Oakland, CA

Caldecott Improvement Project

California Department of Transportation (Caltrans)


The three original Caldecott highway tunnels are located on State Route 24 and pass through the Berkeley Hills, connecting the California cities of Oakland and Orinda. The Caldecott Fourth Bore provides another two lanes dedicated to westbound traffic. This horseshoe-shaped tunnel is approximately 3,399 feet long (1,036 m), 50 feet wide (15.2 m), and 32 feet high (9.8 m) (excavated dimensions).

Fast Facts
  • 3,399 feet long (1,036 m), 50 feet wide (15.2 m), and 32 feet high (9.8 m) (excavated dimensions)
  • Sequential excavation method (SEM) tunneling
  • Excavated through weak rock and unstable fault zones
  • Completed on schedule and under budget

Scope of Work

As the technical design lead on the design team, Delve Underground was responsible for geomechanical characterization, as well as design of initial tunnel support, final lining, portal excavation support, and seismic analysis. In addition, we were responsible for cost estimating and managing the technical work performed by other consultants for drainage, waterproofing, ventilation, fire/​life safety, traffic monitoring, and control systems design. Delve also provided design support services during portal development, tunnel mining, and final lining operations.

Challenges & Innovations

Project challenges included designing a safe and efficient way to mine through weak rock and unstable fault zones. The Fourth Bore encountered steeply dipping, blocky to crushed sedimentary rock formations consisting of mudstone, shale, sandstone, and chert, which have undergone extensive folding and faulting. The design team based its approach on SEM, which provided the required flexibility to construct a wide-span tunnel in weak and variable ground conditions. Using detailed numerical analyses to predict ground behavior in response to tunnel excavation, the team developed efficient support systems tailored for specific reaches along the alignment.