Safety of Big Dig tunnels in Boston

After inspecting the leaks, United States highway authority confirms the safety of Boston's Big Dig tunnels.

Bechtel/Parsons Brinckerhoff, construction manager for the $15 billion Boston Central Artery/Tunnel project, has been vindicated by the United States Federal Highway Administration in its claim last November that despite leaks occurring while the tunnels were still under construction, the 'Big Dig' was safe and sound.

At the time the joint venture described news stories in the Boston Globe and Mail as vastly exaggerating the impact and significance of the tunnel leaks and accused a number of figures in the Massachusetts State administration of rushing to judgment.

Now the B/PB management welcomes what it regards as an objective and fact-based review of its performance as it works with the turnpike authority and the contractors to finish the project.

What the FHWA report basically says is that the assessment team are confident that the program to address leaks is on target and that it should be continued.

The Highway Administration's tunnel leak assessment team, known as the A-team, said there were two types of leak in the Boston tunnels: slurry wall panel leaks such as occurred on 2004-09-15 when one of the wall panels was breached resulting in water flowing into the tunnel, initially at rates estimated to be as high as 300 gallons per minute; and roof/wall interface leaks of lesser magnitude, ranging from dampness to dripping but occurring at a greater number of locations.

Of the panel where the big outflow happened, generating alarm about the hydraulic security of the tunnel works, the A-team report said that it contained defects and inclusions caused by poor quality control and quality assurance during slurry wall construction.

Every one of these panels is being inspected project-wide: at the date on which the FHWA report was finalised, some 60 per cent of them had been examined with the result that one additional panel had been identified as in need of major repair similar to the one that failed in September last year.

The repair scheme adopted in that case was likely, the team thought, to serve as the model for repair of the second defective panel.

The consultant firm MRCE which was closely involved in design work for the slurry walls proposed that the defective panels should be reconstructed to provide a repair that most closely resembled what would have been in place had the original work not been faulty.

The contractor's solution was to inject concrete grout behind the panels, then attach a steel plate over the interior wall face to seal the opening; to provide added strength to support any forces that might be imposed upon the wall, a 10 in.

structural concrete slab could be secured over the steel plate.

The A-team agreed that any one of these proposals would provide an adequate repair, subject to clear demonstration that it would prove structurally sound and provide a durable and effective barrier against moisture entering the tunnel.

It should also, they insisted, be maintenance free, not interfere with other tunnel systems nor detract from the tunnel's finished appearance.

Whatever system is used - and the team did not regard the proposals outlined above as exclusive of other ideas - the essential factors that should be adequately addressed prior to commencing repairs include safety, potential for adverse impacts on abutting property and traffic disruption.

The Highway Administration's Massachusetts Division will approve repair proposals "only after being convinced that the repair is adequate and does not pose unacceptable consequences during construction." Roof/wall interface leaks The more numerous roof/wall interface leaks are referred to as 'chronic low level leaks', which the report accepts are bound to happen to some degree because the tunnel is being constructed significantly below the water table.

Even though the tunnel design incorporates a waterproofing membrane applied to the roofing slab, and 12 in.up the slurry wall, water is still finding its way to and through the roof and slurry wall connections, whereas the specifications require that the system be watertight.

Only dampness is to be tolerated.

As the report points out, an inventory maintained by the project originally identified more than 720 locations for this type of leak for the entire Central Artery Tunnel, and this is being used as a base to monitor the sealing work.

"The number of leaks changes on a daily basis as leak sealing efforts progress; the inventory is updated accordingly".

"The Team is comfortable with the project's methodical approach to sealing all leaks and their effort to seal all leaks by the end of September 2005, and will continue to monitor the work." As to its closing remarks, the A-Team said it believed that the project is addressing the tunnel leaks appropriately.

The slurry wall breach of 2004-09-15, they said, appears to be isolated to a discrete section of the tunnel and was primarily the result of poor quality control during construction.

"The project has successfully installed an interim repair and is actively designing the permanent fix while completing the investigation of all suspect slurry wall panels".

"The A-Team found that the project is continuing repair of all the chronic low level leaks and is committed to completing all repairs before the project completion date to meet specifications for the tunnel".

"The Team will continue to monitor the work." As Bechtel/Parsons Brinckerhoff has previously pointed out to those concerned about the cost of these repairs, the project will hold contractors accountable for the quality of their work and for grouting and sealing leaks at their expense.

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