Big Dig

The Big Dig — officially known as the Central Artery/Tunnel Project (CA/T Project) — was a landmark infrastructure megaproject in Boston, Massachusetts that rerouted the elevated Central Artery (Interstate 93) into an underground tunnel system and extended Interstate 90 to Logan International Airport. The largest and most expensive highway infrastructure project in the history of the United States, it rerouted Boston's Interstate 93 through an underground tunnel. Planning began in 1982 and construction was carried out between 1991 and 2006. The project also produced two celebrated civic landmarks: the Leonard P. Zakim Bunker Hill Memorial Bridge over the Charles River and the Rose Kennedy Greenway, a linear park system running through the heart of downtown Boston. Despite chronic cost overruns and controversies, the Big Dig fundamentally reshaped the city's urban form and transportation infrastructure.

Background and Origins

Boston had a world-class traffic problem called the Central Artery — an elevated highway running through the center of downtown. When it opened in 1959, the Central Artery carried about 75,000 vehicles a day. By the early 1990s it carried upwards of 200,000, making it one of the most congested highways in the United States. Traffic crawled for more than 10 hours each day, and the accident rate on the Central Artery was four times the national average.

The social costs of the elevated highway extended beyond traffic. The construction of the highway had displaced 20,000 residents during its original construction and cut off Boston's North End and Waterfront neighborhoods from its downtown. Without major improvements, Boston expected a traffic jam for up to 16 hours a day by 2010. The annual cost to motorists from this congestion was an estimated $500 million.

Engineer Bill Reynolds was the first to propose the idea of building a wider highway in an underground tunnel, during a meeting of the Boston Transportation Planning Review. Engineer Fred Salvucci joined Reynolds and began lobbying heavily to replace I-93, and Massachusetts Gov. Edward King instead pushed to extend Interstate Highway 90 (I-90) to Boston's Logan Airport, another site of heavy congestion. Dukakis was elected governor again in 1982, and Salvucci returned to his secretary of transportation post, this time lobbying for a joint project that would include the I-93 tunnel and the I-90 extension. Congress approved federal funding and the project's basic scope in April 1987.

Construction

Construction began in September 1991 on a bypass road through South Boston, which took truck traffic off neighborhood streets. A third tunnel to cross Boston Harbor was also begun at that time. In order to avoid disrupting the nine rail lines operating above the I-93/I-90 interchange near the tunnel's southern terminus, sections of the underground expressway were built in an adjacent area before being moved into their final positions using a method called tunnel-jacking. Hundreds of pipes carrying cold brine froze the soil beneath the rail lines to keep it from collapsing while the area was excavated and the concrete tunnel was forced into place.

The scale of the engineering effort was immense. The project placed 3.8 million cubic yards of concrete — enough to build a sidewalk three feet wide and four inches thick from Boston to San Francisco and back three times. The project installed more than 26,000 linear feet of steel-reinforced concrete slurry walls, which formed the walls of the underground highway as well as the supports for the elevated highway during construction — five miles of slurry walls, the largest application of this technique in North America. The project also included the largest geotechnical investigation, testing, and monitoring program in North America, the purpose of which was to identify conditions in the path of tunneling work and help prevent buildings from settling during the digging.

The Ted Williams Tunnel beneath Boston Harbor was the first completed milestone of the Central Artery/Tunnel Project. It opened on schedule and within its $1.3 billion budget on December 15, 1995. Named for the Boston Red Sox legend, the tunnel doubled Boston's cross-harbor tunnel capacity from four lanes to eight. The 0.75-mile underwater portion of the 1.6-mile tunnel used a dozen steel tube sections, each longer than a football field, which were placed into a trench on the Boston Harbor floor and then connected.

Slurry wall construction began in the mid-1990s, which required underpinning of the existing elevated Central Artery before excavation. Once I-93 North opened under the old Central Artery, Big Dig crews began demolishing the aging elevated highway. That work finished in 2004, when all traffic shifted underground and the Artery became devoid of vehicles for the first time in 50 years.

The Massachusetts Turnpike (I-90) Extension opened to traffic on January 18, 2003, meaning I-90 now runs from Seattle, Washington, to Logan International Airport in East Boston. This direct 3.5-mile route to the airport saves drivers as much as 45 minutes off the previous route.

Key Infrastructure: Tunnels and Bridges

The Big Dig produced several major pieces of new infrastructure that remain central to Boston's transportation network.

