Photo Courtesy of San Francisco Public Utilities Commission

Two ancient landslides were discovered during excavation for the new Calaveras Dam, requiring an additional 3.5 million cu yd of excavation and adding significantly to the project's budget.

With a budget of $718 million, the Calaveras Dam Replacement dwarfs other projects in SFPUC's Water System Improvement Program. Located in the Sunol Valley southeast of Fremont, the project replaces the original dam, built in 1925.

The reservoir provides most of the water storage capacity for SFPUC. However, in 2001, the seismically unsafe dam's proximity to the Calaveras Fault—just 1,500 ft away—led officials to lower its capacity by more than 60%. A joint venture of Dragados USA, Flatiron West and Sukut Construction began work on the new, 220-ft-high zoned earth-and-rock-fill dam in 2011. Completion was originally scheduled for 2015. However, in 2012, during excavation of the left abutment, the project team uncovered two ancient landslides that had not been found during earlier geotechnical investigations. Despite this discovery, a third-party review found the design team had performed proper due diligence, Wade says.

An additional 3.5 million cu yd of excavation is required to remove the landslides, significantly hiking the original $416-million budget and pushing the completion date to late 2018. In total, nearly 10 million cu yd will be excavated. Naturally occurring asbestos in some of the earth adds another hurdle. In these areas, the contractor sets up restricted zones in which workers must continuously wear asbestos and protective clothing and, at the end of their shifts, go through decontamination, says Alberto Benlloch, project manager.