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 is responsible for geologic characterization, as well as design of initial tunnel support, final lining, portal excavation support, and seismic analysis. In addition, we are 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 has developed efficient support systems tailored for specific reaches along the alignment.

It is anticipated that the sequential excavation of the tunnel will be performed by roadheader, with presupport measures ranging from spiling to pipe canopies. A waterproof membrane will be placed on the inside face of the initial support—a mix of reinforced shotcrete, lattice girders, and rock bolts—followed by the construction of a cast-in-place reinforced concrete final lining.