Boston’s Central Artery/Tunnel (CA/T) Project (The “Big Dig”) the most complex and technologically challenging highway project in U.S. history, can now see light at the end of the tunnel.
The $14-billion largely underground project is entering the final stages of construction, it is 71 percent complete and nearing the end after 15 years of fumes, dust, sweat and tears.
Overcoming a sea of troubles, the project represents an amazing achievement involving thousands of pieces of construction equipment.
The city’s six-lane elevated Central Artery Highway (I-93) is being replaced by an eight-to-10 lane underground expressway which has burrowed beneath it without disturbing the roadway, historic buildings, subways or trains.
The Massachusetts Turnpike (I-90), meanwhile, is being extended from its present terminus south of downtown Boston, through a tunnel beneath South Boston and Boston Harbor to Logan Airport.
The result will be smooth uninterrupted traffic beneath the city, and a vastly improved urban landscape.
“This is about as easy as building the English Channel Tunnel [the “Chunnel”] under London or Paris or as doing open-heart surgery while the patient is driving to work,” said Sean O’Neill, director of media relations of the Massachusetts Turnpike Authority (MTA), which manages and operates the project. “We are going under one of the most historic metropolitan districts in the country. It’s extremely unstable and unknown territory, contending with topographical changes, centuries of landfill, seawalls, animal bones, historic artifacts and old sewer lines that were never mapped.”
Construction began on the immense project in 1991 and is expected to be completed in 2004. If everything works out as planned, participants will deserve to throw a Big Dig Bash to celebrate the conclusion of a project which has been compared to the Panama Canal and the Chunnel.
“We are definitely now in the final phases of work on I-90 and I-93, with the final configuration, including all ramps, fully operating in 2004,” Rich Lynt, assistant area construction manager for the downtown work, told Construction Equipment Guide.
Lynt is part of Bechtel Corporation’s contribution to the projects integrated management organization, which includes a Bechtel/Parsons Brinckerhoff joint venture and the MTA. “I would think it will be quite a celebration when everything is completed,” he said.
“With anything so complex and challenging, there are always issues, always problems,” Lynt added. “The single biggest issue has been maintaining traffic. The I-93 viaduct is directly above the tunnel alignment. We had to support the raised highway and keep traffic flowing, not through a desert, but through downtown Boston. We also had to work through a metropolitan area, including the financial center, and try to keep people sleeping at night.”
The elevated highway carried about 75,000 vehicles a day when it opened in 1959. Today it carries upwards of 200,000 — making it one of the most congested highways in the United States, with an annual cost to motorists of an estimated $500 million from accidents (four times the national average for urban interstates), wasted fuel and late deliveries. To alleviate this, the CA/T project is completing 161 lane mi. (259 km) of highway in a 7.5-mi. (12 km) corridor, about half in tunnels, including four major highway interchanges.
The old elevated road, which will begin to be demolished in 2003, has 27 on or off ramps. The new one will have just 14, and is expected to carry about 245,000 vehicles a day by 2010.
Gone by the end of 2004 will be the unsightly raised artery, a green barrier through the city, which blemished downtown. Gone, too, will be the 5,000 construction workers, the battalions of equipment, including 150 cranes, the choking, air-polluting traffic tie-ups. In their place: a much more beautiful city with 260 acres (105 ha) of new parks and open space, far-cleaner air, 21st century traffic flow management under a city, much better access to the downtown waterfront/port area and greatly stimulated development.
The Big Dig, in fact, will provide an intermodal transportation infrastructure with links to air, sea, rail, bus and subway, supporting economic growth well into the century.
An idea of the magnitude of the accomplishment:
• The Big Dig is excavating more than 16 million cu. yds. (12 million cu m) — approximately 541,000 truckloads — of dirt.
• The project is placing 3.8 million cu. yds. (2.9 million cu m) of concrete, enough to build a sidewalk 3 ft. (.9 m) wide and 4 in. (10 cm) thick from Boston to San Francisco and back three times.
• It’s installing more than 26,000 linear ft. (7,925 m) of steel-reinforced concrete slurry walls, which form the walls of the underground highway as well as the supports for the elevated highway during construction. This is the largest application of this construction technique in North America, all resting on bedrock up to 120 ft. (37 m) beneath the streets of the city.
• The project has moved 29 mi. (47 km) of gas, electric, telephone, sewer, water and other utility lines maintained by 31 separate companies.
• During the current peak production (1999 through 2002), about $3 million worth of work is completed each day.
• The Big Dig includes 118 separate construction contracts and 26 geotechnical drilling contracts.
• The project has required the largest geotechnical investigation, testing and monitoring program in North America to identify conditions in the path of tunneling and help prevent buildings from settling during the digging.
Central Artery
Lynt told CEG that initial traffic will start to flow in northbound lanes of the Central Artery in November 2002, while the first southbound lanes will open in November 2003.
“We are opening the highway in stages,” he said. “Not all ramps will be completed initially. Staged construction will be complete in 2004.”
Opening of the southbound lanes is taking a year longer because southbound traffic will use the existing Dewey Square (South Station) tunnel, which is being completely refurbished to carry southbound traffic only. The transition from an above-ground to an underground approach to the tunnel requires a more complicated construction sequence.
Spectacular Bridge
As the northbound lanes of the Central Artery open, this traffic is to begin using the new 10-lane $100-million Leonard B. Zakim Bunker Hill Bridge across the Charles River, which has just been completed this summer and will be opened to traffic to handle the northbound lanes.
