There are 280 different species of migratory birds in the Don Edwards San Francisco Bay National Wildlife Refuge. Just a stone’s throw away is the main work site for the five-mile (8km) Bay Tunnel; its perimeter outlined by special fencing that features a one-way valve, much like a ‘doggy door’ or ‘cat flap’ to allow the elusive Salt Marsh Harvest mouse to leave but not return.

“One of the challenges during design was choreographing the borings because if you get too close to the wildlife refuge area, the impacts to the species could lead us to a minor EIR, or what we call a mitigated negative declaration,” explains Johanna Wong, project manager for the Bay Tunnel and regional project manager for all Bay Division regional projects for the San Francisco Public Utilities Commission (SFPUC). And if that were to happen, she says, the environmental assessment alone could take a whole year.

The alignment does go around the refuge, but will pass through environmentally sensitive salt ponds, marshlands and mudflats. Once the project is completed, in 2015, Bay area residents can be reassured in the event of an earthquake, they will have potable water—and that biological resources have been protected.

SFPUC is undertaking its USD 4.6bn Water System Improvement Project to repair and replace the Hetch Hetchy Regional Water System, which serves 2.5 million people. This program focuses specifically on seismic upgrades to ensure high-quality drinking water after an earthquake or during a drought. One element of this involves decommissioning older pipelines where they cross the San Francisco Bay and replacing them with the USD 313M Bay Tunnel.

It’s only a matter of months before the TBM will be launched to excavate the first tunnel under the San Francisco Bay—the Transbay Tube built in the late 1960s was immersed. A joint venture of Michels/Jay Dee/Coluccio must construct the 5-mile tunnel, 15ft (4.6m) in diameter, without any intermediate access shafts.

Late summer launch
Notice to proceed was issued to the contractor on 1 April 2010 and set-up on the 12 acre Ravenswood site is moving along quickly, including the construction of a temporary electrical substation that will power the Hitachi Zosen earth pressure balance machine. Unfortunately with the March earthquake in Japan, shipping has been delayed by two weeks.

Members of the project team had been in Osaka, Japan, to inspect the fully-assembled machine when the earthquake struck. Everyone returned safely to California, and the TBM passed inspection. Delivery is expected in late May following two weeks of travel time and two weeks in customs. Despite the delay, the delivery still fits within the project’s baseline and Jim Stevens, project manager for contractor Michels/Jay Dee/Coluccio, plans to launch the TBM in August.

An EPBM had been recommended to the contractor, though a slurry machine had been an option as well, to deal with the San Antonio formation, a silty clay with some sand. Toward the end of the alignment the tunnel is anticipated to encounter Francisco bedrock for roughly 700ft (213m), and for this the machine has the ability to be outfitted with a full set of 10in disc cutters.

Depths vary between 110ft (33.5m) on the western end of the alignment at Ravenswood where the TBM will start, and 86ft (26.2m) to the east at the second shaft (Figure 1). The tunnel will cross under two existing pipelines built in the 1920s and ‘30s. There is also an abandoned railroad near the Ravenswood shaft and a live railroad spur and a sewer main near the other shaft in Newark.

“We cross at a very far depth,” says Isabelle Pawlik, senior project engineer, Jacobs Associates, the tunnel’s designer. “We’ve done some extensive analysis and we believe they will handle it well, as long as we monitor the earth pressure.”

Several alignments had been looked at and it was really the limitations on where borings could be done that determined the final option. Although Wong notes, “Even though this railroad is abandoned, there has always been a plan to reconnect that bridge as commuter rail. So we had to cross inland, and not where they’re going to potentially be putting in deep piles.”

Another notable challenge of the project is a gap in the borings along the alignment in the salt ponds. “They’ve got a mile or more of material out there we don’t really know what it is. We think it’s clay, but there is nothing to prove it right now,” Stevens says. “We expect it to be the same, but in underground work you don’t know.”

One thing that is for certain is seismic activity. The tunnel is between the two most active fault zones in the Bay area. To the west is the San Andreas fault, and to the east is the Hayward fault.

Lining the tunnel
Subcontractor Bencor Corporation completed the 3ft (0.9m) thick diaphragm walls for the 58ft (17.7m) diameter Ravenswood shaft and the 28ft (8.5m) diameter Newark shaft. At the time of T&TI’s visit to the Ravenswood site, the shaft had been excavated and dewatered—with the exception of sludge at the bottom, which was being mucked out with a crane and a skip box—and would be fully cleaned out just days later.

