July/August
2001
Learning
From the Big Dig
by Daniel C. Wood
It may
not look much like a school, but Boston's Central Artery/Tunnel (CA/T)
Project - the Big Dig - is providing plenty of lessons for transportation
planners and engineers from all over the world.
The sheer
scope of the project, of course, is enough to attract international
attention. Designed to replace 12 kilometers (7.5 miles) of aging
urban highway through the tangled heart of Boston, the Big Dig has
been compared in its extent and complexity to such landmark engineering
projects as the Panama Canal and the Chunnel.
Within
the United States, nothing like the CA/T Project has been attempted
before. It involves replacing the elevated Central Artery highway
(I-93) with an eight-to-10-lane expressway, building a 10-lane cable-stayed
bridge across the Charles River, extending the Massachusetts Turnpike
(I-90) to Logan Airport, and constructing four major highway interchanges.
And all this must be done while keeping traffic and commerce moving
through one of the nation's oldest and most historic cities.
Because
of the innovative and unprecedented nature of the Big Dig, there has
been broad interest in nearly every aspect of the project. To help
facilitate the transfer of CA/T Project technology, the Federal Highway
Administration (FHWA) has established the Innovations and Advancements
Program, designed to share knowledge gained from the Big Dig with
the national and international transportation communities. The program
focuses on specific categories of topics: items that represent a cost
or time savings, topics that exemplify superior quality, new and/or
innovative technology, and items that would prevent others from "reinventing
the wheel."
Discussed
below are some of the areas that are attracting the most interest
among transportation professionals who are eager to learn the lessons
being taught by Boston's Big Dig.
Tunnel
Jacking
Problem: Construct an underground roadway without disrupting traffic
on nine active railroad tracks - including commuter tracks that carry
150,000 people into and out of Boston every workday - right above
the roadway.
Solution:
Construct the tunnel adjacent to where you want it to go and shove
it into place using a technique known as tunnel jacking.
The CA/T
Project extends the Massachusetts Turnpike under the Fort Point Channel
into South Boston, where it meets the Ted Williams Tunnel. The turnpike
(I-90) goes underground where it crosses the Southeast Expressway
(I-93) at the South Bay interchange and passes beneath the tracks
carrying Amtrak and commuter trains into South Station, Boston's busiest
rail station.
To carry
out the tunnel-jacking operation, three concrete jacking pits were
dug alongside I-90 just east of I-93. Tunnel boxes 24 meters (80 feet)
wide and 12 meters (40 feet) high were built inside the pits. The
plan was to break the head ends of the concrete pits and push the
tunnel boxes into place with massive hydraulic jacks.
But the
Big Dig tunnel-jacking operation, the largest such operation ever
attempted, faced a special problem - the poor quality of the soil.
Pushing the tunnel boxes into place without stabilizing the soil could
cause the railroad tracks to settle, threatening train service. The
solution was to freeze the soil ahead of the tunnel boxes, using hundreds
of steel pipes that were driven into the ground between the tracks.
A brine mixture that stayed liquid below 0 degrees Celsius (32 degrees
Fahrenheit) was pumped into plastic pipes within the steel pipes by
a freezing plant located near the railroad tracks. The brine was circulated
back to the freezing plant and returned to the pipes again; the circulated
brine over a period of several weeks froze the ground outward from
the pipes.
The freezing
allowed the ground to be excavated without settling. (It also caused
the ground to expand, but allowances had already been made for this
movement, and the track operations were unaffected.) The frozen soil
ahead of the tunnel box was excavated by a machine called a road header.
The soil was chewed up by the machine's rotating grinder, removed
out of the back of the tunnel box, and carried to the surface by a
crane. The tunnel boxes were then pushed into place by two sets of
hydraulic jacks.
Slurry
Walls
Planners of the Big Dig promised the people of Boston that the mammoth
construction project could be accomplished without bringing the life
of the city to a halt. Traffic would continue to flow, they vowed,
and business would go on with little or no disruption. Slurry walls
have helped the builders of the Big Dig keep that promise, and in
many ways, these walls are the foundation of the CA/T Project. In
fact, the Big Dig represents the largest single use of the slurry-wall
technique in North America.
|
A
three-dimensional schematic of the final build shows (from
bottom to top) the new I-93 northbound lanes passing under
the existing red line subway tunnel, new silver line tunnel,
new station lobby, and restored surface street intersection. |
|
Slurry
walls, which are similar to drilled shafts, are concrete walls that
run from the surface of the ground down to bedrock, defining the area
to be excavated for underground highway construction. Their immediate
purpose is to keep construction trenches from collapsing while the
soil is being removed. They are also used on the CA/T Project to support
temporary traffic decking above the excavation. In the final stage,
the walls are incorporated into the permanent tunnel structure.
