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Big Dig

The Big Dig is the unofficial term for the Central Artery (I-93)/Third Harbor Tunnel (I-90) Project, a massive undertaking to replace the existing elevated Central Artery (Interstate 93) through the heart of Boston, Massachusetts, with an underground freeway. The project included a new tunnel to Logan International Airport and a new bridge, the Zakim Bunker Hill Bridge, over the Charles River.

The Big Dig is the most expensive single highway project in American history. When the last major highway section opened in December 2003, over $14.6 billion had been spent in federal and state tax dollars; in 2003 dollars, that cost is twice what was spent on the Panama Canal a century earlier. Cost overruns were so high that the chariman of the Massachusetts Turnpike Authority, James Kerasiotes, was fired in 2000 and his replacement had to commit to a cap in federal contributions of $8.549 billion.

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. Business leaders were more concerned about access to Logan Airport, and pushed instead for a third harbor tunnel. In their second terms as governor and secretary of transportation, respectively, Michael Dukakis and Frederick P. Salvucci came up with the strategy of tying the two projects together -- thereby combining the project that the business community supported with the project that they and the City of Boston supported.

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.

The nature of the Charles River crossing was a source of major controversy throughout the design phase of the project. Many environmental advocates preferred a river crossing that was entirely in tunnels, but this (and 27 other plans) were rejected as too costly. Finally, with a deadline looming to begin construction on a separate project that would connect the Tobin Bridge to the Charles River crossing, Transportation Secretary Salvucci overrode the objections and chose a variant of the plan known as "Scheme Z". This plan was considered to be reasonably cost-effective, but it had the drawback of requiring highway ramps stacked up as high as 100 feet (30 m) immediately adjacent to the Charles River. At the time construction began, the whole project (including the Charles River crossing) was projected to cost $5.8 billion. The city of Cambridge, objecting to the visual impact of the chosen Charles River crossing design, sued to revoke the project's environmental certificate, and force the project to redesign the river crossing yet again. Meanwhile, construction continued on the Tobin Bridge approach. By the time the I-93 design was finally settled to the satisfaction of all parties, the construction of the Tobin connector (today known as the "City Square Tunnel" after the intersection in Charlestown which it bypasses) was already so far along that significant additional expense would be incurred to stage construction of the US 1 to I-93 interchange and eventually retrofit the tunnel; in the new design, not all of the traffic movements originally envisioned would be possible.

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. Boston blue clay and other soils extracted from the path of the tunnel was used to cap many local landfills, fill in the Granite Rail Quarry in Quincy, and restore the surface Spectacle Island in Boston Harbor Islands National Recreation Area.

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 US 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 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 opened (in a temporary configuration) on December 20, 2003. Remaining work still to be completed includes the refurbishment of the Dewey Square Tunnel, demolition of the old elevated structure and Charlestown High Bridge, and final ramp configurations in the North End, at Leverett Circle, and in the South Bay interchange, plus reconstruction of the surface streets and establishment of new city and state parks.

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 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.)

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