California Construction News staff writer
A proposed $2.5-billion-to-$3.5-billion tunnel project on U.S. Highway 101 in Northern California has advanced into detailed design, moving a long-awaited solution to one of the state’s most persistent landslide problems closer to construction.
Caltrans announced Aug. 20 that it has completed the environmental and project approval phase for the Last Chance Grade project in Del Norte County. The milestone clears the way for detailed engineering and design work on what would become the longest tunnel in Caltrans history.
The selected alternative calls for an approximately 6,000-foot tunnel east of the existing U.S. 101 alignment, bypassing a section of highway that has been repeatedly damaged and threatened by landslides.
The project is the largest and most complex ever undertaken by Caltrans District 1. Current California Transportation Commission materials identify about $225 million for design and right-of-way support, while construction costs are estimated at between $2.5 billion and $3.5 billion. Construction funding has not yet been secured, although Caltrans plans to pursue a mix of federal and state funding and financing opportunities during the design phase.
Construction is currently anticipated to begin in 2031.
“Last Chance Grade is more than a highway — it is a lifeline for the people, businesses and emergency services of California’s North Coast,” Caltrans Director Dina El-Tawansy said. “Advancing this tunnel into the design phase brings us closer to providing the safe, reliable connection Del Norte County and travelers deserve.”
At roughly 1.1 miles long, the tunnel would be about 1,800 feet longer than each of the twin Tom Lantos Tunnels at Devil’s Slide and would surpass them as the longest tunnel constructed in Caltrans history.
During the design phase, Caltrans will develop plans for the tunnel, portals, connecting roadway, bridge, drainage systems, safety systems and operations and maintenance facilities. The agency will also continue geotechnical investigations, permitting, mitigation work and cost refinements.
Caltrans expects to use advanced underground construction technologies, including precision excavation equipment, robotic shotcrete and real-time monitoring, to improve safety and efficiency while reducing surface disturbance.
The project’s delivery strategy will include specialized tunnel designers, early Construction Manager/General Contractor involvement, independent cost estimating and phased design checkpoints intended to manage risk and refine costs before construction begins.
The unstable highway section has required repeated repairs, traffic delays and emergency closures over several decades. A long-term closure could force drivers onto an eight-hour detour through Oregon and inland Northern California, disrupting commercial freight, emergency services, local businesses and tourism.
Construction is expected to generate temporary regional employment in heavy civil construction, earthwork, tunnel support, hauling, material supply, traffic control and restoration. Caltrans has not yet developed a specific job estimate.
Caltrans is also working with local tribes to expand Native American hiring and workforce development opportunities, including coordination with the Yurok Construction Corporation and exploration of educational opportunities for tribal youth.
“Projects of this magnitude require a bold, shared vision from all stakeholders, and we appreciate their support as we advance this effort to the design phase,” Caltrans District 1 Director Matt Brady said.
