 Part of the Big Dig seen from the air. The Big Dig is the unofficial name of the Central Artery/Tunnel Project (CA/T), a massive undertaking to replace the elevated Central Artery (Interstate 93) through the heart of Boston, Massachusetts with an underground freeway. The project included the Ted Williams Tunnel (Interstate 90) to Logan International Airport and the Zakim Bunker Hill Bridge over the Charles River.
At the time, the Big Dig was 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.
Historical background
Boston's historically tangled streets were laid out long before the advent of automobiles. By mid-20th century, car traffic in the inner city was extremely congested, with north-south trips especially so. Commisioner of Public Works William Callahan pushed through plans for an elevated expressway which eventually was constructed between the downtown area and the waterfront. This so-called Central Artery displaced thousands of residents and businesses, produced an eyesore for those who remained, and physically divided the historical connection between the downtown and market areas and the waterfront. Governor John Volpe interceded in the 1950s to send the last section of the Central Artery underground, through the South Station Tunnel, but while traffic moved somewhat better the other problems remained.
Built before strict federal standards for interstate highways were developed during the Eisenhower administration, the expressway was plagued by tight turns, entrance ramps without merge lanes, and continually escalating vehicular loads. Local businesses and residents again wanted relief and historians sought a reuniting of the waterfront with the city. M.I.T. engineers Bill Reynolds and (eventual state Secretary of Transportation) Frederick P. Salvucci envisioned moving the whole expressway underground.
Early planning
The project was conceived in the 1970s to replace the rusting elevated six-lane freeway (officially the John F. Fitzgerald Expressway) that separated 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 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 U.S. 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.
Major obstacles
In addition to these political and financial difficulties, the project faced several environmental and engineering obstacles.
The downtown area through which the tunnels were to be dug was largely landfill, and included existing subway lines and innumerable pipes and utility lines. Before excavation could begin for the tunnels, the lines had to be replaced or moved. Tunnel workers encountered many unexpected barriers, ranging from glacial debris to foundations of buried houses and a number of sunken ships lying within the reclaimed land.
The project received approval from state environmental agencies in 1991, after satisfying concerns including release of toxins by the excavation and the possibility of disrupting the homes of millions of
rats, and causing them to roam the streets of Boston in search of new housing. By the time the federal environmental clearances were delivered in 1994, the process had taken some seven years, during which time inflation greatly increased the project's original cost estimates.
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 also 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.
Construction phase
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 to 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.)
The nature of the Charles River crossing had been a source of major controversy throughout the design phase of the project. Many environmental advocates preferred a river crossing entirely in tunnels, but this, along with 27 other plans, was 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, 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 had the drawback of requiring highway ramps stacked up as high as 100 feet (30 m) immediately adjacent to the Charles River.
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.
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 of Spectacle Island in the Boston Harbor Islands National Recreation Area.
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 cable-stayed bridge, the crossing is supported by two forked towers, which are connected to the span by cables and girders.
At the time construction began, the whole project (including the Charles River crossing) was projected to cost $5.8 billion. Eventual cost overruns were so high that the chairman 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. Total expenses to date have surpassed $15 billion.
The project today
On January 17, 2003, the opening ceremony was held for the I-90 Connector Tunnel, extending the Massachusetts Turnpike (Interstate 90) east into the Ted Williams Tunnel, and onwards to Logan Airport. (The Williams tunnel itself had been completed and in use since late 1995.) The westbound lanes opened in the afternoon of January 18 and the eastbound lanes early January 19.
The next phase, moving the elevated Interstate 93 underground, was completed in two stages: the northbound lanes opened in March 2003 and the southbound lanes opened (in a temporary configuration) on December 20, 2003. A tunnel running underneath Leverett Circle connecting eastbound Storrow Drive to I-93 North and the Tobin Bridge opened December 19, 2004, easing congestion at the circle.
Remaining work to be completed includes refurbishing the Dewey Square Tunnel (set for completion in March 2005) and construction of final ramp configurations in the North End and in the South Bay interchange, plus reconstruction of the surface streets and establishment of new city and state parks.
As of late 2004, minor and major leaks have sprouted in the tunnel. The minor ones resulted from gaps in the roof of the tunnel; the major ones from structural weaknesses in the tunnel walls, which lie below the water table. Some of the leaks in the walls resulted from contractors' failure to remove gravel or other debris before pouring concrete.
The Big Dig is, as of December 2004, at a 95% state of completion. It is expected to be completely finished in 2005, with the exception of some of the parks.
References
- Don McNichol and Andy Ryan, The Big Dig. Silver Lining Press, 1991
External links
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