Progress has been made on the tunnel as part of a $1.2 billion earthquake project

A $1.2 billion project to rebuild Santa Clara County’s largest dam to reduce the risk of it failing in a major earthquake is finally making headway, as construction crews begin digging a massive new outlet tunnel at the Anderson Dam near Morgan Hill.

The tunnel, which would be 24 feet high and 1,700 feet long, would allow the sprawling reservoir east of Highway 101 to be drained more quickly in the event of an earthquake, severe storm, or other accident.

After completing the tunnel in late 2024, and installing a 13-foot pipe inside, the crews will demolish the existing 240-foot-tall earthen dam, built in 1950. Then they will build a new dam and concrete culvert, capable of surviving 7.2 magnitude earthquake. This project isn’t finished until 2032. Dam experts and safety officials across the state are watching.

“This is definitely one of our largest and most complex projects,” said Sharon Tapia, director of the state’s Division of Dam Safety in Sacramento.

Last month, about 30 construction workers with backhoes and rock-cutting machines called roadheads began carving away on a hillside near the left side of the old dam.

Crews supervised by the contractor, Flatiron West Construction, a Colorado company with offices in Benicia and San Diego, progress about 5 feet to 10 feet a day, boring through sandstone, mudstone, and other types of bedrock. They are installing steel support brackets and spraying concrete mixture to harden the tunnel. In all, workers plan to remove about 30,000 cubic yards of rock and dirt from the tunnel—enough to fill about 3,000 dump trucks.

The total cost of the tunnel: $168 million.

“I think it’s going well,” said Ryan McCarter, a civil engineer with the Santa Clara Valley Water District, which owns the dam and was ordered by the Federal Energy Regulatory Commission to do the work. “We have a really good rhythm of dealing with problems.”

Anderson Dam is an essential part of the Silicon Valley water system. When its reservoir is full, Anderson has 89,278 feet of water—more than the other nine reservoirs in the water area combined. Because of the construction project, water has been drained, now down to only 3%, over the past two years, exacerbating local water shortages during the ongoing drought, and forcing the water district to pump more groundwater and import expensive water from farmers. in the Sacramento Valley.

“We’re grinding on this,” McCarter said. “Anderson is a huge undertaking. It is our No. 1 most important project.”

Crews work on a $168 million project to build a new main outlet tunnel at the Anderson Dam near Morgan Hill, California (Photo: Santa Clara Valley Water District)

Usually in the Bay Area, most people encounter tunnels while driving. In 2013, crews excavated the fourth bore of the Caldecott Tunnel, on Highway 24 between Oakland and Orenda, to more than 160,000 drivers daily.

That same year, crews opened two new massive tunnels on the San Mateo coast, at Devil’s Slide, allowing drivers on Highway 1 to head up the Coast Range to Pacifica, ending decades of road closures caused by rockslides. Named after the late Congressman Tom Lantos, it is the second and third longest road tunnel in California at 4,149 feet north and 4,008 feet south.

The longest road tunnel in California is the Wawona Tunnel on Highway 41 in Yosemite National Park, which cuts 4,233 feet.

But water agencies tunnel regularly, too. The San Francisco Public Utilities Commission completed a massive tunnel under San Francisco Bay in 2014 to secure the water pipes that carry Hetch Hetchy’s water to the peninsula as part of the $5 billion earthquake to its system.

Near Los Gatos, the Santa Clara Valley Water District built a similar outlet tunnel at the Lexington Reservoir Dam in 2008. The agency is currently building a tunnel near Morgan Hill as part of a project to improve flood protection at Llagas Creek and direct higher-flow flows around vulnerable areas. .

The Anderson Dam project has been a marathon with many setbacks and delays.

When the area built the dam in 1950, scientists thought the nearby Calaveras fault was inactive. Officials believe the dam was firmly embedded in the rock.

But an engineering firm that conducted tests in December 2008 found that the foundation of the dam contains sand and gravel, which can shift in a major earthquake. A magnitude 6.6 earthquake on the Calaveras Fault directly in the Anderson Reservoir, or a magnitude 7.2 earthquake a mile away, could cause the massive earthen dam to collapse and fail.

Engineers concluded that a complete failure of the Anderson Dam when the reservoir is full, though unlikely, could send a 35-foot wall of water into downtown Morgan Hill within 14 minutes. The water would be 8 feet deep in San Jose in three hours, which could kill thousands of people.

Crews work on a $168 million project to build a new main outlet tunnel at the Anderson Dam near Morgan Hill, California (Photo: Santa Clara Valley Water District)

Initially, in 2011, the region planned to strengthen the existing dam. But new faults were found in the area that required an entire new dam to be built, doubling the price by 2016 to $400 million.

A very small Anderson outlet pipe contributed to flooding in downtown San Jose during major storms in 2017.

That same year, major storms caused the Oroville Dam in Butte County to collapse. Federal regulators, concerned about the Santa Clara Valley Water District’s slow progress to improve safety in Anderson, ordered the dam drained, and the tunnel project implemented first, before building the new dam, which district officials say augmented the project. Time from 5 to 10 years and raise the price. Further delays, they say, have been caused by the slow pace of obtaining permits from federal fish and wildlife agencies.

Tapia called this effort a “megaproject” and it’s really three or four big projects. But she said it must be done because climate change is causing more extreme weather swings.

“Our dry and wet cycles are getting more intense,” she said. “So it is more important than ever that our dams are resilient.”

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