The Thomas P. O'Neill Jr. Tunnel carries Interstate 93 through 1.5 miles of underground roadway beneath downtown Boston. From Kneeland Street to Causeway Street it connects to the Leonard P. Zakim Bunker Hill Bridge. The northbound side opened to traffic on March 29, 2003, and the southbound side opened on December 20 of the same year.

The Leonard P. Zakim Bunker Hill Memorial Bridge has become one of Boston's most recognizable landmarks. The bridge is the widest cable-stayed bridge in the world. Swiss bridge designer Christian Menn conceived the bridge with inverted Y-shaped towers, in reference to the shape of the Bunker Hill Monument in neighboring Charlestown. Along with the Battle of Bunker Hill, the bridge is also dedicated to the life of civil rights activist Lenny Zakim, serving as a permanent memorial to a man who bridged many gaps. The bridge, at 1,432 feet long, emerges from the underground Central Artery near the TD Garden at Causeway Street, then crosses the river to connect with both I-93 and Route 1, carrying 10 lanes of traffic.

The Ted Williams Tunnel interface in East Boston, between the land-based approach and the underwater section, is 90 feet below the surface of Boston Harbor — the deepest such connection in North America. The project's seven-building ventilation system is one of the largest highway tunnel ventilation systems in the world.

Cost Overruns and Controversies

The Big Dig became notorious for its cost overruns and mismanagement. The project was originally scheduled to be completed in 1998 at an estimated cost of $2.8 billion (approximately $7.4 billion adjusted for inflation as of 2020). The project was ultimately completed in December 2007 at a cost of $14.6 billion — a cost overrun of about 97 percent when adjusted for inflation.

The Big Dig was plagued by cost overruns, delays, leaks, design flaws, accusations of poor execution and use of substandard materials, criminal charges and arrests, and the death of one motorist. As a result of a death, leaks, and other design flaws, the Parsons Brinckerhoff and Bechtel consortium agreed to pay $407 million in restitution, and several smaller companies agreed to pay a combined sum of approximately $51 million.

Water infiltration from Boston Harbor remained a persistent operational problem after the tunnels opened. The problem of leaks was aggravated by the fact that many involved corrosive salt water, caused by the proximity of Boston Harbor and the Atlantic Ocean, creating a mix of salt and fresh water leaks in the tunnel. The situation was made worse by road salt spread in the tunnel to melt ice during freezing weather, or brought in by vehicles passing through.

Despite the controversies, the project's general contractor structure was defined from the outset. The project's general contractor was Bechtel, with Parsons Brinckerhoff as the engineers, who worked as a consortium, both overseen by the Massachusetts Highway Department.

Legacy and Urban Impact

The removal of the elevated Central Artery opened considerable land for public use and permanently altered Boston's relationship with its waterfront. The project created more than 300 acres of new parks and open space, including 27 acres where the existing elevated highway stood, 105 acres at Spectacle Island, 40 acres along the Charles River, and 7 acres as part of an expanded Memorial Stadium Park in East Boston.

In the path of the old elevated Central Artery now stands the Rose Kennedy Greenway, a tree-lined boulevard with several miles of new and refurbished sidewalks, 600 streetlights, roughly 900 trees, and a number of plazas. The 1.5-mile-long park runs above the tunnels and features green spaces, gardens, and pathways. The Greenway has become a gathering place in downtown Boston for hundreds of outdoor community events each year.

Excavated material from the project was also put to use elsewhere in the region. In the process of creating more than three miles of multi-lane underground tunnels, the Big Dig excavated more than 16 million cubic yards of soil, and almost a quarter of it went to Spectacle Island, where it remains today. Much of the remaining soil went toward the transformations of Spectacle Island, Millennium Park, and Memorial Stadium Park.

In terms of traffic and environmental outcomes, the results were measurable. Travel times on the project highways dropped 62 percent between 1995 and 2003, and carbon monoxide levels were reduced by 12 percent over the same period. The Big Dig also reduced the urban heat island effect with the addition of an estimated 900 trees along the downtown corridor, and a total of 4,800 trees and 31,000 shrubs were planted in all of the resulting green space following project completion.

The project created more than 45 parks and plazas while increasing connectivity between downtown and adjacent neighborhoods, and included shoreline restoration along the Charles River Basin, Fort Point Channel, Rumney Marsh, and Spectacle Island, as well as the Boston Harborwalk.

The Big Dig stands as one of the most consequential — and contentious — public works undertakings in American urban history. It remade Boston's downtown, reconnected its neighborhoods, and produced infrastructure that will define the city for generations. At the same time, it serves as a cautionary case study in the challenges of managing large-scale, publicly funded construction projects.