The slurry
in slurry wall construction is polymer or bentonite clay mixed with
water that is pumped into the excavation as the soil is removed. The
mixture is heavy enough to keep the trench walls intact before reinforcing
steel beams are lowered into the trenches and concrete is pumped in.
The concrete fills the holes, displacing the slurry.
Once
the slurry walls are in place - there will be almost 8,000 meters
(about 26,000 linear feet) of them in the CA/T Project - massive steel
beams are placed between them at ground level; concrete decking is
placed on the beams to support traffic and construction equipment
while excavation continues below. As the excavation proceeds, large
steel beams, known as struts, are installed between the slurry walls
to counter pressure from the ground and nearby buildings. When the
excavation reaches the proper depth, the new concrete roadbed is constructed.
The struts are removed, and the excavation is backfilled to the surface
once the tunnel is completed.
|
Active
railroad tracks are placed on top of a temporary steel curved
bridge over cut-and-cover tunnel construction. |
|
Immersed
Tubes
The Ted Williams Tunnel connects Logan Airport to South Boston. The
12 binocular-shaped steel tunnel sections, each longer than a football
field, were built in a Maryland shipyard and sent by barge to the
harbor. The tubes came to rest temporarily at Black Falcon Pier along
the South Boston waterfront, where they were outfitted with steel-reinforced
concrete walls and roadbed. Meanwhile, a huge dredging machine was
digging a 1.2-kilometer-long (three-quarter-mile-long) trench in the
harbor between the South Boston waterfront and the airport. When the
preliminary work on the tubes was completed, they were moved by barge
into the harbor, lowered into the trench, and connected. The tunnel
was then completed with tile, lighting, ceiling panels, emergency
systems, and other features.
But the
method of floating in the tubes couldn't be used for the tunnel under
the Fort Point Channel, a narrow extension of Boston Harbor into South
Boston that lies just east of the South Bay interchange. The four
existing bridges across the channel were too low for a tube to float
underneath. To solve the problem, project planners turned to a European
technique and decided to build a concrete immersed tube tunnel. The
six tubes were manufactured in a casting basin - a hole 305 meters
(1,000 feet) long, 91 meters (300 feet) wide, and 18 meters (60 feet)
deep. The basin, dug on the South Boston side of the channel, was
sealed off from the water by a series of steel cofferdams filled with
crushed stone. When the sections are completed, the basin will be
flooded and the tunnel boxes - the largest weighing 45,350 metric
tons (50,000 short tons) - will be floated out of the basin and put
in position to be lowered into a trench dug in the channel bottom.
Positioning the tubes must be done precisely (13 mm [1/2 inch] tolerance)
because they can't be moved once they're in place.
But exact
positioning isn't the only issue. The highway tunnel will pass just
a few feet above the Red Line subway tunnel, and to prevent any damage
to the subway tunnel, the new highway tunnel will be supported by
110 concrete shafts on both sides of the subway tunnel, drilled as
much as 44 meters (145 feet) into bedrock.
|
The
casting basin used to construct the concrete, immersed-tube
tunnels in the Fort Point Channel is separated from the
ocean by 18-meter- (60-feet-) diameter cofferdams. |
|
And as
a final challenge, the westernmost portion of the concrete immersed
tube tunnel will serve as the foundation for a ventilation building.
In both cases, these are first-of-their-kind engineering solutions.
Cable-Stayed
Bridge
In many ways, the 444-meter-long (1,457-foot-long) cable-stayed bridge
over the Charles River is the public face, the most visible element,
of the CA/T Project. With its thick pearl-colored cables swooping
dramatically from roadbed to twin concrete towers, the span - recently
named the Leonard B. Zakim Bunker Hill Bridge - looks more like a
piece of sculpture than a structure designed to carry thousands of
vehicles daily between Boston and Charlestown.
Its towers,
in the shape of inverted Y's, resemble the design of the nearby Bunker
Hill Monument. At 56 meters (183 feet), it is the widest cable-stayed
bridge in the world. It is also the first asymmetrical cable-stayed
bridge in the United States - two northbound lanes are cantilevered
outside the towers on the bridge's east side. And it is the first
hybrid cable-stayed bridge in the country that uses a steel center
span and concrete back span superstructures. Conceived by Swiss bridge
designer Christian Menn, the $86.4 million bridge, which replaces
an aging and unsightly double-decked six-lane span, is intended to
be a new northern gateway into the city of Boston.
|
Pipe
struts are used to hold open the massive excavations through
downtown Boston. |
|
The design
and engineering of the bridge needed to overcome a number of obstacles,
including physical constraints caused by the bridge's location. These
included an existing Orange Line subway ventilation building adjacent
to the south main pier; the Orange Line tunnel below the bridge site;
a steep 5-percent grade entering a tunnel at the south end of the
bridge, tying into a three-level interchange at the north end; a planned
north-south rail link under the location of the south tower; a water
main under the south tower; and the existing Charles River lock-and-dam
system abutting the bridge on the east side. The bridge also had to
meet the objectives of numerous state and federal agencies, including
FHWA.
