California's landslide bypass tunnel moves into design

Caltrans has advanced the Last Chance Grade tunnel project into detailed design, moving closer to a long-term solution for one of the most unstable sections of US Highway 101 in northern California.

The project will bypass a landslide-prone coastal section of highway in Del Norte County that has required repeated repairs, traffic restrictions and emergency closures for decades. The selected alternative involves an approximately 1.83 km long tunnel east of the existing road alignment. If delivered as planned, it would become the longest highway tunnel built by Caltrans.

The project has completed the Project Approval and Environmental Document phase, which included environmental studies, engineering analysis, public and agency coordination and approval of the preferred alignment. Construction is currently targeted to begin in 2031, subject to funding, permitting and completion of design.

Last Chance Grade is part of a critical north-south transport corridor on California’s North Coast. A major landslide-related closure could force traffic onto a detour through Oregon and inland northern California, adding around eight hours to journeys. That would affect residents, emergency services, freight, tourism and local businesses.

The scale of the engineering challenge is significant. Current California Transportation Commission materials estimate construction costs between USD2.5bn and USD3.5bn, while around USD225M has been identified for design and right-of-way support. Main construction funding has not yet been secured.

During the design phase, Caltrans will develop the tunnel, portals, connecting highways, bridge works, drainage, safety systems and operations and maintenance facilities. Further geotechnical investigations, mitigation works, permitting and cost refinement will also be undertaken.

The most important aspect is that the tunnel is not only a transport project. It is a ground risk management project. The purpose is to move the highway away from a coastal slope section affected by long-term instability and place the route through a more controlled underground alignment.

That shifts the design focus to portal stability, tunnel excavation method, groundwater behaviour, ground support, drainage, monitoring, emergency systems and long-term maintenance access. The project is expected to use advanced underground construction technologies, including precision excavation equipment, robotic shotcrete application and real-time monitoring.

The delivery strategy will include specialist tunnel designers, early Construction Manager/General Contractor involvement, independent cost estimating, additional geotechnical investigations and phased design reviews. Barnard-Traylor-Myers has been selected as the Construction Manager/General Contractor.

The project follows earlier California tunnel precedents, including the Tom Lantos Tunnels at Devil’s Slide, the Caldecott Fourth Bore and the Doyle Drive and Presidio Parkway tunnels. The Tom Lantos Tunnels are especially relevant because they also bypassed a landslide-prone section of coastal highway.

The Last Chance Grade tunnel shows how infrastructure resilience can require relocation rather than repeated repair. In this case, the design challenge is to create a safer and more reliable highway connection by understanding the ground, avoiding the unstable slope and managing the risks of a major rural coastal tunnel project.