Seismic Stability Evaluation of Lenihan Dam
Santa Clara County, California
Owner:
Santa Clara Valley Water District
Lenihan Dam is a “High Hazard” dam located in a highly seismic area, with the San Andreas Fault 1.3 miles away.
TERRA is leading a multi-disciplinary team of consultants retained by the Santa Clara Valley Water District (SCVWD) to complete seismic stability evaluations of Chesbro, Lenihan, Stevens Creek and Uvas Dams. The general scope of the project consists of field, laboratory, and office studies required to evaluate the seismic stability of the dams.
Lenihan Dam is one of the first two dams being studied. The earthfill dam constructed across Los Gatos Creek is classified by DSOD as a “High Hazard” because of the extensive urban development close by. The dam is 1.3 miles from the San Andreas Fault capable of generating a 7.9 magnitude earthquake.
After a comprehensive review of existing field and laboratory data, TERRA designed a field exploration program to make maximum use of existing information and use the most up-to-date methods available to fill in the gaps and ensure sound engineering analyses. One challenge was the collection of intact samples of embankment materials for laboratory testing because of the significant amounts of gravel in the materials. Thus, Pitcher barrel samples were obtained in mud rotary borings and the samples were X-rayed prior to being extruded to allow for sample selection considering indications of gravel and sample disturbance. Key results of the site characterization was the verification that the dam was built directly on bedrock with no potentially liquefiable alluvium left in the foundation, as previously believed, and that the shear wave velocity of the bedrock was much higher than originally anticipated.
A challenge of the analyses was to develop a good understanding of the seepage pattern within the embankment (a critical input to the stability analyses) because of the complex geometry of the valley. The results of the non-linear dynamic deformation analyses showed deformations during the design earthquake will be similar to those during the 1989 Loma Prieta earthquake, and that seismic upgrades are not required.