The home stretch at last!

June 19, 2002

Bostonians celebrated Mother’s Day by taking a close look at the new Leonard P. Zakim Bunker Hill Bridge over the Charles River. The event’s planners expected maybe 40,000 people to attend the bridge open house; instead, they got 200,000, despite bad weather. People stood in line for three hours in a cold rain to walk across the new landmark on the Boston skyline.

Bostonians celebrated Mother’s Day by taking a close look at the new Leonard P. Zakim Bunker Hill Bridge over the Charles River. The event’s planners expected maybe 40,000 people to attend the bridge open house; instead, they got 200,000, despite bad weather. People stood in line for three hours in a cold rain to walk across the new landmark on the Boston skyline.

The bridge is named in honor of Leonard P. Zakim, the former chairman of the New England Anti-Defamation League, who died in 1999 at age 46. Zakim was a popular civil rights activist and influential in bringing together different communities within Boston.

The view from the bridge encompasses several of Boston’s neighborhoods. When the 10-lane bridge is officially in service, pedestrian traffic will not be allowed, so Mother’s Day was the only chance for citizens to take in the sights at less than traffic speeds, which the city is hoping will be faster than a leisurely walk.

At the other end of the $14.6 billion Central Artery/Tunnel Project (a.k.a. the “Big Dig”) that snakes under the heart of Boston, Slattery Skanska Inc. was putting the finishing touches on the tunnels at the interchange of I-93 and I-90.

Slattery Skanska, Whitestone, N.Y., led the joint venture of Slattery, Interbeton, J.F. White and Perini (SIWP) that was in charge of the tunnel jacking effort that was designed to carry I-90 under the Amtrak rail lines near South Station. From there, the jacked tunnel segments meet up with the immersed-tube tunnel segments that take the highway under Fort Point Channel, connect to another tunnel under South Boston and another immersed-tube tunnel, the Ted Williams tunnel, under the Inner Harbor and on to Logan Airport. (See Non-stop to Boston, R&B, December 2000, p 28, for a detailed description of the tunnel jacking.)

Buried in layers

The Big Dig is notorious for its swelling costs. It includes 162 lane miles of interstate highway, mostly underground, and seems to have started about a generation ago. The latest estimate is that it will cost $14.6 billion.

Any such gigantic undertaking that inconveniences an entire major city is bound to provoke the ire of many people. It also is bound to run into unanticipated obstacles. In the case of the Big Dig, the obstacles are literal as well as figurative.

SIWP’s tunnel jacking operation encountered 10-12 times as many underground obstructions as were expected in the original contract. “That hampered the entire jacking operation,” Alaeden Jlelaty, a project manager at Slattery Skanska, told Roads & Bridges.

The obstacles were manmade—archaeological remnants of previous generations of human construction—not natural geology. They included granite beds from old rail lines and wooden and steel support structures.

“They used to have an oil tank in there,” Jlelaty remarked, “some sort of reservoir tank with a forest of wooden piles in there.” The tank apparently was aboveground and was removed long ago. “When the tank was removed and tracks were laid out they left a forest of wood underneath.”

One of the reasons Slattery Skanska used the technique of soil freezing to stabilize the earth under the railroad tracks was that there were too many underground obstructions to do effective jet grouting. Stabilizing the soil was the only way the contractor could dig under the tracks without disrupting rail traffic.

As a result of the many obstacles to digging, it took SIWP a year to push the three tunnel boxes into place, twice as long as expected. The jacking part of the job was completed in December 2000. Because the joint venture added manpower, equipment and overtime, the overall contract stayed on schedule.

The obstructions did not hamper the contractor’s accuracy in grinding through the earth and shoving the concrete tunnel boxes into position using hydraulic jacks. The specifications allowed for a deviation of up to 6 in. in the vertical and horizontal location of the boxes. Slattery Skanska positioned them within 2 in. of their design location. Jlelaty commented, “That was a very comforting thing at the end of the job.”

The contractors are now installing tile on the inside of the tunnels all along the Central Artery/Tunnel, as well as lighting and the cameras, loop detectors and other equipment of the Integrated Project Control System to be employed by the Massachusetts Turnpike Authority (MTA).

The Big Flood

Interbeton Inc., the Rockland, Mass., contractor that worked on several sections of the Central Artery/Tunnel, did not have trouble digging through a forest of obstructions on its other contracts, but for those Interbeton was digging cut-and-cover tunnels rather than mining underground.

Perhaps the biggest dose of excitement for Interbeton came last fall when an adjacent contract sprung a leak and water from the Fort Point Channel started coming in. The channel runs roughly northeast between downtown Boston and the area known as South Boston.

Interbeton built a section of cut-and-cover tunnel in South Boston that picked up from the immersed-tube tunnel that crossed the bottom of the channel, built by Modern Continental Cos., Cambridge, Mass. The so-called I-90 extension carries traffic from the I-90/I-93 interchange under South Boston to the Ted Williams Tunnel under the Inner Harbor to Logan Airport.

“We’d just started working in what was called the casting basin where Modern Continental had built the immersed-tube tunnel boxes,” Geoff Collins, a vice president at Interbeton recalled for Roads & Bridges. “They’d floated the last boxes out, closed off the basin and handed it over to us to build tunnel in the basin” when the flood started.

“We had water from the channel coming in underneath—not through but underneath—one of the immersed-tunnel tubes,” Bob Bliss, a spokesman for the MTA, explained to Roads & Bridges.

