Crystal Springs Dam Bridge Replacement Project

Location

Crystal Springs Road & Skyline Boulevard (Highway 35)

City or County Responsible for Project

County of San Mateo, Department of Public Works

Category

Bridge: Efficient and Sustainable Bridge Maintenance, Construction and Reconstruction Projects

Author

Carter Choi, PE

Organization

County of San Mateo, Department of Public Works

Address

555 County Center, Redwood City, CA 94063

Phone

(650)363-4100

Project Description

The Lower Crystal Springs Dam (Dam) across San Mateo Creek is owned and operated by the San Francisco Public Utilities Commission (SFPUC) and forms the Crystal Springs Reservoirs which have a combined capacity of 22.5 billion gallons of water and receives water from the Hetch Hetchy Reservoir. The Dam, which was constructed in 1888, is a gravity arch dam and served as a model for Hoover Dam. San Mateo County owns and operates the bridge that carries traffic over the dam along Skyline Boulevard, also known as Highway 35, and leads to the recreational Sawyer Camp Trail. In 1988 the California Division of Safety of Dams reevaluated the Probable Maximum Flood for Crystal Springs Reservoir and deemed the Dam’s existing spillway to be too small. In order to construct the spillway improvements, it was necessary to remove the bridge, widen the spillway, and then construct a new bridge on the dam structure. The County of San Mateo completed demolition of the 1920’s era bridge from the top of Dam in 2011. Subsequently, the SFPUC modified the spillway, parapet wall, and stilling basin to enable the Dam to accommodate the Probable Maximum Flood, and the water transmission systems adjacent to the dam, which were completed by 2014. The bridge replacement project occurred thereafter and was primarily funded by Federal Highway Bridge Replacement and Rehabilitation funds. Due to the presence of endangered species (California Red Legged Frogs, San Francisco Garter Snakes, and Dusky Footed Woodrats) in the work area, it was necessary to complete many environmental studies and receive multiple approvals from respective regulatory agencies. Engineering design approvals were also required from Caltrans, State Department of Water Resources Division of Safety of Dams, and in close coordination with the SFPUC. Construction of the new bridge began in February 2016 and was completed in October 2018. The cast in place, post tensioned, concrete box girder bridge is 626 feet long and 51.5 feet wide and was constructed approximately 7 feet higher than the former bridge to accommodate the SFPUC dam parapet wall modifications. The road approaches had to be raised, and the retaining walls at the Scenic Vista Point parking area were also constructed due to the change in bridge elevation. The work also included grading, drainage facilities, paving, and security fencing. PG&E relocated their overhead 230kV electrical transmission lines to the bottom of the new bridge in connection with this project. The new bridge also includes a 15-foot wide recreational trail that is separated from vehicular traffic to provide improved connectivity for the Crystal Springs Regional Trail users for years to come. Lastly, the sustainability goal was to replace the former seismically deficient bridge with one able to withstand the maximum credible earthquake considering its location within 1000 feet relative to the San Andreas fault and its location relative to the Dam (a lifeline structure).