After seven years of construction, the $823-million Calaveras Dam Replacement Project has hit a major milestone as the new replacement dam at Calaveras Reservoir reached its full height of 220 ft earlier this month.
With the earth and rock fill dam now topped-out, the dam construction portion of the project is complete. Crews still need to construct access roads, automate instrumentation and controls, restore the site, and place rock slope protection before the construction of the overall project is complete, which is slated to happen in spring of next year.
The project is being oversaw by the San Francisco Public Utilities Commission (SFPUC), the California Department of Water Resources Division of Safety of Dams and the Bay Area Water Supply and Conservation Agency (BAWSCA). Construction is being led by a tri-venture between Dragados, Flatiron and Sukut. Black & Veatch is serving as construction manager.
Since 2001, reservoir levels behind the 93-year-old Calaveras Dam have been reduced to 40 % of capacity due to seismic concerns. Calaveras Reservoir, located on the border between Santa Clara and Alameda counties, is the largest of the SFPUC’s five Bay Area reservoirs, capable of storing 31 billion gallons of water at full capacity. With the dam now at full height, the SFPUC says it will begin refilling the reservoir this winter.
While the Hetch Hetchy Reservoir in the Sierra Nevada Mountains supplies 85 percent of the drinking water to the SFPUC’s 2.7 million customers, the remaining 15 percent is delivered through the SFPUC’s five water supply reservoirs in the Bay Area. SFPUC says that restoring the Calaveras Reservoir to full capacity is key to ensuring a reliable water supply for customers in four Bay Area counties.
The new dam is composed of seven zones of different materials, with the majority of the earth, rock, sand and clay used for the structure being sourced from onsite. The new dam, which took two years to construct, is located directly adjacent to the old dam, and has been built to withstand a 7.25 magnitude earthquake on the nearby Calaveras Fault. Earlier this month, a 3.4 magnitude temblor shook the area, and no damage occurred to the new dam or its accompanying structures.
Under the State of California Department of Water Resources, the Division of Safety of Dams (DSOD) provides oversight to the design, construction, and maintenance to nearly 1,250 dams in California, including the Calaveras Dam.
“Given California’s complex geology and tectonic regime, many of the dams in California, such as the Calaveras Dam, are located near major faults and can be subject to severe loading conditions,” said DSOD Division Chief Sharon K. Tapia in a recent news release. “Ensuring the seismic stability of the dams in California is paramount for public safety. As one of California’s largest seismic retrofit projects, the Calaveras Dam Replacement Project is a model project in its robust design features and construction standards.”
Chris Mueller, Black & Veatch’s project director, told me that main challenges on the dam portion included differing geologic site conditions in the dam foundation’s left abutment that required excavation and disposal of more than 2 million cu-yds of additional rock from what was indicated in the construction contract.
“The additional work required development of new disposal sites and resequencing of the spillway construction,” said Mueller. “The team – owner, contractor, engineering of record and construction manager – overcame the challenge by working collaboratively to find solutions to the problem in the left abutment that minimized impacts to project costs and schedule.”
The Calaveras Dam Replacement Project is the largest project of the $4.8 billion Water System Improvement Program (WSIP) to repair, replace, and seismically upgrade key components of the Hetch Hetchy Regional Water System. The SFPUC, together with its 26 wholesale customers, launched the WSIP in 2002. One of the largest water infrastructure projects in the country, the WSIP is now more than 96 % complete.
Crews working on the Calaveras Dam Replacement Project moved about 12 million cu-yds of earth and rock to construct the new dam. Of that total, roughly four million cu-yds of material was used for the new dam, while the remainder was placed in other areas on site.
Construction on the project began in 2011. It is scheduled to conclude in spring 2019. It is being funded from a bond measure that was approved by San Francisco voters in November 2002.