The three original Caldecott highway tunnels on State Route 24 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’s excavated dimensions are approximately 3,399 feet long (1,036 m), 50 feet wide (15.2 m), and 32 feet high (9.8 m).
As the technical design lead, McMillen Jacobs Associates was responsible for geomechanical characterization, initial tunnel support design, 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 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 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 provided the flexibility needed 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.
McMillen Jacobs Associates provided design support services during portal development, tunnel mining, and final lining operations. Tunnel excavation was performed using a top heading and bench sequence, and the initial support consisted of a combination of fiber reinforced shotcrete, rock dowels, face dowels, spiling or pipe canopy presupport, and lattice girders, depending on the ground conditions. Tunnel excavation was performed using a roadheader and excavators fitted with roadheader attachments and hydraulic hammers. The final lining consists of cast-in-place reinforced concrete with a waterproof membrane. The Caldecott Fourth Bore opened to traffic on November 16, 2013, after four years of construction. The project was completed on schedule and under budget.