The area known as Last Chance Grade runs along an unstable stretch of the Pacific coastline in Del Norte County, CA. The project site is the most seismically active in the conterminous U.S., with tectonic features capable of generating damaging high-magnitude earthquakes. The existing US 101 alignment has long been subject to instability and slope failures, with four active landslide complexes within site boundaries.
The site is also environmentally sensitive, surrounded by Redwood National and State Parks, as well as First Peoples’ lands and is a designated UNESCO World Heritage site. Redwood National and State Parks encompass a majority of the affected highway, with trees exceeding 300’ in height and over 1,000 years old. The area is of particular importance to a number of Native American tribes that have maintained a presence in the region for centuries.
HNTB is the prime consultant leading a team of design and environmental experts and working closely with Caltrans and its partners, stakeholders, and North Coast communities, to develop a permanent solution. Our shared goal is to provide a safe and reliable route, while balancing the critical elements of geotechnical/seismic complexity, environmental sensitivity, stakeholder concerns, and funding availability.
Before engaging the HNTB team, Caltrans prepared a Project Study Report and a Supplemental which identified 16 potential alternatives that were combined and screened down to seven alternatives to move forward into the environmental phase. Our scope included developing effective alternatives analysis screening criteria to rank and weight the performance and impacts of alternatives across several categories. Key criteria include level of environmental impacts, time required to study and build the alternative, and operational performance.
Based on analysis against these criteria, we were able to screen down to two alternatives that performed significantly better than the other five alternatives – saving millions of dollars and cutting a year off the project schedule.
Alternative F: Tunnel Bypass
Alternative F creates a tunnel at a location that enables the highway to avoid much of the landslide area, reducing risk of interruption and minimizing potential environmental impact. We are providing Caltrans with conceptual design options for the tunnel and its approach structures, drawing on lessons we learned working together in District 4 to design the Tom Lantos (Devil’s Slide) Tunnels in in San Mateo County. Leveraging our expertise in tunnel engineering, tunnel ventilation/fire-life safety, and tunnel construction methods, we are maximizing utilization of the limited geotechnical information and optimizing tunnel design for the significant site constraints.
Alternative X: End-to-End Re-Engineering
Alternative X is quickest to construct, least expensive, and has the second smallest potential impact on the environment and resources. Alternative X re-engineers the entire length of the existing route to minimize the project footprint and environmental impacts and mitigate the landslide hazard. Improvements include an underground drainage system to reduce excessive stormwater (a major cause of damage) and retaining structures with tiered and steel-reinforced walls, supports and ground anchors both uphill and downhill.
Location: Del Norte County, CA
Client: California Department of Transportation, District 1