The Caldecott Fourth Bore Project is a four-year, $417 million project to build a fourth tunnel bore at the Caldecott Tunnel on Route 24 through the Oakland Hills. The new bore will be built to the north of the existing three bores and will reduce congestion in the off-peak direction (eastbound in the morning and westbound in the afternoon).
The construction of the new bore will eliminate the need to reverse the direction of the traffic at the middle bore, something that is done twice a day on weekdays and several times a day on weekends.
The new bore will have a 10-foot shoulder adjacent to the right lane, allowing disabled vehicles a safe area to wait for help and providing additional space for emergency vehicles. A 3-foot walkway will be located off the left lane.
State Route 24 passes through the Oakland Hills between Oakland and Orinda, carrying about 160,000 vehicles per day through the three bores of the Caldecott Tunnel. Four lanes approach the tunnel in each direction, but each bore only provides two lanes – meaning that one direction of traffic (eastbound or westbound) will be favored with four lanes, while the opposite direction must merge down to two lanes.
The center bore is reversible, meaning that the direction of traffic flow can be changed to eastbound or westbound with the assistance of pop-up delineators, which open or close lanes to traffic depending on whether the bore will accommodate eastbound or westbound traffic. Keeping traffic moving has become more challenging over the years as a result of increased off-peak traffic. Also, traffic has become notoriously unpredictable during weekends, and it is not uncommon for Caltrans to reverse the direction of traffic in the center bore a half dozen times or more to manage congestion on Saturdays or Sundays.
In order to perform the work as expeditiously as possible, tunneling will simultaneously occur from the Oakland and Orinda sides. Most of the excavation will be performed with roadheaders, machines that have a long, extendable arm and a barrel-shaped, high-velocity rotating cutting head with hardened steel teeth that grind into the rock. When rock that is too hard to grind is encountered, blasting may be used.
The tunnel will be excavated using the New Austrian Tunneling Method (NATM), a sequential excavation process in which the length of each excavated section is based on the surrounding geology. Rock at the advance face of the tunnel is graded for its competency to be self-supporting. Hard unfractured rock, for example, would allow work crews to tunnel for about seven feet before stopping. Broken shale, by contrast, would allow crews to dig for only two feet before becoming too unstable.
Once a section has been excavated, work crews will install the initial support by applying shotcrete to the freshly excavated walls and installing steel rods around the perimeter of the excavation. An arch-shaped steel lattice will be put into place and sprayed with shotcrete. After installation of the initial support along the entire length of the tunnel is completed, a waterproof liner will be placed on the tunnel walls and a final layer of concrete will be applied. Tiles will be installed on the lower portion of the tunnel walls
The new fourth bore will accommodate westbound traffic through an arch-shaped tunnel 3,348 feet in length and 41 feet in width at the base, with two 12-foot lanes, a 10-foot shoulder on the right and a 2-foot shoulder and 3-foot walkway on the left.
Seven cross passages will connect to the existing northern bore that will serve as an emergency escape route for both tunnel bores.The maintenance building above the west portal will be demolished and replaced with a new two-story operations and maintenance center.
Several retaining walls will be constructed near the east and west sides, and a sound wall will be built on the west side of the tunnel, parallel to Caldecott Lane. Upon completion of the project, the staging areas will be landscaped with a variety of plants, including trees, to replace those which had to be removed to facilitate construction.