Earthquake Analysis and Retrofit Solution for Bradbury Dam Spillway

The existing spillway structure at Bradbury Dam was designed for a less significant earthquake than can be expected. It must be strengthened so that collapse or large deflections of the spillway walls or piers won't lead to a failure of the spillway gates and a sudden uncontrolled reservoir release. The spillway crest structure will be modified so that the spillway walls, hoist deck and highway bridge behave as a frame during an earthquake, allowing the structure to deflect while preventing failure of the gates. The piers will be supported by this frame so that failure of a pier will not result in failure of the gates. The spillway crest structure modifications are the subject of this paper. Bradbury Dam and reservoir are located on the Santa Ynez River, approximately 18 miles northwest of Santa Barbara, California. Construction of Bradbury Dam began in 1949 and was completed in 1953. The dam is a zoned, rolled earthfill structure with a crest length of 3350 feet, a structural height of 279 feet, and a hydraulic height of 198 feet. Cachuma Lake has an active conservation storage capacity of 172,360 acre-feet. It is the main storage feature in the Cachuma Project, which provides water for irrigation and municipal use. The spillway is located on the left abutment and consists of a concrete lined overflow crest, four 50- by 30-foot radial gates, a concrete-lined spillway chute, and a stilling basin. The spillway walls are a combination of cantilevered retaining walls and counterforted retaining walls. In addition to the modifications to the spillway crest structure, other Safety of Dams modifications will be performed at Bradbury Dam [1]. The spillway gates and hoists will be modified. The upgraded hoist system will include automatic controls, which will replace the existing floatwell system. Also, steel plates will be welded to the spillway gates, where the gate arms attach to the gate leaves, in order to stiffen the gates at this location. The embankment dam will also be modified to improve stability under seismic loading. 10 pp., 8 references.