Void Occurrence and Subsequent Safety of Dams Investigations at Keechelus Dam, Washington

Keechelus Dam, a zoned earthfill embankment constructed at the outlet of a natural glacial lake, is located just west of Easton, Washington near Snoqualamie Pass. The dam was constructed from 1912 through 1916, and has a crest length of 6,550 feet and height of 128 feet at the maximum section. Foundation units consist of variably impervious to pervious glacial till, glacial outwash, and alluvial fan materials. Archival design and construction records reveal considerable use of wooden trestles in the embankment to support borrow placement by rail cars; transverse and longitudinal rockfill drains with vitrified pipe placed for outfalls; an abandoned lake tap consisting of deep outlet works tunnel with upstream and downstream headings; and evidence of drainage conveyances to control and remove water during and after construction of the dam.

The Bureau of Redamation's Safety of Dams investigations began in June 1998 to increase instrumentation monitoring and surveillance capabilities for the dam. Excavation for a common telephone/ instrumentation trench along the crest intercepted a void, measuring six feet in diameter by two feet in depth, near the maximum section. Subsequent investigations at the void included excavations, underwater examinations with tracer dye, and a ground-penetrating radar (GPR) survey that included the entire embankment. The GPR survey identified 42 sites where voids were suspected. Further excavations at five of these sites revealed additional voids and evidence of embankment deterioration was found.

One large test trench was excavated from the crest of the dam at station 21+00 to expose the entire downstream portion of the embankment and underlying foundation. Numerous zones of openwork were noted, as were several six-to twelve-inch diameter voids. Considerable sediments were evident in the rockfill drains. Wood trestles were present in varying stages of decomposition, and void spaces were observed where the wood had rotted away. Exploration test excavations along the toe of the dam intersected seven previously unknown outfall drains. Video inspections of the transverse outfall pipes revealed varying amounts of sediments and coarse materials. Historically, seepage was monitored at five weirs at the toe of the dam. Additional flumes and weirs have been installed for seepage monitoring. Limited piezometric installations in drill holes show artesian conditions in the downstream right abutment area, but the piezometric conditions within the embankment are poorly understood.

Results of geotechnical field investigations and findings, engineering and risk analyses, and operational capabilities and constraints have determined that dam safety deficiencies exist at Keechelus Dam due to the potential for dam failure from piping and/or internal erosion of embankment materials. In order to reduce risks associated with the operation of the dam, a reservoir operating restriction of seven feet has been set for the 1999 irrigation season. Additional investigations and analysis are required to determine the extent of the deficiency and scope of corrective action alternatives. Monitoring and surveillance efforts will be greatly increased during and after the 1999 irrigation season. Rehabilitation construction is targeted for initiation in the year 2000. 12 pp., 15 references.