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<br />043:) <br /> <br />reservoir area near the right abutment of the dam. It also appears most of the material <br />could be borrowed below the joint-use pool. <br />DECEMBER 1977 INSPECTION <br />Our inspection of the damsite and vicinity was made on December 6, 1977. The <br />weather was cold and partly cloudy. A thin mantle of snow was present over the area, <br />but it was insufficient to obscure significant details. The inspection party consisted of <br />Messrs. C. W. Griffin, N. B. Bennett III, J. M. Walker and G. Rappl for USBR, and <br />independent consultants Howard A. Coombs, George F. Sowers, and Thomas M. Leps. <br />The party drove to various points of interest on the North and South abutments, ranging <br />from the spillway area on the North to Bijou No. 2 Reservoir on the South. It viewed <br />backhoe excavation of a shallow test pit in the proposed core borrow area on the north <br />side of the river. It made a brief aerial reconnaissance of the damsite and nearby reach <br />of the river, using the USBR airplane. The party was unable, because of reported prob- <br />lems in obtaining Owner approval, to visit the river bottom area along the proposed dam <br />axis. <br /> <br />In our judgment, the inspection was adequate to provide us with the following <br />general impressions relative to the design of Narrows Dam. <br />1. The damsite topography is gentle and rolling, except for modest bluffs adja- <br />cent to the main river bed. <br />2. The required axis length of the dam is uncommonly long (over 4 mi.), with <br />the important consequence that exposure to the problems of foundation defects is rela- <br />tively great, and ability within economic limitations to discover such defects is rela- <br />tively slllllll. <br />3. All surficial materials along the dam axis, including the weathered Pierre <br />siltstone to the North, and the Bijou Flat alluvium and aeolian sands to the South are <br />highly erodible. <br />4. No evidence of firm, massive Pierre Shale was seen in outcrops to the North <br />of the river, but samples of such material taken from drill holes at depth were viewed <br />. in the USBR laboratory. <br />5. None of the borrow material pointed out to us as potential pervious shell fill <br /> <br />5. <br />