Gibraltar Dam: Roller Compacted Concrete Buttress Construction

The original Gibraltar Dam was a constant radius concrete arch dam built in 1920 on the Santa Ynez River approximately 9 miles north of the City of Santa Barbara, California. The dam is owned and operated by the City of Santa Barbara and provides storage for the City's municipal water supply. In 1948, the dam was raised 15 feet (increasing dam's height to 195 feet) to provide additional storage. This required, a new thrust block on the right abutment, enlargement of the left abutment thrust block, and installation of three 52-foot wide spillway radial gates. The spillway is located on the east (left) abutment adjacent to a 162-foot long, 35-foot high gravity concrete structure. Due to the spillway being severely taxed during the 1969 floods the spillway capacity was increased in 1971 by adding a 58-foot wide radial gate and 25-foot wide skimmer gate. A plan and sections of the existing dam prior to the construction of the roller compacted concrete (RCC) buttress and statistical data of the dam are provided. In 1979, the California Division of Safety of Dams (DSOD) conducted an investigation of Gibraltar Dam as part of the National Dam Inspection Program. Subsequently DSOD directed the City of Santa Barbara to perform a detailed investigation of the stability of the dam under extreme seismic and reservoir loadings corresponding to the Maximum Credible Earthquake (MCE) and Probable Maximum Flood (PMF). The City hired Woodward-Clyde Consultants (WCC) to perform the detailed study which included field investigations, lab testing, finite element and stability analyses, and evaluation. Based on WCC's investigation results DSOD agreed that the existing dam was stable under the PMF loadings but that the stability of the dam was questionable under seismic loads. WCC concluded that stresses developed in significant portions of the dam under extreme seismic loads would be higher than the allowable tensile strength of the existing concrete. Based on WCC's study results and discussions between the City, WCC, and DSOD it was agreed that the dam would be strengthened. Based on technical and cost evaluations it was decided that a RCC buttress would be the best method of strengthening the dam. The alteration application, plans, and specifications for the strengthening of Gibraltar Dam prepared by WCC were approved by DSOD in November 1989. The project was also approved by the Federal Energy Regulatory Commission (FERC). The City of Santa Barbara awarded the construction contract to Ball, Ball, & Brosamer, Inc. in January 1990. Construction started in April 1990 and was completed near the end of February 1991. Placement of approximately 93,000 cubic yards of RCC began on October 1, 1990 and was completed by December 22, 1990.