The proposed Caldecott 4th Bore, located along State Route 24 in Oakland, California is a 15 m wide, 9.7 m high, highway tunnel that will be constructed using the New Austrian Tunneling Method (NATM). Rock mass strength and deformability properties including local effects due to strain softening, intact rock softening and strength degradation due to erosion and softening of joint infillings are key design considerations for the project. Ground/structure interaction issues modeled in numerical analyses include stress relaxation ahead of the tunnel heading, arching of loads across weak zones, face stability, effects of early age creep in shotcrete and stress redistributions due to the formation of plastic hinges in the shotcrete lining. The design also evaluated forces, moments and rotations in the shotcrete lining to determine ground support requirements.
1.1. Project Background
The existing Caldecott Tunnel complex includes three bores along State Route 24 (SR 24) through the Berkeley Hills in Oakland, California. The California Department of Transportation (Caltrans) and the Contra Costa Transportation Authority (CCTA) propose to address congestion on SR 24 near the existing Caldecott Tunnels by constructing a fourth tunnel that will provide two additional traffic lanes. The proposed horseshoe-shaped fourth bore is 1,036 m (3,399 ft) long, with a span of 15 m (50 ft), and a height of 9.7 m (32 ft). The fourth bore will be constructed using the New Austrian Tunneling Method (NATM). The project will include short sections of cut-and cover tunnel at each portal, seven cross-passageway tunnels between the fourth bore and the existing third bore, electrical substation buildings, and a new operations and control building. State Route 24, considered a lifeline route by Caltrans, is required to be open to emergency vehicles 72 hours after an earthquake with a return period of 1,500 years and a peak ground acceleration of 1.2 g. Construction of the fourth bore is anticipated to begin in the summer of 2009 and be completed in 2014.
The geology of the alignment is characterized by northwest-striking, steeply-dipping, and locally overturned marine and non-marine sedimentary rocks of the Middle to Late Miocene age. The western end of the alignment traverses marine shale and sandstone of the Sobrante Formation. The Sobrante Formation includes the First Shale, Portal Sandstone, and Shaly Sandstone geologic units as identified by Page . The middle section of the alignment traverses chert, shale, and sandstone of the Claremont Formation. The Claremont Formation includes the Preliminary Chert, Second Sandstone, and Claremont Chert and Shale geologic units . The eastern end of the alignment traverses non-marine claystone, siltstone, sandstone, and conglomerate of the Orinda Formation. Major formations and geologic units within these formations are shown Figure 1. The geological structure of the project area has been characterized as part of the western, locally overturned limb of a broad northwest-trending syncline, the axis of which lies east of the project area. The fourth bore alignment will encounter four major inactive faults, which occur at the contacts between geologic units. These faults strike northwesterly and perpendicular to the tunnel alignment.