After years of evaluation and public input, the California Department of Transportation selected HNTB to design a safe replacement for a cliff-side stretch of U.S. Highway 101, dubbed Devil’s Slide, long plagued by land slippages and rockslides.

The project included two 4,200-foot tunnels, a 1,000-foot bridge, and a system that monitors and reports road and environmental conditions and suppresses fires.

Challenges included boring through multiple rock types, draining 10 million gallons of water, the potential for seismic activity, and requirements to protect endangered species.

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.

Winner of multiple awards, the tunnels can withstand a magnitude 7.5-8.0 seismic event and meet every requirement of the National Fire Protection Association’s 502 Code and the Americans with Disabilities Act.

•    Award-winning construction surmounts seismic, ground and environmental challenges
•    Project introduces special tunnel-mining technique, sets new standards
•    Experts, public applaud project for outstanding safety

Location: San Francisco, California

Client: California Department of Transportation