Big Dig

CA/T logo

CA/T logo

The Big Dig is the unofficial name of the Central Artery/Tunnel Project (CA/T), a massive undertaking to reroute the Central Artery (Interstate 93), the chief controlled-access highway through the heart of Boston, Massachusetts, into a 3.5 mile (5.6km) 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 "Central Artery" (known officially as the John F. Fitzgerald Expressway) displaced thousands of residents and businesses 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 Interstate Highway standards 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.

Zakim Bunker Hill Bridge over the Charles River under construction, looking north. The old elevated Central Artery crossing is to the right.

Cancellation of the Inner Belt

Another important motivation for the Big Dig in its final form was the abandonment of the Massachusetts Highway Department's intended expressway system through and around Boston. The Central Artery, as part of MassHighway'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 Brookline, Cambridge and Somerville. However, earlier controversies over impact of the Massachusetts Turnpike Boston extension, particularly on the heavily populated neighborhood of Brighton, and the large number of additional homes that would have had to be destroyed 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 one most recently added to the city, in 1912). 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 on 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 a severely overstressed expressway system. With ever-increasing traffic volumes funneled onto I-93 alone, the Central Artery became chronically gridlocked. 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 and a new right of way for the Orange Line subway and Amtrak. Parts of the planned I-695 right of way remain unused and under consideration for future mass transit projects.

The original 1948 Master Plan included a Third Harbor Tunnel plan that was hugely controversial in its own right because it would have disrupted the Maverick Square area of East Boston. It was never built.

Commingling of traffic

A major reason for the all-day congestion was that the Central Artery carried not only north-south traffic, but much east-west traffic as well. Boston's Logan Airport lies across Boston Harbor in East Boston, and before the Big Dig, the only access from downtown was through the paired Callahan and Sumner tunnels. Traffic on the major highways from west of Boston, the Massachusetts Turnpike and Storrow Drive, mostly traveled on portions of the Central Artery to reach these tunnels. Getting between the Central Artery and the tunnels involved short stretches on city streets, increasing local congestion.

The final Big Dig plan, then, combined several projects—the depression and improvement of the Central Artery, the construction of a third Harbor tunnel (now known as the Ted Williams Tunnel), and massive interchange improvements to the Massachusetts Turnpike and several other major routes in the area. While only one net lane in each direction was added to the north-south I-93, several new east-west lanes were added to untangle the traffic. East-west traffic on the Massachusetts Turnpike now proceeds directly through the Ted Williams Tunnel to Logan Airport and Route 1A beyond, with new exits in South Boston along the way. Traffic between Storrow Drive and the Callahan and Sumner Tunnels uses a short portion of I-93, but additional lanes and direct connections are provided for this traffic.

Mass transit

A number of public transportation projects were included as part of an environmental mitigation for the Big Dig. Perhaps the most ambitious was the building of the Phase II Silver Line tunnel under Fort Point Channel, done in coordination with Big Dig construction. Silver Line buses now use this tunnel and the Ted Williams Tunnel to link South Station and Logan Airport. Several other mitigation transit projects promised have not been completed as of 2005 and litigation has been threatened.

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 dropped 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 Central Artery. The expressway separated downtown from the waterfront, and 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 as well as innumerable pipes and utility lines that would have 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 dig beneath 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' Incorporated, 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 U.S. Route 1 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 were 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 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

Interstate 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 limited use for commercial traffic and high-occupancy vehicles 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.

By the end of December 2004, 95% of the Big Dig was completed. Major construction remained on the surface, including construction of final ramp configurations in the North End and in the South Bay interchange, and reconstruction of the surface streets. Many impact-mitigation projects (transit, pedestrian, bicycle, and parks) also remain, but some are in danger of cancellation due to cost overruns on the rest of the project.

In late 2004, leaks 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 are a result of Modern Continental and other subcontractors failing to remove gravel or other debris before pouring concrete. Bechtel/Parsons Brinkerhoff is blamed for failing to detect and fix the problem during construction. The two companies are responsible for finding and repairing the leaks, mostly at their own expense, and this work is ongoing.

On August 11, 2005, it was announced that the Massachusetts State Police searched the offices of the Big Dig's largest concrete supplier in June and found evidence of faked records that hid the poor quality of concrete delivered for highway project. However, it is not believed that the low-quality concrete is connected to the hundreds of leaks discovered in the tunnels that take vehicles under Boston.

The final ramp downtown — exit 20B from I-93 south to Albany Street — opened January 13, 2006.

The Big Dig has led to a marked reduction in traffic congestion. 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.

Trivia

• The total amount of earth excavated is 15 million cubic yards (11 million m³ or 540,000 truckloads of dirt).
• Enough steel (25,800 m³) was used in the project to make a one-inch steel bar long enough to wrap around the earth (24900 miles).
• The Zakim Bunker Hill Bridge has seven main-line travel lanes, plus a two-lane entrance ramp cantilevered off the east side, making it the widest cable-stayed bridge in the world.

External links

• Official site
• Boston CA/T Project History at MIT Rotch Library
• Steve Anderson's BostonRoads.com
• PBS.org – Central Artery