PUC staff: Calaveras Dam excavation reveals additional ancient landslides, likely raising costs and schedule

The San Francisco Public Utilities Commission received a technical update from staff on the Calaveras Dam repair project that identified newly exposed ancient landslide features requiring substantial additional work and likely increasing costs and schedule.

At the commission meeting Julie Labonte, a project lead, said crews have completed roughly half the excavation and the project is about 58% complete by expenditure. She reported that a lower geologic feature recently exposed has been confirmed as an ancient landslide and will require over-excavation of roughly 25 feet beneath the new spillway footprint. That work is likely to add about 135,000 cubic yards of excavation and about 7,000 cubic yards of low-density concrete placed below the spillway foundation to create competent ground, Labonte said.

Labonte added that another upstream area of the site is ‘‘much wider than first anticipated’’ and may require removal of approximately 300,000 cubic yards of landslide material. She said staff and the contractor are conducting parallel analyses of schedule impacts and contractor claims, and the PUC expects a more precise estimate of the magnitude of cost and time impacts by January 2014.

"We are now down at the foundation of the spillway," Labonte said. "This lower feature now has been confirmed as a landslide as well. In order to have this new spillway on competent ground, we will need to over-excavate underneathup to about 25 feet." The presentation also noted current mitigation steps elsewhere on the project, including use of rock bolts and shotcrete where smaller features were detected.

Commissioners pressed staff on downstream inundation risks. Dan Wade, senior project manager for the Sonoma Valley projects, said an inundation study done during environmental review shows that a catastrophic dam failure could inundate parts of Fremont and areas downstream toward the bay with several feet of water in places. Wade cautioned that the study models a complete dam disappearance and that actual failure modes could be partial overtopping or partial collapse.

The PUC staff emphasized that contingencies included in the approved budget were intended for events like this but that the newly quantified impacts are ‘‘quite significant’’ and may exceed those contingencies. Labonte said the commission will see the effects trended in the next quarterly financial report and reiterated the need for schedule-management and cost-control measures across the Water System Improvement Program.

There was no motion or formal vote tied directly to the Calaveras update at the meeting; staff committed to return with refined cost and schedule estimates once the contractor and PUC analyses are complete.

The commission did not adopt new project authority or appropriations at this session; next steps are further analysis, inclusion of impacts in the quarterly forecast, and a staff report with updated estimates.