In November 2011, the Los Angeles Times reported that to halt a competition project at the University of Southern California, Conquest Student Housing filed a lawsuit against the developer Urban Partners using California’s landmark environmental law to protect endangered animals, then they sued the firm’s other projects too. The suit was withdrawn only after the developer filed a federal racketeering lawsuit. The case is a cool reminder of the tough environmental laws open to project opponents in the state of California.
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For the contractors behind the five-mile (8km) Bay Tunnel, the first bored tunnel beneath the bay, the strict environmental laws have been a regulatory headache – the alignment passes through environmentally sensitive salt ponds, marshlands and mudflats and the team had to be cautious choreographing the borings because if they get too close to the wildlife refuge area, the potential impacts to the species could require the project to undergo a costly environmental impact report. "We haven’t had any challenges with the alignment in the city, our biggest challenge is environmental," says Bob Mues, construction manager with US firm Jacobs Associates, the tunnel’s designer. "It’s something we have to consider with everything that we do, whether that is sinking the shaft, launching the TBM or our daily activities on the Ravenswood and Newark sites."
Shaft sinking and TBM launch
The team has just started to construct the retrieval shaft at Newark on the eastern end of the alignment, where there is a live railroad spur and a sewer main nearby. It is considerably more restrictive than the western end at Ravenswood; depths vary between 110ft (33.5m) on the western end where the TBM launched, and 86ft (26.m) at the eastern end with the Newark shaft. The diameter of the Newark shaft is 28ft (8.5m), whereas the diameter of the Ravenswood shaft is 58ft (17.7m).
The team is also facing environmental issues at the Newark site. "The retrieval shaft was designed as a slurry wall system and there are serious environmental restrictions on both sides of the bay. Given the size of the eastern site, we believe that freezing the shaft is less risky," says Jim Stevens, project manager for contracting joint venture Michels/Jay Dee/Coluccio.
The Hitachi Zosen EPBM was launched from the 58ft (17.7m) diameter shaft at the Ravenswood site on 15 August 2011 without complications and the project is still on target to be completed in 2015. Michels/Jay Dee/Coluccio is constructing the tunnel, 15ft (4.6m) in diameter, without any intermediate access shafts. In addition, the TBM has 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.
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By GlobalDataSan Francisco Public Utilities Commission (SFPUC) is undertaking its USD 4.6bn Water System Improvement Project to repair and replace the Hetch Hetchy Regional Water System, serving 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 involved decommissioning older pipelines and replacing them with the USD 313M Bay Tunnel. "We’re almost a mile in," says Stevens. "We’ve just started tunneling under the bay and we’ve passed our first curve, which was a 1,000ft (304m) radius curve that was 400ft (121m) long."
"The launch all went according to plan," Stevens adds. "We started off in the shaft with a big thrust frame, and temporary segments, and as the machine progressed we were able to eliminate this. The ground was found to be much better than anticipated, the material was very stiff, relatively impermeable clay. The 40ft (12m) long jet grout block was removed in exchange for alternative sealing mechanisms to enable successful launching of the TBM. "
Coping with mixed ground
A slurry machine had also been an option to handle the San Antonio formation, a silty clay with some sand. But toward the end of the alignment the tunnel is anticipated to encounter Francisco bedrock for roughly 700ft (213m) and the team felt an EPBM could better cope with the mixed ground conditions. When they hit rock the machine has the ability to be outfitted with a full set
of 10in disc cutters.
Originally, the team expected squeezing ground would be an issue, and the TBM has been designed with features to handle this including increased thrust and higher torque, and ports on the outside of the shield to pump bentonite, if needed. "The TBM functions are all useful for their purposes," Stevens says. "However, squeezing ground has not been a problem. We know the sand and gravel is there and prepared to deal with the messy situation it can create."
Several alignments had been looked at and it was really the environmental limitations on where boreholes could be placed that determined the final option; the team had to bear in mind the wildlife refuge areas. Geotechnical investigations included a series of marine-based borings across the San Francisco Bay, as well as landbased borings and cone penetrometer tests on both site locations. In addition to the conventional exploration, a geophysics program was performed along the majority of the proposed alignment corridor.
"At the beginning of the design phase, Jacobs Associates launched a geotechnical investigation program that included a total of 34 borings along the tunnel alignment," says Isabelle Pawlik, senior project engineer, Jacobs Associates. "Seventeen land borings are located on either side of the bay where the shafts are. We actually went out into the bay and drilled 17 marine bores to give us an overview of what to expect along the alignment. The marine borings are generally spaced about 1,000 – 1,500ft (152 – 213m) apart. So far the geology has been what we’ve expected."
Stevens says that the learning curve for using the TBM, and the initial performance, also went to plan. "Just like any project there is a learning curve. The team must learn how the machine operates and how to react to the different types of ground encountered — a very stiff clay and/or sand, gravel, and water," say Stevens. "But it went well and nothing happened that shouldn’t have happened."
A delayed delivery of the TBM has not slowed the project, thanks to a better than expected advance. The team is averaging around 75ft (22m) per day in comparison to the originally estimated 50ft (15m) per day during two 10-hour shifts. Much of the improved performance can be attributed to more favourable ground conditions than anticipated in design but Stevens says that this is also a reflection on the crew: "We haven’t made any tweaks to the machine to improve advance rates but we focus on safety and training and the team gets better together as they go."
Pawlik agrees: "There are three operating modes: the excavation, installation and grouting process, and everything has to work in sync. So the better the team synchronizes the work, the more production they will get. In the beginning, it was a little bit difficult due to the learning curve the team had to work through, but it keeps getting smoother and smoother as the work progresses."
Another measure to speed up the process was the decision to use a continuous conveyor belt for muck removal to avoid the ever-extending haul times and passing switches required with trains. There is a take-up unit on the surface, and sections of 1,000ft (305m) of belt can be added. A vertical conveyor is bringing spoil to the surface, which is transferred to a muck disposal pit before it is trucked off site. "It’s unusual," says Mues. "Not many contractors use that system for an EPB tunnel; it’s more commonly used outside of the US."
Stevens is currently running two 10- hour shifts five days a week, with maintenance performed on Saturdays. He also explains that maintenance is carried out during and between shifts. "We maintain it as it goes and we have a comfortable production," he says. The TBM is designed so we can check the cutterhead under pressure, however that has not been necessary as we’re in good clay ground and we’ve been able to go in at atmosphere. We’ve checked for wear on the cutterhead and cutters a few times and they are wearing extremely well."
Stevens adds that continually maintaining the machine is the key to the project’s success. "My philosophy is that you never want to push a machine until it breaks. It’s important to develop a comfortable cycle plan and keep that steady," Stevens says. "It’s like the race between the tortoise and the hare. This machine can go very fast, working like mad for a day or two and set all sorts of records, but then you’re down for three weeks repairing it so the high speed run didn’t really do any good. The idea is to have consistently good production."
On not-so shaky ground
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," says Pawlik. "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 analyses 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 (see Figure 1, far left, top). 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 and giving a 9ft (2.7m) internal diameter.
