Three-Dimensional Post-Earthquakes Stability Analysis for Casitas Dam, California

he U.S. Bureau of Reclamation (Reclamation) has recently completed modification designs to alleviate potential seismic dam safety concerns for Casitas Dam. Analyses completed by Reclamation had identified the existence of potentially liquefiable soils in the foundation beneath the dam. Three-dimensional post-earthquake stability analyses were completed as part of the engineering effort to support the modification designs.
Casitas Dam, the main feature of the Ventura River Project, was completed in 1959, and is located on Coyote Creek about 6.5 miles northwest of Ventura, California. The dam is a zoned earthfill embankment, with a structural height of 335 feet and a capacity of 250,835 acre-feet at the top of active conservation, elevation 567.0 feet. The foundation of the dam consists of alluvial deposits with a thickness of 40 to 45 feet, overlying bedrock. The dam is in a highly active seismic area, and recent studies have identified several potential seismic sources. The controlling source is the Red Mountain Fault, which is capable of producing a maximum credible earthquake (MCE) of magnitude (Mw) 6.9, at a distance of 3 kilometers from the site. In situ testing has identified loose alluvial silt, sand, and gravel materials beneath the upstream and downstream shells of the embankment, portions of which are likely to liquefy in the event of a large earthquake. Failure of the dam could result in severe economic damage and loss of many lives in communities downstream.
The modifications that have been designed by Reclamation include construction of a downstream buttress and filters. No modifications are planned for the upstream section of the dam. Two-dimensional post-earthquake stability analyses of the upstream slope indicated factors of safety in the range of 1.1 to 1.2 and two-dimensional deformation analyses indicated large, but acceptable deformations. However, it was recognized that the geometry of the alluvial valley and the configuration of failure surfaces through the liquefied alluvium could offer three-dimensional benefits to post-earthquake stability. Reclamation contracted with URS Greiner Woodward Clyde (URSGWC) to complete three-dimensional post-earthquake stability analyses of the upstream slope.