The 41-ft. (12.5 m) wide and 3,389-ft. (1,033 m) long tunnel will be constructed by a roadheader using the sequential excavation method (SEM), also known as the New Austrian Tunneling Method (NATM).
The Berkeley Hills, separating California’s Alameda and Contra Costa counties, have always presented a challenge in terms of transportation. In the early 1900s, travelers braved the Broadway Tunnel, built with timber supports 320 ft. (97.5 m) below the summit. At 1,040 ft. (317 m) long and 17 ft. (5.2 m) wide, the tunnel was only wide enough for one-way traffic.
The Broadway Tunnel was eventually replaced by a two-bore tunnel, a.k.a. the Caldecott Tunnel.
Opened in 1937, the twin bores are still in use today, one for each direction of traffic traveling along state Route 24 between Orinda and Oakland. They are augmented by a third bore, completed in 1964, with “pop-up” lane control that makes it possible to reverse the traffic flow of the middle bore depending on commute conditions.
Over the years, off-peak traffic has increased, making it difficult to keep vehicles moving. Weekend traffic, in particular, has become so unpredictable that it’s not uncommon on Saturdays and Sundays for the direction of traffic in the center bore to be reversed a half dozen times or more.
A fourth bore has been in the works for years; California’s department of transportation, Caltrans, said that the first public meeting on the topic took place in 1992. But until recently, funding problems have repeatedly stalled construction of a fourth bore.
Finally, in January, the Caldecott Tunnel Fourth Bore Project broke ground due to nearly $200 million of American Recovery and Reinvestment Act federal funding — the largest single-project ARRA investment to-date.
The entire project, from scoping through construction, will cost about $420 million and take four years to complete. It is a partnership between Caltrans and Contra Costa and Alameda agencies.
The construction contract went to Tutor-Saliba Corporation, a California company with a 50-year history in construction and engineering jobs. The project’s first steps, which consist of tree removal and construction of a temporary soundwall 1,000 ft. (305 m) long and 35 ft. (10.7 m) high, are now underway.
Trees were removed primarily by chainsaw, instead of the more typical method of knocking them down with excavators; the loosening effect excavators’ wheels can have on soil was deemed a risk to storm water restriction compliance.
Tunnel excavation is scheduled to begin this summer. At present, the complete details of the excavation process are undetermined. The contractor has submitted options for review to Gall Zeidler Consultants LLC, a Virginia-based company specializing in geotechnics, tunnel design, and engineering that is supporting Caltrans on the project’s technical aspects.
What is known at this point is that the 41-ft. (12.5 m) wide and 3,389-ft. (1,033 m) long tunnel will be constructed by a roadheader using the sequential excavation method (SEM), also known as the New Austrian Tunneling Method (NATM).
“The major piece of equipment, the thing that everybody will see, is the roadheader,” said Axel Nitschke, a senior tunnel engineer with Gall Zeidler Consultants who is acting as NATM engineer for the Caldecott Tunnel project.
The 120-ton (108.9 t) roadheader will be constructed in Germany by Aker Wirth, said Nitschke; the roadheader type is T3.20, which is specifically designed for tunneling in medium to hard rock. The machines in the T3 series have 405 hp (300 kW) cutting power and are able to excavate tunnel sections up to 8.4 yds. (7.69 m) high and 9.8 yds. (8.96 m) wide from a single central position.
Cost was a major factor in the decision to go with a boom-type machine, as opposed to a Tunnel Boring Machine.
“If you have a tunnel boring machine, that is a huge, custom-made investment,” said Nitschke. (Although roadheaders aren’t manufactured in great numbers, neither are they typically built to order like TBMs are.)
Nitschke said that tunnel engineers generally don’t even consider using a TBM for a tunnel that isn’t considerably longer than .6 mi. (1 km); the cost of investment rarely will be recouped during the construction of a shorter tunnel.
In addition, a roadheader is more adaptable and thus better suited to a job encompassing a variety of rock types, of which the Fourth Bore project is an example. In comparison, the cutter head of a TBM is very difficult to change once the machine has been manufactured.
“If you have very changeable geology… you can run into trouble with a TBM because you can’t change anything,” said Nitschke.
One of the advantages of SEM is the ability to adjust the excavation process depending on the conditions of each tunnel section. This is possible because each portion of tunnel is excavated and supported, one at a time, before moving on to the next. Holes will be drilled in each upcoming section so engineers “can get an idea of what the ground will look like,” said Nitschke.
In some sections, the roadheader’s cutting head will be able to grind and excavate the rock; in other sections, controlled blasting may be required.
Tunnel support includes rock bolts installed in a radial pattern around the perimeter of the tunnel, lattice girder composed of steel rods bent into an arch-shaped structure, and shotcrete sprayed into place on the roof and sidewalls.
Nitschke pointed out that successfully working with shotcrete in a tunneling situation is a fine art. Not only does the concrete need to be properly mixed to achieve the necessary strength and drying time, but it also must be applied expertly using specialized equipment that holds and manipulates the spray nozzle.
Nitschke said, “[Applying shotcrete] requires a lot of skill and experience; not everybody can do it… Even with high-quality material, if you have a bad nozzle man he can screw up the shotcrete.”
Months of preparation lie ahead before it is time for shotcrete specialists to prove their skills in the Caldecott Tunnel. In the meantime, crews continue to build sound and retaining walls while the contractor, project owners, and consultants work together to decide which construction methods and equipment will be used later.