By: Allen Zeyher
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.
Venting
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
