The Modeling and Analysis of Three Multiple-Arch Dams

An inclined multiple-arch dam is a complex structure compared to a single arch dam in that it resists imposed loading using both arch and gravity type actions. The thin arches transfer load by arch action to the buttresses, which are located between the arches. The buttresses, acting like thin gravity sections, transfer the load to the foundation. The gravity sections are aided by the incline of the arches into the reservoir with depth, which increases their structural stability. The Southern California Edison Company (Edison) has over the past two years been reassessing the capability of its three multiple-arch dams to withstand a maximum credible earthquake (MCE). During this reassessment, various technical issues were encountered. This paper discusses these issues and describes how they were handled. This has also generated some thoughts among the authors about how future analyses of this type of dam might be performed. The three dams, which were reassessed, are Florence Lake Dam, Gem Lake Dam and Agnew Lake Dam. Florence Lake Dam is Iocated on the western slope of the Sierra Nevada mountain range near Fresno, California. It has 58 arches across its breadth and has a crest length of3156 feet. The dam is 150 feet high at its deepest section and impounds about 64,400 acre-feet of water. Gem and Agnew Lake Dams are located on the eastern slope of the Sierra Nevadas near Bishop, California. Gem Lake Dam has 18 arches, is 688 feet long, is 80 feet high and impounds about 17,200 acre-feet of water. Agnew Lake Dam has 7 arches, is 278 feet long, is 30 feet high and impounds about 800 acre-feet of water. The reassessments of Florence and Gem Lake Dams were requested by Edison's FERC Part 12 Board of Consultants in their most recent reviews of these dams. The request was a result of their reevaluation of the seismic ground motion hazard at the Florence Lake Dam and Gem Lake Dam sites, and their opinion that a more current day analysis should be performed. Their reevaluation of the seismic hazard concluded that the peak ground acceleration for the MCE would have to be raised. Agnew Lake Dam was recently changed to a high hazard dam by FERC, and therefore, required a new Part 12 review. Reevaluation of the dam's seismic capability to withstand a maximum credible earthquake is a requirement of this type of review and was completed in the same time frame as that of Gem Lake Dam. The following discusses the procedures used in the analyses, the structural modeling and a summary of the results of the evaluation obtained thus far. As part of our attempt to understand how multiple-arch dams could be tested under dynamic loading, some testing was performed on Florence Lake Dam. A benefit of the testing was that the results were useful in the development of the finite element model for the reevaluation of the dam. This limited testing is also discussed in this paper. 9 pp., 2 references.