Replacing an infamous coastal stretch of California’s State Route 1—coined Devil’s Slide due to its lengthy closures from land slippages and rockslides—the Tom Lantos Tunnels project provides a safer and far more reliable mobility solution for motorists dependent on the route.

The solution, one of the largest and most complex projects in California’s history, include:

  • Twin tunnels, with the longest bore of approximately 5,000 feet, making them the longest highway tunnels in California.
  • A southern portal approach, measuring 1,000 feet long, with a tall rock slope held in place by soil nails.
  • Northern portal approaches, measuring 1,500 feet long, with separated twin cast-in-place segmental concrete box girder bridges.
  • An off-site operations and maintenance center.

While the distinguished, well-awarded project was successful on all accounts, innovative approaches to the tunnel’s design and delivery were critical to addressing unavoidable obstacles. Challenges included boring through multiple rock types, draining 10 million gallons of water, the potential for seismic activity, and requirements to protect endangered species.

The tunnels’ length and the challenging ground conditions did not lend themselves to a tunnel boring machine technique. Selected by Caltrans to design the replacement and provide construction support services, HNTB’s approach included the New Austrian Tunneling Method, a sequential excavation and support tunnel-mining technique that facilitated unprecedented in-field collaboration and set a standard for future designers and contractors.

Now, with a tunnel design that can withstand a magnitude 7.5-8.0 seismic event, the Tom Lantos Tunnels are continuing to provide a safe and reliable travel option for the surrounding community and motorists of State Route 1.

Location: San Francisco, California

Client: California Department of Transportation