The Solution

The project had two major components. One was replacing the six-lane elevated highway with an eight-to-ten-lane underground expressway directly beneath the existing road culminating at the northern point with a 14-lane, two-bridge crossing over the Charles River. The other was the extension of I-90, the Massachusetts Turnpike, from south of downtown Boston with a tunnel beneath South Boston and the Boston Harbor to Logan Airport. The first link of the connection, the four-lane Ted Williams Tunnel under the harbor, completed the 3,089 miles of Interstate 90 from Boston to Seattle, WA.

 

The Challenges

The Central Artery/Tunnel Project became known as the Big Dig. It is public works on a scale comparable to some of the great projects of the last century, the Panama Canal, the English Channel Tunnel (the "Chunnel"), and the Trans-Alaska Pipeline. Each of these projects presented unique challenges. The Central Artery/Tunnel Project's unique challenge was to construct the roadway in the middle of Boston without crippling the city. The work of the project and its magnitude and duration had never been attempted in the heart of an urban area. Unlike any other major highway project, it was designed to maintain traffic capacity and access to residents and businesses, keeping the city open for business throughout construction.

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Building an expressway underground in a city like Boston proved to be one of the largest, most technically difficult, and environmentally challenging infrastructure projects ever undertaken in the United States. The project's 7.8 miles of highway had close to 50 separate designs divided into 118 separate construction contracts, with 26 geotechnical drilling contracts. At the peak of construction, five thousand construction workers were on the project, and workers did about $3 million of work each day. About 150 cranes were in use project-wide. The deepest point is 120 feet and runs beneath the Red Line subway tunnel at Dewey Square. The highest point is at State Street, where the highway passes over the Blue Line subway tunnel, and the roof of the highway is the street above. The Leonard P. Zakim Bunker Hill Memorial Bridge, a ten-lane cable-stayed bridge over the Charles River, is the widest ever built and the first to use an asymmetrical design.

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Achievements

Along with improved mobility in a notoriously congested city, the Central Artery/Tunnel Project reconnected neighborhoods severed by the old elevated highway and improved the quality of life in the city. Boston's carbon monoxide levels dropped 12 percent citywide. The Big Dig was an engineering feat like no other. It made significant advances in roadway construction and urban planning. It created more than 300 acres of new parks and open space, including the 27 acres of The Rose Kennedy Greenway, more than 100 acres on Spectacle Island, where dirt from the project capped the abandoned dump, and 40 acres along the Charles River.

 

The nation's most complex and costliest highway project officially ended on Dec. 31, 2007, marking the end of the joint venture that teamed megaproject contractor Bechtel/Parsons Brinckerhoff with the Massachusetts Turnpike Authority to build the dizzying array of underground highways, bridges, ramps, and a new tunnel under Boston Harbor—all while the city remained open for business.