SOD Modification Update for Keechelus Dam, Yakima Project, Washington

On June 9, 1998, excavation across the crest of the dam for a telephone line/instrumentation trench intercepted a void. Initial investigations at the dam included: excavations to determine the extent of, and to characterize the void; underwater examination of the upstream face of the dam in the vicinity of the void; ground-penetrating radar (GPR) survey; seismic reflection survey; test pits; test trenches; and drill holes. The GPR survey identified 42 sites where voids were likely to be found within the embankment. Void and/or very loose, open-work gravel and cobbles were encountered at a number of test pits and trenches. Loss of all drill fluid within the embankment was encountered in 6 holes, 3 at the maximum section and 3 scattered elsewhere. Enhanced monitoring of the toe drains has revealed that material (fines) are being piped from the embankment and/or the foundation into the drains. The decision to modify Keechelus Dam to prevent potential failure was made in the fall of 1998. The need for modification was based primarily on past performance, forensic investigations, and review of embankment design data. A reservoir restriction was placed on the structure in 1999 with enhanced monitoring and additional exploration for design data. Modifications to the dam will begin in the summer of 2002 and are expected to be completed within two field seasons. Modifications to the dam will include a downstream seepage interceptor drain; removing and replacing a large portion of the existing earth embankment; a cutoff wall constructed on the right abutment; and modifications to the spillway. This paper reports the current investigations and status of the dam; the understanding of the deficiencies at the dam; as well as the design modifications to the dam. 12 pp., 10 references.