Construction crews are halfway finished on a project to dig a 1,740-foot outlet tunnel — roughly as long as five football fields — through bedrock near Anderson Dam in Morgan Hill.
The new tunnel, 24 feet high, will allow water to be released from the reservoir faster in the event of an earthquake, massive winter storms or other emergency. It is part of a $1.4 billion project ordered by the federal government to tear down the 240-foot earthen dam, originally built in 1950, and rebuild it to modern seismic standards to reduce the risk of it failing in an earthquake.
As of Tuesday, workers had finished digging 830 feet of the new tunnel. When the work began last November, they used excavators and rock-cutting machines called road headers to carve away at a hillside on the left side of the dam. This month they have used TNT to blast through some of the hardest bedrock.
“We have made a lot of progress this year,” said Ryan McCarter, a civil engineer with the Santa Clara Valley Water District, who is overseeing the project. “We’re pleased. We’ve had some setbacks, but we’ve had weeks of really good production. It’s still on schedule.”
VIDEO: TNT explosions carve out new tunnel for Santa Clara County’s largest reservoir
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.
After the tunnel is finished in late 2024, and a 13-foot high pipe installed inside, crews will tear down the existing Anderson Dam, the largest in Santa Clara County. They will then build a new dam and concrete spillway, capable of surviving a 7.2 magnitude earthquake. That project won’t be finished until 2032.
The Santa Clara Valley Water District recently released a 1,000-page draft environmental impact report spelling out details of the project. The district plans to hold a public meeting to solicit comments on the document at 6:30 pm on Oct. 4 at the Morgan Hill Community and Cultural Center, 17000 Monterey Highway, in Morgan Hill. The meeting also can be watched via Zoom at this link: https://valleywater.zoom.us/j/87331718480