Camanche Reservoir has a capacity of 136 billion gallons at spillway level and was created in 1964 by the construction of a 170-foot high zoned earth dam across the Mokelumne River and six dikes ranging in height form 40 to 90 feet around the reservoir rim, for a total embankment length of 4.4 miles.
TERRA led a multi-disciplinary team of subconsultants that was retained by EBMUD to perform a comprehensive safety review of the dam and dikes. This safety review followed completion of a Potential Failure Mode Analysis (PFMA) that had been mandated by FERC. Key issues to be addressed included potential piping within the embankments, potential instability associated with uplift pressures, and potentially large deformations that could be induced by liquefaction of tailings beneath the dam. The scope of the project included site characterization using existing and new field and laboratory geotechnical data; installation of new piezometers; evaluation of seepage and uplift pressures under normal and extreme (PMF) reservoir levels; evaluation of piping potential using a detailed risk-based approach; analysis of embankment stability under normal and extreme reservoir levels; development of site-specific ground motions; dynamic analyses of embankment performance during the design earthquake; and evaluation of alternative remedial measures, as necessary.
TERRA’s finite element seepage model was key to evaluating potential uplift pressures, assessing performance of relief wells, and providing input to static and dynamic analyses.
Significant interaction with DSOD and FERC was required to obtain regulatory approval of the site-specific NGA-based ground motions with acceleration-time histories that were consistent with DSOD requirements and yet not overly conservative given the tectonic environment of the site region.
The safety review was completed in September 2010 and identified a small number of potential issues to be addressed as part of EBMUD’s long term program for operation, maintenance, and seismic upgrade but did not reveal any safety concerns requiring immediate attention.