wide slurry trench <br />to the shale formation to provide seepage control below the active <br />conservation water surface elevation of 4399 feet. The slurry trench <br />would be terminated before reaching the south end of the embankment. <br />Concern was expressed that excessive seepage rates around the end of <br />the slurry trench would undermine the toe of the embankment causing an <br />embankment failure. <br /> <br />A stability study was requested on July 28, 1976, by the Lower Missouri <br />Region to be conducted by the Division of Design and Construction of <br />the Engineering and Research (E&R) Center. The stability study was <br />initiated August 30, 1976, by a memorandum from the Director of Design <br />and Construction and completed in November 1977. <br /> <br />Stabil ity Study <br /> <br />The study area selected by the E&R Center was centered at the south end <br />of the proposed slurry trench. <br /> <br />" <br /> <br />Field studies consisted of Denison-type drilling with continuous <br />sampling, geologic logging, laboratory testing of the Denison samples, <br />geophysical logging, permeability testing for both horizontal and <br />vertical permeabilities and finally, computer analysis. The culmination <br />of the field work was a 2-day pump-out water test which included the <br />participation of the U.S. Geological Survey staff. <br /> <br />The results of the drilling and permeability testing program were <br />utilized in a seepage analysis using'iI numerical' model. The results <br />of the seepage and stability analyses are covered in an E&R Center <br />memorandum report of November 8, 1977, subject: "Seepage Analyses for <br />Stability of Narrows Dam." The conclusions and recommendations of <br />this report are: <br /> <br />9 <br />