Digging a Highway Revolution in Boston

The Big Dig
Written by Nate Hendley

Three decades ago, Boston had a reputation for being one of the most car-congested cities in North America. The cause of Boston’s traffic woes was glaringly obvious.

“The main obstruction was the Central Artery (I-93), an elevated six-lane highway through the center of downtown which was like a funnel full of slow-moving or stopped cars. Opened in 1959, this 1.5 mile (2.4 km) highway at first carried approximately 75,000 vehicles a day. This had grown torturously to 190,000 vehicles by the 1990s, when its traffic crawled for 10 hours a day,” reported a June 2007 article in Construction Equipment Guide, a Pennsylvania-based media outlet.

In addition to traffic jams, the elevated highway featured an accident rate four times higher than the national average and was a financial drain for motorists and the city alike. Traffic was equally congested in a pair of tunnels that led from downtown to Boston Logan International Airport.

“The annual cost to motorists from this congestion was an estimated $500 million. Costs included a high accident rate, wasted fuel from traffic and late deliveries,” stated a report from the Highway Division of the Massachusetts Department of Transportation.

Inconvenient for commuters, I-93 was also an eyesore for Bostonians. The elevated highway cut through neighbourhoods and tortured local residents with constant noise and exhaust fumes.

Something had to be done. Boston officials coalesced around a seemingly simple solution: construct a new highway system, consisting of underground tunnels, roads, interchanges, and bridges to accommodate traffic then knock down I-93. It was hoped such a scheme would “unclog the throat of a city choking with traffic,” as Construction Equipment Guide put it.

In 1982, planning began on what was officially called the Central Artery/Tunnel Project. There were nightmarish challenges to be overcome. Not only does Boston have lots of narrow, old streets which are difficult to navigate, but workers had to create new traffic infrastructure around old infrastructure. The existing elevated roadway would remain in use while the new highway system was created in the ground beneath it.

The difficulty was to achieve the construction of The Central Artery Project while minimizing the disruption to businesses and people living in the city. An advanced traffic monitoring system aided in this venture.

The proposed project would be both complex and massive. “The Central Artery/Tunnel Project was public works on a scale comparable to some of the great projects of the last century. Projects like the Panama Canal, the English Channel Tunnel (the ‘Chunnel’) and the Trans-Alaska Pipeline,” stated the Highway Division, Massachusetts Department of Transportation.

In 1990, Congress provided $755 million in initial funding for what was dubbed ‘the Big Dig.’ The project would be run by the Massachusetts Turnpike Authority (MTA) with the firms Bechtel/Parsons Brinckerhoff doing design and construction management consultation in a joint venture.

Construction began in 1991 with a bypass road going through South Boston. Shortly thereafter, work started on the 8,500-foot-long Ted Williams Tunnel underneath Boston Harbour. To build the tunnel, 890,000 cubic yards of material was dredged from the harbour to create a fifty-foot deep and one-hundred-foot wide trench. The dredging process took almost two years. Meanwhile, giant steel tubes were prefabricated for the tunnel structure be placed in the trench.

“Assembly teams at the site placed 25,000 tons of concrete roadway deck and 33,000 tons of steel reinforcement bars inside each [tunnel] section. After the sections were barged out to the trench, a lay-barge lowered them into place by cable, using a laser beam and a global positioning system to determine exact location. Divers then aligned their final placement,” reports Construction Equipment Guide.

Pumps removed water and debris to create air-tight seals, then the tunnel sections were welded together. Engineers also had to build land approaches to the Ted Williams Tunnel, which involved liquefying and freezing the soil, much of which was loose landfill.

The Ted Williams Tunnel became the first major segment of the Central Artery/Tunnel Project to be completed. The tunnel was opened to vehicle traffic, in late 1995.

Other engineering milestones achieved during the Big Dig included of two new bridges spanning the Charles River. The Leonard P. Zakim Bunker Hill Memorial Bridge was the first bridge in the country employing an asymmetrical design of a hybrid of steel and concrete and is the widest cable-stayed bridge in the world.

The project also saw a 1.5 mile underground Thomas P. O’Neill, Jr. Tunnel, new roadways, an extension to Logan Airport, and the installation of “26,000 linear feet of steel-reinforced concrete slurry walls. These formed the walls of the underground highway as well as the supports for the elevated highway during construction. That is five miles of slurry walls, the largest application of this technique in North America. This is all resting on bedrock up to 120 feet below the streets of the city,” noted the Highway Division of the Massachusetts Department of Transportation.

During peak construction, 5,000 workers using 150 cranes toiled on the Big Dig. Crews excavated over sixteen million cubic yards of soil, added 3.8 million cubic yards of concrete, and “relocated 29 miles of gas, electric, telephone, sewer, water, and other utility lines,” reported Construction Equipment Guide.

Such work was enormously expensive and the Big Dig was plagued with cost-overruns. “In 1985, officials assigned a $2.5 billion price tag to the Big Dig. By the early 1990s, it had risen to $7.5 billion. It hit the $10 billion mark before 2000. To date, the Big Dig has cost $14.6 billion,” reported the November 19, 2004 Washington Post.

“In 2000, former Big Dig head James Kerasiotes resigned after failing to disclose $1.4 billion in overruns. A frustrated Congress capped the federal contribution,” added a December 25, 2007 Associated Press article.

In September, 2004, seawater began spurting through a wall in a northbound tunnel under construction. The leak was fixed but an investigation discovered hundreds of other leaks which could have caused a catastrophe. These too were fixed.

There was a human cost involved in the project, with four workers killed during construction. On July 10, 2006, a three-ton concrete ceiling panel fell on a car traveling on a ramp in South Boston, killing a woman and injuring her husband.

According to the National Transportation Safety Board “The probable cause of the July 10, 2006, ceiling collapse in the D Street portal of the Interstate 90 connector tunnel in Boston, Massachusetts, was the use of an epoxy anchor adhesive with poor creep resistance, that is, an epoxy formulation that was not capable of sustaining long-term loads.” Thousands of bolts were subsequently replaced at a cost of $54 million.

Difficulties aside, work went on. The original elevated I-93 was torn down in 2003, with green space taking its place. Four years later, work on the Central Artery/Tunnel Project, which would be known as “the largest, most challenging highway project in the history of the United States,” in the words of the Highway Division, Massachusetts Department of Transportation, was finally done.

Coming in at a total price tag of around $14.8 billion, the new highway system drastically reduced commuter time. “A study by the Turnpike Authority found the Big Dig cut the average trip through Boston from 19.5 minutes to 2.8 minutes,” reported the December 25, 2007 Associated Press.

It also enhanced Boston’s quality of life and established more than three hundred acres of parks and green space. “For those who grew up with the noise and clutter of the old Central Artery, the transformation of downtown Boston is still a wonder to behold,” noted the December 25, 2007 Associated Press article.

The Big Dig also offers several lessons for anyone contemplating future mega-construction projects. Lesson number one: anticipate that the project will take drastically longer and cost a lot more money than initially expected. Other lessons: expect the unexpected (such as leaking walls, as in the case of the Big Dig) and strongly emphasize safety. More focus on this might have saved the lives of the workers who died during the Big Dig and the motorist killed by falling concrete.

And, perhaps most importantly, recognize that is sometimes possible to achieve what might seem impossible, such as building a new highway system in a crowded old city without interrupting traffic on the old highway system.



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