Big Dig

From Academic Kids

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CA/T logo

The Big Dig is the unofficial name of the Central Artery/Tunnel Project (CA/T), a massive undertaking to route the Central Artery (Interstate 93), the chief controlled-access highway through the heart of Boston, Massachusetts, into a tunnel under the city, replacing a previous elevated roadway. The project also included the construction of the Ted Williams Tunnel (extending 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. Commissioner 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 Dewey Square (or "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.

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Zakim Bunker Hill Bridge over the Charles River under construction.

Another important motivation for the Big Dig in its final form was the failure of the Massachusetts Highway Department to complete the intended highway system around Boston. The Central Artery, as part of the Massachusetts Highway Department's Master Plan of 1948, was originally planned to be (and signed as) the downtown Boston stretch of Interstate 95, with a bypass road called the Inner Belt (officially Interstate 695) to pass around the downtown core to the west, through the neighborhood of Roxbury and the cities of Cambridge and Somerville. However, earlier controversies over the construction of the Massachusetts Turnpike, particularly through the heavily populated neighborhood of Brighton, led to massive community opposition to both the Inner Belt and the Boston section of I-95.

Clearances for I-95 through the neighborhoods of Roxbury, Jamaica Plain, and Roslindale led to secession threats by Hyde Park, Boston's southernmost neighborhood (and the most recently consolidated neighborhood, having been added to the city in 1910.) By 1972, however, with only a minimum of work done on the I-95 right of way and none on the potentially massively disruptive Inner Belt, Governor Francis Sargent put a moratorium of almost all highway construction within the MA-128 corridor, except for a short stretch of Interstate 93. In 1974 the remainder of the Master Plan was canceled, leaving Boston with an unfinished highway system. Without the bypass/distribution capability of the Inner Belt, Boston was left with a chronic gridlock problem in the heart of the urban core. The Sargent moratorium led to the rerouting of I-95 away from Boston around the MA-128 beltway and the conversion of the cleared land in the southern part of the city into the Southwest Corridor linear park.

A final point was enhanced airport access—Boston's Logan Airport lies in East Boston, and until the completion of the Ted Williams Tunnel in 1995 the only access from downtown was through the narrow Callahan Tunnel, which did not directly connect to the Central Artery. The original Third Harbor Tunnel plan that was part of the 1948 Master Plan would have disrupted the Maverick Square area of East Boston and was hugely controversial in its own right.

The final Big Dig plan, then, was a combination of several projects—the depression and improvement of the Central Artery, the construction of the third Harbor tunnel, and interchange improvements to the Mass Pike and several other points in the area. (Yet another plan, the North-South Rail Link that would have connected North and South Stations, the major passenger train stations in Boston, was part of the original Big Dig but was ultimately ruled out by the Dukakis administration as an impediment to acquiring Federal funding for the project.)

Early planning

The project was conceived in the 1970s to replace the rusting elevated six-lane expressway (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 Ronald 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 was 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 Corporation, 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.

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The Big Dig master plan.

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.

Similar issues occurred with the Leverett Circle Connector, a rather unassuming companion bridge to the Zakim that carries traffic from Interstate 93 to Storrow Drive along the Charles River. The project had been under consideration for many years, opposed largely by the residents of Boston's wealthy Beacon Hill neighborhood, and finally came to fruition as a way to funnel the traffic bound for Storrow Drive and the northern part of downtown Boston away from the mainline roadway. Ultimately the Leverett Connector wound up using a pair of ramps originally constructed for Interstate 695, ironically making it possible for the mainline I-93 to carry more of the through traffic that was supposed to use I-695 in the original Master Plan.

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

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Interstate I-93 Tunnel

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 had been completed and in use since late 1995.) The westbound lanes opened on the afternoon of January 18 and the eastbound lanes on January 19.

The next phase, moving the elevated Interstate 93 underground, was completed in two stages: northbound lanes opened in March 2003 and southbound lanes (in a temporary configuration) on December 20, 2003. A tunnel underneath Leverett Circle connecting eastbound Storrow Drive to I-93 North and the Tobin Bridge opened December 19, 2004, easing congestion at the circle. All southbound lanes of I-93 opened to traffic on March 5, 2005, including the left lane of the Zakim Bridge, and all of the refurbished Dewey Square Tunnel.

As of December 2004, 95% of the Big Dig was completed. Work is expected to be finished in 2005, with the exception of some of the new parks. Other remaining work to be completed includes construction of final ramp configurations in the North End and in the South Bay interchange, plus reconstruction of the surface streets.

As of late 2004, leaks have sprouted in the tunnel. Minor ones resulted from gaps in the roof of the tunnel; major ones from structural weaknesses in the tunnel walls, which lie below the water table. Many of the leaks resulted from Bechtel Corporation failing to remove gravel or other debris before pouring concrete.

The Big Dig has led to a marked reduction in gridlock. The combination of the Mass Pike extension to the Ted Williams Tunnel and the extensive use of feeder roads to remove interchange traffic from the mainline has drastically reduced the headaches of Boston highway traffic.


  • Dan McNichol and Andy Ryan, The Big Dig. Silver Lining Press, 1991

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