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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation

Public Roads - 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 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 (