Crews make progress on major tunnel project
Federal safety regulators ordered county's largest reservoir drained
A $1.2 billion project to rebuild the largest dam in Santa Clara County to reduce the risk of it failing in a major earthquake finally has begun to make significant visible progress as construction crews have started to dig a huge new outlet tunnel at Anderson Dam near Morgan Hill.
The tunnel, which will be 24 feet high and 1,700 feet long, will allow the sprawling reservoir east of Highway 101 to be drained more quickly in the event of an earthquake, extreme storm or other incident.
After the tunnel is finished in late 2024, and a 13-foot-high pipe installed inside, crews will tear down the existing 240-foot earthen dam, built in 1950. They then will build a new dam and concrete spillway, capable of surviving a 7.2 magnitude earthquake. That project won't be finished until 2032. Dam experts and safety officials around the state are watching.
“This definitely is one of our biggest, and most complex projects,” said Sharon Tapia, manager of the state Division of Safety of Dams, in Sacramento.
Last month, about 30 construction workers with excavators and rock-cutting machines called road headers began carving away on a hillside near the left side of the aging dam.
Crews overseen by the contractor, Flatiron West construction, a Colorado company with offices in Benicia
and San Diego, are progressing about 5 feet to 10 feet a day, boring through sandstone, siltstone and other types of bedrock. They are installing steel support arches and spraying a concrete mix to harden the tunnel. Altogether, 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 is $168 million.
“I think it is going pretty 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 working through the issues.”
Anderson Dam is a key part of Silicon Valley's water
system. When its reservoir is full, Anderson holds 89,278 acre-feet of water — more than the water district's other nine reservoirs combined. Because of the construction project, it has been drained, now to only 3% full, for the past two years, which has exacerbated local water shortages during the ongoing drought, and forced the water district to pump more groundwater and import expensive water from farmers in the Sacramento Valley.
“We are grinding on this,” Mccarter said. “Anderson is a massive undertaking. It's our No. 1 most important project.”
Usually in the Bay Area, most people encounter tunnels when they are driving. In 2013, crews dug a fourth bore of the Caldecott Tunnel, on Highway 24 between Oakland and Orinda, for more than 160,000 motorists a day.
That same year, crews opened two huge new tunnels on the San Mateo Coast, at Devil's Slide, allowing drivers on Highway 1 to head through the Coast Range to Pacifica, and ending decades of road closures due to rock slides. Named for the late Congressman Tom Lantos, they are the secondand third-longest road tunnels in California at 4,149 feet northbound and 4,008 feet southbound.
The longest road tunnel in California is the Wawona Tunnel on Highway 41 in Yosemite National Park, which traverses 4,233 feet.
But water agencies regularly tunnel also. The San Francisco Public Utilities Commission completed a huge tunnel under San Francisco Bay in 2014 to secure water pipes that deliver Hetch Hetchy water to the Peninsula as part of a $5 billion earthquake hardening of its system.
Near Los Gatos, the Santa Clara Valley Water District built a similar outlet tunnel at Lexington Reservoir's dam in 2008. And the agency currently is building a tunnel near Morgan Hill as part of a project to improve flood protections on Llagas Creek and route high stream flows around vulnerable areas.
The Anderson Dam project has been a marathon with multiple setbacks and delays.
When the district built the dam in 1950, scientists thought the nearby Calaveras Fault was inactive. Officials believed that the dam was anchored in bedrock.
But an engineering company performing tests in December 2008 found the dam's foundation contains sand and gravel, which could shift in a major quake. A 6.6 magnitude quake on the Calaveras Fault directly at Anderson Reservoir, or a 7.2 quake centered 1 mile away, could cause the huge earthen dam to slump and fail.
Although unlikely, a complete failure of Anderson Dam when the reservoir is full could send a 35-foot wall of water into downtown Morgan Hill within 14 minutes, engineers concluded. The waters would be 8 feet deep in San Jose within three hours, potentially killing thousands of people.
At first, in 2011, the district planned to strengthen the existing dam. But new trace faults found in the area required an entirely new dam to be constructed, doubling the price by 2016 to $400 million.
Anderson's very small outlet pipe contributed to the floods in downtown San Jose during major storms in 2017.
That same year, big storms caused the near failure of Oroville Dam in Butte County. Federal regulators, concerned about the slow pace of the Santa Clara Valley Water District's progress to improve safety at Anderson, ordered the dam drained, and the tunnel project to be done first, before construction of the new dam, which district officials say increased the project time from five to 10 years and raised the price tag. More delays have occurred, they say, because of the slow pace of obtaining permits from federal fish and wildlife agencies.
Tapia called the effort “a megaproject” that is really three or four large projects. But she said it has to be done due to climate change driving more extreme weather swings.
“Our wet and dry cycles are getting more intense,” she said. “So it is more important than ever that our dams are resilient.”