The flood shut down construction work on the Fort Point Channel contract for a month or so, and at press time the connection of the jacked tunnel and the immersed-tube tunnel was still a couple of weeks away.

Collins also said there were cost increases on Interbeton’s contracts, because of differing site conditions, additional work and design changes. One design change was to add features to one section of the tunnel in South Boston to make it into a sort of dam so that future accidental flooding of the casting basin would be confined to the area around the casting basin.

The change involved building steel bulkheads in the tunnel bores where the traffic goes through, building a wall on top of the tunnel up to the level of the surrounding ground and filling up the side gaps between the tunnel and the excavation support system with a concrete wall.

Constant change

Other design changes that affected  Slattery Skanska’s contracts has included putting a different overlay on the pavement. The overlay originally specified was not performing well, so the Central Artery/Tunnel Project changed it.

“It is now much more suitable for the work,” said Jlelaty. “It is a much more durable product. It will last longer.” But of course it costs more to change the job partway through.

The owner also changed the waterproofing specification for the underground artery. The original specifi- cation called for the tunnels to be coated with polyurethane to prevent water from penetrating the concrete and corroding the rebar, according to Jlelaty, but the polyurethane became brittle and tended to break.

The new waterproofing system requires the tunnels to be either painted with a polyurea coating or wrapped in a polyurea membrane.

For the tunnels that Slattery Skanska jacked into place, either waterproofing method would be impractical. Those tunnel sections were cast with a concrete additive, called a direct corrosion inhibitor, that prevents the reinforcing steel inside from corroding and, according to Jlelaty, adds more than 50 years to the life of the structure.

Changing the waterproofing specification was a good decision, based on experience, but it added time and money to the job.

The original bid price for the Slattery Skanska joint venture contract that included the tunnel jacking was $397 million. In late May, with changes and the job basically done, the contract value was $430 million.


On the opposite end of the artistic spectrum from the elegant cable-stayed Leonard P. Zakim Bunker Hill Bridge are the massive ventilation buildings. The Big Dig has the world’s largest such ventilation system for its miles of underground roadway.

The Boston Globe reported on May 7 that some residents of Boston are not pleased with the appearance of the concrete monoliths, one of which rises 221 ft tall. Some of the ventilation buildings in downtown Boston are obscured by other buildings and so are not as obtrusive. Others are set in more open areas and inspired one member of a local artists organization to liken the stark concrete tower to a nuclear power plant.

“They are not finished,” Bliss pointed out. “The brick surface is not on them yet. I think people are reacting to the fact that these are large buildings, at least a couple of them are. And they’re in areas right now where there’s very little around them. And they are not finished. So, they don’t look particularly attractive.”

When the buildings are finished, they should look better, he said. And the buildings were not meant to stand alone: “Other structures are going to be built around them, especially in the South Boston area, which is a very hot area for development in the city now that the Central Artery is going to deliver the Mass Pike interchange to what otherwise had been a pretty hard to get to area.”

Ventilation Building 7, on the other hand, has won several architectural and design awards. The building, which was opened in 1995, sits in East Boston and ventilates the Logan Airport end of the Ted Williams Tunnel. The stacks of Vent Building 7 are capped in stainless steel, which is said to shimmer at sunrise and sunset, making the building more appealing.

The Globe reported that money was cut a few years ago for such stainless steel accents on some unfinished vent buildings.

The ventilation buildings not only pump stale air out of the tunnel and pump fresh air in, they also purify the air coming out. Even though the stale air is treated, it still is exhausted through stacks well above ground level where more people are walking and working.

Finishing landscaping

At press time, the Big Dig overall was 80% completed, MTA’s Bliss said. it was possible to walk through the tunnel the length of the project from the I-90/I-93 interchange north to the Charles River cable-stayed bridge. The follow-on contract work was under way installing the tile, lighting and Integrated Project Control System.

“We’ve been handing over bits of tunnel as we progressed for over a year now,” said Interbeton’s Collins, “and the follow-on contractors are doing that [finishing] work. As far as building tunnels are concerned, we are finished.” Interbeton finished its tunnel construction work in March of this year.

The final piece of the overall construction feat is to decide what to do with the strip of land that used to be taken up by the old Central Artery and will now be open space above the new Central Artery/Tunnel. It will not be an easy decision, according to Bliss.

“Like everything else in Boston, it’s a subject of considerable debate and controversy as to who should be in charge of it and who is going to pay for maintaining these public spaces/parks when they are finished.”

The MTA is moving ahead with gathering ideas from designers. And then there will be lots of public input on what the final design should be.

There also was some question about whether there was money left to pay for landscaping, but Bliss said the financing was settled.

“The financial situation is quite good. We now have an approved finance plan as a result of this agreement with the DOT inspector general. We’re working on already coming up with a new finance plan that will be sent to the Federal Highway at the end of August. Federal Highway and DOT IG actually said some very positive things about our finance plan, too, and said it should be used as a model for large construction projects of this type nationwide.”

The schedule is for the northbound lanes of I-93 to be open for traffic by the end of the year. All traffic will then be transferred to the northbound lanes. Before the southbound lanes can be opened, the old overhead artery structure must be dismantled and new ramps built. That will take another year to accomplish. After that is done, the permanent southbound I-93 lanes will be opened and Boston will be able to appreciate the full ramifications of the new artery.

The next chance Boston’s citizens will have to take in the view from the Zakim Bunker Hill Bridge will be from behind the wheel toward the end of this year as they emerge from the Big Dig tunnel driving north.

About The Author: Allen Zeyher is the Associate Editor for Roads & Bridges

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