The new 1,457-ft. (444 m) long bridge, the widest 76-ft. (23 m) cable-stayed bridge in the world, incorporates an asymmetrical design. It has been compared to “a gigantic two-masted schooner sailing into the city’s skyline.” One of its design features is that an extra two lanes are cantilevered off the east side of an eight-lane main span.
The bridge is named for a Jewish civil rights leader who died last year.
Some wags, noting the wide stance of the tower legs, had suggested naming it for a Boston Red Sox first baseman who allowed a ground ball to slip between his legs, costing the team the World Series in 1986.
It’s the first “hybrid” cable-stayed bridge in the United States, using steel in the main span and concrete in back spans. The main span consists of a steel box girder and steel floor beams, while back spans contain post-tensioned concrete.
With its inverted Y-shaped 270-ft. (82 m) towers, the shape of the Bunker Hill Monument in neighboring Charlestown, the bridge will serve as a spectacular landmark gateway to downtown Boston.
Traffic also can use a parallel four-lane Leverett Circle Connector Bridge which opened in October 1999, between downtown Boston and I-93 in Charlestown. Nine box girder sections — in cross section the largest in North America — were barged into place and raised by cranes or (in the main span) jacks.
I-90 Extension
The I-90 extension includes construction of a new four-lane highway and connecting ramp system, both eastbound and westbound, through and under Boston. The extension is expected to be opened next fall, with more complete operation in late 2003. It also will include a high occupancy vehicle (HOV) ramp system.
Approximately 3.5 mi. (5.6 km) long, the extension begins at the existing South Bay I-90/I-93 interchanges and proceeds east under the Fort Point Channel to the new South Boston I-90 interchange. Here it connects to the Ted Williams Tunnel, which was the first Big Dig milestone, opened to traffic between South Boston and Logan Airport in 1995.
The extension has to pass under a newly constructed 1-90/I-93 Interchange at the outset. It then descends below ground just west of the railroad tracks at South Station and proceeds under the tracks through a series of jacked tunnels.
The underground highway then briefly passes through a cut-and-cover tunnel section before entering immersed tube tunnels under the Fort Point Channel and above the Massachusetts Bay Transportation Authority Red Line subway tunnel. These tunnels then reconnect to cut-and-cover tunnels before joining the Ted Williams Tunnel.
Previously restricted primarily to commercial vehicles, this tunnel is to be opened to all traffic when the extension is complete. It’s expected to carry 88,000 vehicles a day, compared with the current 25,000. After the tunnel, the highway emerges at the new I-90/Logan Airport Interchange.
The I-90/I-93 Interchange at the southern end of the underground highway is being completely rebuilt on six levels, two subterranean, and is 50 percent complete. The interchange will carry a total of 28 routes, including HOV lanes, and will channel traffic to and from Logan Airport to the East.
The Fort Point work is the most extensive use of concrete immersed tube tunnels in the United States. It also represents the first installation of jacked vehicle tunnels in North America (and one of the largest in the world), and the second use of soil mix construction on the East Coast (the first such use being the Ted Williams Tunnel).
Problems
Problems associated with a project of this magnitude have plagued contractors and motorists and delayed completion. The latest, widely covered in the Boston press, involves leaks under the immersed tubes.
Lynt told CEG the leaks are under the tubes, which are immersed in water and sealed at one end. The leaks must be sealed in order to properly join future sections in the next phase of construction. One of the problems is that workers can’t see well at the underwater depths.
Other problems include: difficult and changing soil conditions, tight working spaces, proximity to construction of huge glass and steel office towers and fragile brick buildings, and, of course, holding up the elevated highway while tunneling directly beneath it.
Costs
Costs just for construction have been estimated at $9.12 billion. Other costs include project management ($1.96 billion), design ($996 million), right of way ($572 million), insurance ($572 million), plus contingency, and other fees.
As the Big Dig winds down from mid-2002 to 2004, expenditures are estimated at $45 million to $75 million per month, compared with $125 million per month at present.
According to the project’s Web site (www.bigdig.com), mitigation costs take up one fourth of the project’s budget. These costs include engineering innovations, community improvement, and working with environmental and other agencies, community groups, businesses and political leaders.
The Big Dig is 80 percent federal and 20 percent state financed. Federal funding for the Big Dig totals $8.55 billion. The state’s Transportation Infrastructure Fund (TIF) is providing $2.2 billion, about half of which comes from the sale of Commonwealth bonds backed by automobile license and registry fees.
Management
The CA/T project is owned and managed by the Massachusetts Transportation Authority (MTA) and is part of the Metropolitan Highway System (MHS).
Design and construction management consulting is provided by Bechtel/Parsons Brinckerhoff, a joint venture of Bechtel Corp. of San Francisco, CA, and Parsons Brinckerhoff Quade & Douglas Inc. of New York, NY.
The joint venture works under an integrated project organization. This provides a chain of command which includes officials from both companies and from the MTA, reporting to each other and communicating.
“I would say it has been a successful approach, a pretty seamless organization,” Lynt told CEG.
End of Big Dig
The elevated highway is to be gone, completely demolished and removed by the end of 2004, along with the existing I-93 bridge over the Charles River. Neighborhoods previously severed by the structure will be reconnected and new urban vistas will be opened.
About 75 percent of the area formerly covered by the highway will be open space, including trees. About 25 percent will be developed.
Structural steel from the demolished structure could build five huge bridges.
Traffic using the new highway system, including the Central Artery, will be monitored by the most advanced traffic management and incident response system in the world, including more than 400 video cameras, 1,400 in-pavement detectors to measure traffic flow, 130 electronic message signs, 25 electronic height detectors and six 24-hour emergency response stations. The Big Dig will be over, but never forgotten.
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