Caldecott Improvement Project

The three existing 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 will provide another two-lane traffic tunnel dedicated to west-bound traffic. This horseshoe-shaped tunnel will be approximately 3,399 feet long (1,036 m), 50 feet wide (15.2 m), and 32 feet high (9.75 m) (excavated dimensions).

As the technical design lead on the Parsons team, Jacobs Associates 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 other consultants for drainage, waterproofing, ventilation, fire/life safety, traffic monitoring, and control systems design.

Project challenges include designing a safe and efficient way to mine through weak rock and unstable fault zones. The Fourth Bore will encounter steeply dipping, blocky to crushed sedimentary rock formations consisting of mudstone, shale, sandstone, and chert, which have undergone extensive folding and faulting. The unconfined compressive strength of the rocks ranges from 100 to 23,000 psi (0.68 to 158.57 MPa).

The design team based its approach on the Sequential Excavation Method, also known as the New Austrian Tunneling Method (NATM). This technique provides 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.

The project is currently under construction, and Jacobs Associates is providing design support services during construction. The top heading excavation has been advanced approximately 591 feet (180 m) from the east portal as of December 2010. A canopy pipe arch is being installed at the west portal, and top heading excavation is anticipated to begin in January. Excavation from the east portal is being performed using a Wirth road header. Initial support consists of fiber reinforced shotcrete, rock dowels, face dowels, spiling or pipe canopy presupport, and lattice girders. Benching, installation of waterproofing membrane, and final lining construction will follow completion of the top heading excavation.