Then,
there was the asymmetry inherent in Menn's concept. A committee of
international bridge experts agreed that only a hybrid - concrete
and steel - structure made sense. Because the back span is short in
comparison to the main span, it would be made of relatively heavy
cast-in-place concrete to balance the lighter main span, which has
steel floor beams and edge girders and a precast concrete deck. The
shape of the tower piers and the arrangement of the cables also were
driven largely by technical considerations although aesthetics played
a significant part. At the south end of the new bridge, the double-deck
ramps of the existing I-93 bridge had to remain in service during
construction, and that meant that the back span cables had to be anchored
in the median of the bridge and not splayed out, as were the cables
for the main span.
"What
was gratifying to everyone was that they were able to come up with
something that could satisfy all the interested parties, provide a
transportation system that would satisfy traffic requirements, and
also fit in the tight constraints of the site," said Larry O'Donnell,
the FHWA Massachusetts Division bridge engineer.
"And
at the same time, they're getting a spectacular landmark structure
for the Boston area that we'll continue to see in photos, on postcards,
and in magazines for years to come," O'Donnell added.
Environmental
Mitigation
For the
CA/T Project, mitigation has a fairly broad meaning. It refers to
ways that planners have found to manage the impact of the project.
The program can be divided into the categories of traffic, community
outreach, and the environment. The environmental record of the Big
Dig is a particularly notable success story.
|
The
completed Ted Williams Tunnel with steel, immersed-tube
tunnel construction, was opened to traffic on Dec. 15, 1995.
|
|
Environmental
planning for the Big Dig began in 1982, eight years before construction
began. Thousands of federal, state, and local environmental permits,
licenses, and approvals were required for the project, and environmental
reviews have continued throughout the course of construction. But
the innovative ways that planners have found to mitigate the environmental
effects of the Big Dig will continue to benefit the Boston area for
decades after the project is completed.
The Big
Dig certainly figures to be good news for the local shellfish population
because of the construction of an artificial reef in Boston Harbor's
Sculpin Ledge Channel between Spectacle Island and Long Island. Created
in collaboration with the National Marine Fisheries Service and the
U.S. Army Corps of Engineers, the reef is designed to compensate for
filling in 0.65 hectares (1.6 acres) of blue mussel habitat in the
harbor during the closing and capping of the former municipal landfill
on Spectacle Island. As the northernmost artificial reef system in
the United States, the complex is expected to become home to lobsters,
crabs, and finfish, as well as the displaced blue mussels.
The reef
consists of 17 terrace-type modules, each 6 meters (20 feet) square
with five layers of panels and six cobble/boulder patch reefs, each
of which is 20 meters (66 feet) long by 10 meters (33 feet) wide.
The modules were placed at a sufficient depth to allow boats to pass
over them, and the locations were approved by the U.S. Coast Guard
so that they would not be a hazard to navigation. The reefs provide
8,175 square meters (88,000 square feet) of surface area for marine
habitat and cover 1,208 square meters (13,000 square feet) of harbor
bottom.
|
The
tunnel contractor used a specially designed scissor lift
truck to transport the ceiling panel modules from off site
and to install them in the tunnel to form the exhaust plenum. |
|
Spectacle
Island, mentioned above, has also been a beneficiary of the CA/T Project's
environmental mitigation program. Before the project began, Spectacle
Island was little more than a mountain of decaying garbage, much of
which was leaking into Boston Harbor. CA/T Project planners, working
with local and state officials, came up with a plan that would benefit
the project by providing a cost-effective place to put 3.47 million
cubic meters (3.8 million cubic yards) of excavated material and help
enhance the city by creating a new island park.
When
completed, the 40-hectare (100-acre) public park will feature a dock
for public ferry and recreational boats, beaches, picnic areas, a
trail system, recreational areas, and a visitors' center. These improvements
will once again attract city residents to an island that once served
as a fishing ground for Native American tribes and as the site of
resort hotels.
