Caldecott Fourth Bore Tunnel

Project Narrative

The Caldecott Fourth Bore Tunnel is an approximately 3,250-ft (991m) long by 49-ft (15m) wide by 40-ft (12m) high two-lane highway tunnel built through the East Berkeley Hills to improve traffic flow on congested SH 24 connecting Alameda and Contra Costa Counties. Constructed by the Sequential Excavation Method (SEM) or New Austrian Tunneling Method (NATM), the tunnel includes seven mined cross passages connecting the third and fourth bores and serve as emergency exits. Other project components are soil nail walls, secant pile walls, and soldier pile walls at the portal approaches. The cover above the tunnel varies from 15- to 20-ft (5m to 6m) at the portals to a maximum of over 500-ft (152m) near the midpoint.

The tunnel was driven through marine and terrestrial sedimentary rocks comprised of shale, sandstone, siltstone, claystone, and conglomerate with altered igneous and sandstone dikes and sills cross-cutting the steeply dipping strata. Four known inactive fault contacts were encountered with associated broken rock and gouge conditions. Support included: rock bolts, rock dowels, shotcrete, lattice girders, grouted and self-drilled spiles, and grouted pipe canopy arch initial support. The final lining is cast-in-place concrete.

Brierley Associates served as the Tunnel Contractor’s Engineer during construction, providing: pre-bid evaluations and successful bid preparation assistance; submittal preparation for tunnel excavation and support related work; full-time on-site engineering for SEM construction including Lead SEM Engineer and Geologist positions for initial support selection; geotechnical documentation and instrumentation, and coordination with the Owner, Designer, and Construction Manager.