International Construction Project – Boston’s Big Dig

Mon, 01 Dec 2008 09:22

The Problem

Boston, Massachusetts had a world-class traffic problem, an elevated six-lane highway called the Central Artery that ran through the downtown city centre. When it opened in 1959, the Central Artery comfortably carried about 75,000 vehicles a day. It has carried upwards of 200,000, quite uncomfortably, making it one of the most congested highways in the world.

Traffic crawled for more than 10 hours each day. The accident rate on the deteriorating elevated highway was four times the national average for urban Interstates. The same problem plagued the two tunnels under Boston Harbour between downtown Boston and Logan Airport. Without major improvements to the Central Artery and the harbour crossings, Boston expected a stop-and-go traffic jam for up to 16 hours a day – every waking hour – by 2010.

The annual cost to motorists from this congestion - in terms of an elevated accident rate, wasted fuel from idling in stalled traffic, and late delivery charges - was estimated at $500 million (R3.6 billion).

And traffic wasn't the only problem the old Central Artery caused in Boston. The elevated highway (which displaced 20,000 residents when it was built) also cut off Boston's North End and Waterfront neighbourhoods from the downtown area, limiting these areas' participation in the city's economic life.

The Solution

This extraordinary traffic mess has represented a continuous economic and quality-of-life drain on Boston and New England. The solution, called the Central Artery/Tunnel Project (CA/T), was constructed under the supervision of, and is operated by, the Massachusetts Turnpike Authority.

The project's two major components are:

Replacing the six-lane elevated highway with an eight-to-ten-lane underground expressway directly beneath the existing road, culminating at its northern limit in a 14-lane, two-bridge crossing of the Charles River. After the underground highway opened to traffic, the crumbling elevated highway was demolished and replaced by modest developments and more than 300 acres of landscaped and restored open space, including over 45 parks and major public plazas.

The extension of the Massachusetts Turnpike from its former terminus, south of downtown Boston, through a tunnel beneath South Boston and Boston Harbour to Logan Airport. The first link in this new connection – the four-lane Ted Williams Tunnel under the harbour – was finished in December 1995.

The Challenges

To put these highway improvements underground, in a city like Boston, proved to be one of the largest, most technically difficult and environmentally challenging infrastructure projects ever undertaken. The project spans 12.5 km of highway, 251 lane miles in all, about half in tunnels. All told, the CA/T placed 2.9 million cubic metres of concrete – the equivalent of 2,350 acres, one foot thick – and excavated more than 12 million cubic metres of soil. The larger of the two Charles River bridges, a ten-lane cable-stayed hybrid bridge, is the widest ever built and the first to use an asymmetrical design.

The project also includes four major highway interchanges to connect the new roadways with the existing regional highway system. At the southern end of the underground highway, the interchange between I-90 and I-93 is being completely rebuilt on six levels – two subterranean – to connect with the underground Central Artery and the Turnpike extension through South Boston. In mid-2005 the interchange will carry a total of 28 routes, including High Occupancy Vehicle lanes, and channel traffic to and from Logan Airport to the east.

The Central Artery/Tunnel Project is public works on a scale comparable to some of the great projects of the last century – the Panama Canal, the English Channel Tunnel, and the Trans-Alaska Pipeline. Each of these projects presented unique challenges: The Panama Canal confronted earth slides, malaria, yellow fever, and Central American jungles. The Channel was dug from either end, 31 miles apart, meeting at a precise point under the channel floor. The Alaska Pipeline contended with vast distances, freezing temperatures, and major environmental concerns.

The Central Artery project's unique challenge was to construct this project in the middle of Boston without crippling the city.  Work of the CA/T project's magnitude and duration has never been attempted in the heart of an urban area, but unlike any other major highway project, the CA/T was designed to maintain traffic capacity and access to residents and businesses – to keep the city open for business – throughout construction. Highway projects of the ‘50s and ‘60s, when the interstates were first built, gave very little consideration to the communities in the path of the new roads, with disruption and dislocation the rule of the day.

Recognizing that failing to maintain Boston's economic viability during construction would damage the city's competitive position for years to come, project planners worked with environmental and various oversight and permitting agencies, community groups, businesses, and political leaders to create consensus on how the project would be built. The process of keeping the city open and making certain that all affected groups are treated fairly is called mitigation, and it has taken up a fourth of the project's budget.

The multilane interstates also had to pass under South Station's 7 train tracks, carrying over 40,000 commuters and 400 trains per day. In order to avoid multiple relocations of the train lines while the tunnelling advanced, as had been initially planned, a specially designed jack was constructed in order to support the ground and tracks to allow the excavation to take place below. The ground was also frozen using heat extractors in order to help stabilise the surrounding ground as the tunnel was excavated. This was the largest tunnelling project undertaken beneath railway lines anywhere in the world. The ground freezing enabled safer, more efficient excavation, and also assisted in environmental issues, as less contaminated fill was exported compared to traditional cut and cover methods.

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