Excavated
material from the Big Dig is also benefiting the city of Quincy, five
miles (eight kilometers) south of Boston, where three adjacent landfills
are being transformed into a major recreational complex that includes
two golf courses, four baseball fields, and two soccer fields. Big
Dig materials are also being used to fill Quincy's Swingles Quarry,
a 122-meter-deep (400-foot-deep) abandoned quarry pit that has been
a public safety hazard for more than 25 years. Other cities in Massachusetts,
Rhode Island, and Connecticut have also been assisted in efforts to
close and redevelop landfills by receiving clay and other excavated
materials from the Big Dig.
Also
benefiting from the Big Dig mitigation program is Rumney Marsh in
Revere, Mass., a wetland habitat that was partly filled in during
the 1960s as part of a later-abandoned plan to extend I-95 through
several North Shore communities. About 229,500 cubic meters (300,000
cubic yards) of sand were removed to restore the 7.3-hectare (18-acre)
marsh; the sand was used on the CA/T and other construction projects.
The newly restored marsh is already being colonized by salt-marsh
vegetation and by various species of migratory birds, fish, and shellfish.
Partnering
Program
Although the Big Dig is called a project, it's actually 90 different
construction projects. In many cases, contractors working on different
projects work within inches of each other. This degree of complexity
creates an enormous potential for contract, schedule, and cost problems.
Managers of the CA/T Project have chosen to deal with these issues
through a partnering program designed to increase communication and
help prevent conflict and misunderstanding.
Partnering
simply means bringing key parties together during a project to discuss
plans and clarify issues. It can either be informal, occurring during
regular project management meetings, or more formal with an outside
facilitator and a definite agenda. The CA/T Project has used both
approaches during the design and construction phase of the CA/T Project.
Partnering
produces numerous benefits. It builds nonadversarial working relationships
through continuous open communication, provides the opportunity to
identify and confront problems before they grow into full-blown disputes,
encourages innovation, and develops a common vision of the project
that can be shared by all participants.
In a
sense, partnering is a form of risk management - one that has been
shown to have real bottom-line benefits. Studies that compared partnered
and nonpartnered projects managed by the U.S. Army Corps of Engineers
and the U.S. Navy showed that partnering reduced the cost of contractor
claims, increased the number of value-engineering savings proposals,
and helped keep projects on schedule.
The Big
Dig has used partnering to an unprecedented extent, and although it
would be difficult to precisely quantify the benefits of the process,
many experts have concluded that because of its scope and complexity,
the CA/T Project would have been simply unmanageable without partnering.
The CA/T
Project has not been without controversy, and it has caused some inconvenience
for Boston's residents and visitors. But the benefits of the completed
project will be enormous. An aging, unsightly elevated highway will
be dismantled, opening up 11 hectares (27 acres) of land. A daily
traffic nightmare will be replaced by a normal urban rush hour. Even
air quality will be improved; carbon monoxide levels are expected
to drop 12 percent citywide because traffic will be kept moving.
The project's
benefits, however, extend far beyond the city of Boston. The Innovations
and Advancements Program, highlighting the topics described in this
article as well as other aspects of the CA/T Project, is helping planners
and urban officials in the United States and throughout the world
develop better and more efficient transportation solutions. That's
a big benefit from a really Big Dig.
Daniel
C. Wood is the CA/T Project structural engineer within FHWA's
Massachusetts Division Office. He has worked on the Big Dig since
October 1995 and serves as the coordinator for the CA/T Project Innovations
and Advancements Program. Wood joined FHWA in 1988, and his career
has included assignments in Arizona, Colorado, Illinois, and Pennsylvania.
He has a bachelor's degree in civil engineering from the University
of Wisconsin - Platteville and a master's degree in structures from
Pennsylvania State University. Wood is a registered professional engineer
in Wisconsin.
For
more information about the Big Dig and the Innovations and Advancements
Program, visit the CA/T Project Web site www.bigdig.com.
Also, see "Big Bridge, Little Bridge: The Big Dig Soars Across the
Charles River" in the September/October 1999 issue of Public
Roads. This article is available on the Internet in the Public
Roads archive section of the Turner-Fairbank Highway Research
Center Web site (www.tfhrc.gov).
Other
Articles in this Issue:
HELP
WANTED - Meeting the Need for Tomorrow's Transportation Work Force
The
Dwight David Eisenhower Transportation Fellowship Program: Preparing
for the Future of Transportation
The
Millennium Manual Matters
QuickZone
Iowa's
Approach to Environmental Stewardship
Moveable
Barrier Solves Work-Zone Dilemma
Learning
From the Big Dig
A
Light at the End of the Tunnel
International
Cooperation to Prevent Collisions at Intersections
Pay
Attention - Buckle Up: Safe Driving Is a Full-Time Job
A
Light at the End of the Tunnel
International
Cooperation to Prevent Collisions at Intersections
Pay
Attention - Buckle Up: Safe Driving Is a Full-Time Job