First Use of Stone Columns in California Under State Regulatory Jurisdiction - the Seismic Remediation Design of Lopez Dam

Lopez Dam, constructed in 1968, is a 166-foot high embankment dam and impounds a 50,000 acre-foot reservoir serving five communities. The dam, owned and operated by the San Luis Obispo County Flood Control and Water Conservation District, includes a central clay core, a gravel upstream shell, a random downstream shell and filter zones between the core and shells. The original dam foundation included excavation through 120 feet of alluvium to bedrock under the core of the dam, and placement of the shells on alluvium. Engineering studies concluded that the foundation alluvium under the shells of the dam would liquefy under the Maximum Credible Earthquake (MCE), a magnitude 7 event within about one mile from the dam, resulting in a peak ground acceleration at the site of 0.8g. This paper will discuss the seismic hazard assessment, seismic stability evaluations, and design of alternative concepts with associated costs to strengthen the dam. Alternatives included upstream modifications using buttresses, which would require draining the reservoir, and downstream methods using buttresses and/or foundation improvement, all of which had to be designed and constructed considering environmental compatibility, acceptability to the local communities, California Division of Safety of Dams (DSOD) review, and lowest cost, but with the benefit of keeping the reservoir in service. The seismic upgrade method selected included strengthening of the alluvium with 2500 stone columns, up to 90 feet deep, and construction of a widened crest and downstream buttress. This alternative allowed the project to be constructed while the reservoir remained in service. A full-scale stone column test program, design criteria development, and performance requirements for the stone column treatment will be discussed. Dynamic response and deformation analyses were performed to evaluate the configuration of the stone column treatment zone and the earthfill buttress to limit embankment dam deformations to less than 5 feet. Discussion of the effect of foundation alluvium stratigraphy, which included layers of silty soils and sandy gravelly soils, on stone column layout, column diameters, and buttress configuration will be presented. With a total project cost of about $30 million, this innovative seismic strengthening approach resulted in approximately $10 million construction cost savings over a conventional buttress method requiring excavation of the alluvium and reservoir lowering or draining. This project is the first to use stone columns for dam remediation approved by the DSOD in California. The two-year construction period was completed in August 2003. A companion paper presents details on the construction of the stone column remediation. 16 pp., 2 tables, 7 figures, 13 references.