Because the tunnel doesn’t actually cross a fault, along with the shafts, it has only been designed for shaking. “We basically had seismic parameters that were considered for the design. The maximum was 7.5 on San Andreas and 7.1 on the Hayward fault,” explains Pawlik.

She adds, “the tunnel itself is not the problem. We designed the lining for internal/external pressures, as well as seismic stresses. But the key problem areas were the connection points to the, very rigid, shafts. We ran some three-dimensional analysis on these portions and we actually made adjustments to the design. We increased the steel strength of the pipe, and we encased these portions in higher strength concrete.”

The Bay Tunnel will have a two-pass lining, with an initial segmental lining of sealed and gasketed concrete segments (Figure 2). The six-piece rings are being fabricated roughly 80 miles (129km) away by Traylor Shea Precast’s plant in Stockton, California. Segments are 5ft (1.5m) long and reinforced with steel fibers. The secondary lining is the steel pipe required by the SFPUC. The tunnel will be backfilled with cellular concrete, the pipe installed and a 5/8in (127/203mm) cement mortar lining applied for corrosion protection—for a 9ft (2.7m) internal diameter.

Squeezing ground is an issue, and the TBM has features designed to handle this, including increased thrust, higher torque and ports on the outside of the boy to pump bentonite, if needed, explains Bob Mues, construction manager with US firm Jacobs Associates.

The TBM has also been designed to take two rings of the segmental lining at one time because of the project’s limited access. All work must be done out of the Ravenswood shaft.

“The whole machine is processed and designed around the ability to deliver two rings at a time to the heading,” Mues says. “That travel time is an issue on a five-mile tunnel. When you get out there over two and half miles, haul time is important and there are some limits to the speed you can haul in a tunnel.”

Limited by length
A rail system will be set up for the whole length of the tunnel, strictly as a people mover. Michels/Jay Dee/Coluccio will be using electric-powered locomotives rather than diesel to more easily meet ventilation requirements of California, as well as saving money on fuel costs.

One measure to save time is the contractor’s decision to use a continuous conveyor belt for muck removal to avoid the ever-extending haul times and passing switches required with trains. There will be a take-up unit on the surface, and sections of 1,000ft (305m) of belt can be added. A vertical conveyor will bring spoil to the surface, which will be transferred to a muck disposal pit before it is trucked off site.

Trucking is restricted from 19:00 to 7:00, to comply with noise restraints, as there is a residential area on the other side of the site. Once the TBM is installed and excavating, Stevens will run two 10-hour shifts, five days a week with maintenance carried out on Saturdays.

One concern once excavation starts is transporting grout to the face as it moves further away from the shaft. “It’s really difficult to pump over five miles, the power you need,” Pawlik explains, “and to control the set time so it doesn’t set early.”

The contractor is anticipating building its own grout plant on site, and pumping down the shaft and into the tunnel. The line will be continually charged with grout to avoid waiting, and cut down on traffic in and out of the tunnel. An admixture would be injected at the nozzle to make the grout set, so it stays just fluid enough to be pumped, but hardens with the segments.

In late March, the site is bustling with activity as concrete slabs are poured for the grout plant, fan and gantry crane, among other equipment. The temporary electrical substation that will interconnect to a 115,000 volt transmission line is ready to be fired up. The TBM will take 12,800 volts once it’s installed.

Pending its late May delivery, June and July will see assembly of the machine. With more than 700ft (213m) of trailing gear, the installation will go in stages once the machine starts mining. “We’re going to go the length of the gantries,” says Stevens. “We’ll go a long ways before we put it all in, probably about 1,000ft (305m).”

He estimates it will take one and a half to two years to mine the tunnel. The current schedule is based on an excavation rate based on 80ft (24m) per day, although during the design process it was estimated to be around 50ft (15m). If ground conditions are good, a rate of 120ft (36.6m) per day is possible, says Mues. “If it is as anticipated from the geology, it could be 50ft a day.”

The existing pipelines crossing the San Francisco Bay Figure 2, the Bay Tunnel will have a two-pass lining Figure 1, land and marine borings along the tunnel alignment and shafts An excavator hauls sludge from the Ravenwood shaft The conveyor belt can be extended by adding the 305m long rolls