The Interstate 90 connector tunnel needs hundreds of diagonal steel hangers to support its ceilings in case of earthquakes -- the third major safety flaw in the tunnel that will require extensive repairs, Governor Mitt Romney said yesterday.
There are some diagonal hangers in place that, along with vertical hangers, help stabilize the suspended ceilings in the I-90 connector. But state engineers concluded within the past two weeks that there are not enough for the 3-ton concrete ceiling panels to withstand the swaying an earthquake and aftershocks would produce, increasing the possibility of a collapse, Romney said.
The diagonal hangers will be installed throughout the I-90 connector tunnels and ramps. The Ted Williams Tunnel will not require them because of the unique construction of its suspended ceiling. The other Big Dig tunnels do not have suspended ceilings.
State transportation officials took the installation of the hangers into account when they acknowledged Wednesday that it would take several months to reopen the I-90 connector tunnel, a spokesman said last night.
Romney declined yesterday to give an opening date or an estimate. ``I won't make predictions," he said in a telephone interview with the Globe.
But Romney said project engineers have developed a plan to reopen fixed sections of the tunnels and entrance and exit ramps, while repairs continue nearby, by using shoring towers to hold up the ceilings.
The governor said a single lane in the eastbound I-90 connector tunnel would open to traffic soon, allowing eastbound traffic on the Massachusetts Turnpike access to the Ted Williams Tunnel for the first time since the July 10 fatal ceiling collapse in the I-90 connector.
Romney said the connector lane could open by this weekend, as soon as the state gets approval from federal officials.
When the connector lane opens, drivers will go into the tunnel and then on to an exit ramp in South Boston. Drivers will then take surface streets for about 100 feet before re-entering the tunnel at the recently opened Ramp A, and can head to Logan International Airport via the Ted Williams Tunnel. Romney said he plans to announce the opening today.
Though temblors are rare, the immediate Boston area has had serious earthquakes. New England experiences earthquakes regularly, though of low magnitude, according to the Weston Observatory of Boston College, which monitors and researches seismic activity in the region.
A magnitude-1.7 earthquake was recorded about 8 miles northwest of Concord, N.H., on Aug. 20, while a 3.2-magnitude temblor was recorded 37 miles northwest of Presque Isle, Maine, on July 14. Closer to Boston, a 1.3-magnitude quake was recorded in Wakefield in January.
Modern building codes require structures such as bridges and tunnels to be reinforced against the side-to-side movement an earthquake generates.
Because the Big Dig tunnels were constructed in fill dirt and less stable soil, they could be more vulnerable to tremors.
In the I-90 connector tunnel, project managers at Bechtel/Parsons Brinckerhoff called for a concrete ceiling in part because it would resist seismic shifting better than a lighter ceiling. For further reinforcement, ironworkers embedded steel crossbeams in the concrete ceiling when they installed it in 1999 and 2000. Following the design for the tunnel ceiling, workers also installed diagonal hangers to help support the suspended ceiling. Each module of five concrete ceiling panels has diagonal hangers.
But state engineers say that design failed to meet the project's safety standards. The hangers are spaced too far apart and more are needed, they say.
In Ramp D, which leads from the Ted Williams Tunnel to Interstate 93, some 200 more diagonal hangers need to be installed, said Jon Carlisle, spokesman for the Executive Office of Transportation.
Carlisle said last night that he did not know how many hangers are in place or how many additional hangers need to be installed elsewhere.
The seismic bracing could also be necessary for the heavy ventilation fans that line the tunnels, Romney said.
The installation of the diagonal hangers is the third major set of repairs since the ceiling collapse.
Crews are replacing or reinforcing the epoxy-and-bolt system suspected of failing in the collapse, and in the next few days, the state hopes to begin replacing 3,300 weak steel brackets in the I-90 connector and Ted Williams tunnels.
``It's a far more extensive repair process than was originally anticipated," Romney said. ``It's something that gives us concern about the entire project. This is one thing that finally outside eyes have come in and taken a look at, and it has three major failures."
``You can only hope that we're not going to continue to find failure after failure in design, engineering, installation, construction, and oversight," he said.
Because the repairs will last past Labor Day, when Boston's traffic is expected to increase substantially, state officials on Wednesday unveiled what they called a comprehensive transportation plan, relying largely on getting more commuters to use public transit.
But state transportation officials are also starting a new strategy to reopen portions of the tunnels as soon as they are repaired, even as work continues on other parts of the tunnels.
For the planned opening of the I-90 eastbound connector lane, Romney said a series of 118 shoring towers would be installed to protect against a ceiling collapse above the open lane of traffic, while giving workers access to the ceiling for repairs. As of Wednesday night, 74 of the towers had been installed, with the rest to be completed last night, he said.
``We want to be able to work day and night at the same time as having traffic run through the system," Romney said.
Romney said opening Ramp D would be more difficult and could require some creative solutions, including the possibility of traffic jumping into the adjacent westbound I-90 connector tunnel so the ramp can be partially opened and still be accessible to workers.
``When the only problem in Ramp D was bolts, we thought that could be open pretty quickly," Romney said. ``Now that it's also brackets and seismic, Ramp D has some real problems."
Scott Allen of the Globe staff and Globe correspondent Russell Nichols contributed to this report.