BIG DIG

The Big Dig is the unofficial term for the Central Artery/Tunnel Project, a massive undertaking to replace the existing elevated Central Artery (Interstate 93) through the heart of Boston, Massachusetts, USA with an underground highway. The design includes a new tunnel to Logan Airport. Due for completion in 2005, it is the most expensive highway project in American history (roughly $15 billion, or more than $1.5 billion per mile). The Zakim Bunker Hill Bridge at the project's northern end has already become a new symbol of Boston.

The project was first conceived in the 1970s to replace the rusting elevated six-lane Central Artery that splits off downtown from the waterfront, and which was increasingly choked with bumper-to-bumper traffic. Planning for the Big Dig officially began in 1982, with environmental impact studies starting in 1983. After years of extensive lobbying for federal dollars, a 1987 public works bill appropriating funding for the Big Dig was passed by Congress, but it was subsequently vetoed by President Reagan as being too expensive. When Congress overrode his veto, the project had its green light and ground was first broken in 1991. At the time, it was projected to cost $5.8 billion, but that was before its scope was expanded to include more tunnels and bridges.

Reworking such a busy corridor without seriously restricting traffic flow required a number of state-of-the-art construction techniques. Because the old elevated highway (which remained in operation throughout the construction process) rested on pylons located throughout the designated dig area, engineers first utilized slurry-wall techniques to create 120 ft. deep concrete walls upon which the highway could rest. These concrete walls stabilized the sides of the site, preventing cave-ins during the excavation process.

Other challenges included an existing subway tunnel crossing the path of the underground highway. In order to build slurry walls past this tunnel, it was necessary to undermine the tunnel and build an underground concrete bridge to support the tunnel's weight.

The Zakim Bunker Hill Bridge, designed by Swiss designer Christian Menn, represents the terminus of the project, connecting the underground highway with I-93 and Route 1. A distinctive suspension bridge, the crossing is supported by two forked towers, which are connected to the span by cables and girders.

On January 17, 2003, the opening ceremony was held for a 1.3-mile tunnel section of the Dig, extending the Massachusetts Turnpike (Interstate 90) east to Logan International Airport. The westbound lanes opened in the afternoon of January 18 and the eastbound lanes early January 19. The next phase, taking the elevated Interstate 93 and putting it underground, will be completed in two stages: the northbound lanes opened in March 2003 and the southbound lanes are scheduled to open (in a temporary configuration) in December 2003.

The Central Artery/Tunnel Project is managed by the Massachusetts Turnpike Authority with design and construction supervised by a joint venture of Bechtel Corporation and Parsons Brinckerhoff. Due to the enormous size of the project -- too large for any company undertake alone -- the design and construction of the Big Dig were broken up into dozens of smaller subprojects with well-defined interfaces between contractors. Major heavy-construction contractors on the project included Jay Cashman, Modern Continental, Obayashi, Perini Corporation, Peter Kiewit's Sons, J.F. White, and the Slattery division of Skanska USA. (Of those, Modern Continental was awarded the greatest gross value of contracts, joint ventures included.)

External Links

• http://www.bigdig.com/