Seismic Rehabilitation of Echo Lake Dam

Jernigan, Russell L.; O'Connell, Daniel P.; ,

Seismic Rehabilitation of Echo Lake Dam

by Russell L. Jernigan, Pacific Gas and Electric Co, San Francisco, United States,
Daniel P. O'Connell, Pacific Gas and Electric Co, San Francisco, United States,

Document Type: Proceeding Paper

Part of: Geotechnical Practice in Dam Rehabilitation

Abstract:

Echo Lake Dam is an existing dam raising a natural lake in the Sierra Nevada in California, along the western edge of the Tahoe Basin. The dam was an embankment built with steep, hand laid rock faces, on a foundation of glacial till. A geotechnical investigation indicated that the body of the dam is composed of a loose, skeletal arrangement of coarse noncohesive material. The upper five to ten feet of the glacial till underlying the dam is saturated and loose. The nearby West Tahoe Fault Zone has been identified as potentially active, and capable of generating an earthquake of Richter magnitude 7, with a corresponding PGA at the dam site of 0.7 g. Due to the loose material comprising the embankment and the underlying foundation, the dam has been judged potentially seismically unstable. A conceptual study was conducted of ways to rehabilitate the dam. Three basic types of rehabilitation were investigated: Insitu, downstream buttressing, and replacement. After consideration of the site limitations, including the significant impact of the construction on the sensitive environment and the heavy recreation use of the area, and the long term influence of the rehabilitation, replacement with a roller compacted concrete (RCC) structure was chosen. Replacement with RCC was chosen due to the small footprint of the new dam that would be required for seismic stability, the potential for rapid placement allowing construction in a short period of time, its non-erodible properties, and because the finished structure would have a similar `look and feel' to the existing dam. Design of the RCC structure included the development of foundation densification treatment of the loose glacial till underlying the dam site, analysis of the effects of a rigid structure placed upon the relatively softer foundation material, and maintenance of downstream seepage to avoid drying up an established wetland at the dam toe. Preliminary design of the proposed RCC structure was developed by Berlogar Geotechnical Consultants, with input from the owner, Pacific Gas and Electric Company (PG&E). The final design was developed by PG&E.

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