Project Name:  Caldecott 4^th Bore

Location: California, USA

Application: Sprayed Concrete Lining (Shotcrete)

The Caldecott Fourth Bore Project is a 1,030m long, 15.2m wide, US$420 million highway tunnel in the San Francisco Bay Area owned by the California Department of Transportation. The main contractor was the Tutor-Saliba Corporation, and the preliminary and final designs were provided by Jacobs Associates. The tunnel used the New Austrian Tunnelling Method (NATM). Ground support for the initial shotcrete lining comprised macro synthetic fibers and rock bolts.

In NATM tunnels the role of the initial lining is to carry the ground loads, control the ground deformations during tunnel construction, and provide a safe work environment. Typically, this lining is considered redundant after construction. However, the Caldecott 4th Bore utilises a load sharing design method between the initial and secondary lining to reduce the design loads on the secondary lining.

By designing the secondary lining to carry 67% of the ground load rather than 100%, designers could reduce the secondary lining required thickness by 30%, a massive cost saving. An additional benefit of the reduced secondary lining thickness was the reduction in forces in the more flexible lining when subject to seismic racking deformations.

Outcomes:

• Reduced secondary lining required thickness by 30%
• Increased ductility in the overall lining
• Eliminated the risk of corrosion

Author:  Geoff Sedgman: gsedgman@elastoplastic.com