Boston’s Central Artery Tunnel Project, more commonly known as the Big Dig, was one of the largest and most technically challenging highway projects in United States history.
Cutting through the heart of the Massachusetts capital, the massive urban infrastructure project commenced in 1991. It involved moving the city’s six-lane elevated Central Artery (Interstate 93) underground to alleviate traffic congestion, reconnect downtown Boston to the waterfront and improve access to Boston Logan International Airport.
An unprecedented scope
The need for the project was clear. Boston’s Central Artery carried about 75,000 vehicles a day when it opened in 1959 – but that had risen to 200,000 by the early 1990s, making it one of the most congested highways in the US. The highway also effectively cut the city in two. Blocking off the waterfront created an urban eyesore and lack of social and economic utility.
The Big Dig’s enormous scale and complexity have seen it likened to projects such as the Panama Canal and the English Channel Tunnel. At 12.55 km long, with half of that in tunnels, it replaced the six-lane elevated highway with an underground expressway finishing in a 14-lane, two-bridge crossing of the Charles River. It also involved the construction of four highway interchanges.
As well as the incredible scope – more than 8000 km of fibre-optic cable and 322,000 km of copper cable were installed – the project commanded a list of engineering firsts. It saw the deepest underwater connection and the largest slurry-wall application in North America. Hundreds of pipes carrying cold brine were used to freeze the soil beneath nine existing rail lines; these kept it from collapsing while the area was excavated before a concrete tunnel built off-site was “tunnel-jacked” into place.
The world’s widest cable-stayed bridge, the Zakim Bunker Hill Bridge, spanned the Charles River, and its largest tunnel-ventilation system was installed underground.
A success – at what cost?
The project was hailed a success after its completion, with a study by the Massachusetts Turnpike Authority finding it cut the average car trip through Boston from almost 20 minutes to under three minutes.
However, the construction saw Boston suffer through years of traffic jams, and blowouts led to the project taking more than 15 years to complete. Construction began in 1991 and was officially finished in 2007, while the initial cost estimate escalated from just under AUD$4 billion to more than $23 billion.
Reported reasons for this included inflation, the failure to assess unknown subsurface conditions, and extensive environmental and mitigation costs.
Multiple stakeholders were another complicating factor. One Big Dig program required the relocation of more than 46 km of utility lines such as gas, electricity, telephone, sewer and water, which were maintained by 31 separate companies.
Boston University’s Mega-Project Research Program concluded that no single event caused Big Dig costs to escalate.
“The critical cause was a lack of experience and knowledge about dealing with the complexity and uncertainty that giant projects bring with them,” it said. “The sheer size of this project and the fact that construction occurred in a busy city resulted in having to deal with many unanticipated conditions and a large volume of claims and changes.”
Surprises encountered on the project included uncharted utilities, ground-water conditions, environmental problems, weak soil and hazardous materials. The discovery of Revolutionary-era sites and Native American artifacts was another delaying complication.
A simple error, devastating results
A small detail led to a fatal accident that occurred when the concrete ceiling panels in one tunnel collapsed on a car in 2006, killing a passenger.
The subsequent investigation by the National Transportation Safety Board (NTSB) found builders had used the wrong epoxy to hold the anchor bolts in place.
The builders had tested the strength of the bolts during construction; however, when some failed, the problem was attributed to installation errors rather than the breakdown of the epoxy.
A variety of other problems were identified by the NTSB in how the epoxy was mixed and used, but these did not reduce the strength enough to cause the bolts to pull out of the concrete, the investigation found.
“It’s kind of ironic [that] in a $14 billion project about $1.50 per anchor is what ended up bringing the ceiling down,” one NTSB member told The New York Times.
The supplier of the epoxy later noted the specialised fast-set product was not intended for long-term use, but that information was “in the fine print”, the NTSB said.
In a devastating failure of engineering, no one noticed that the product would weaken over time, and no regular safety inspections of the panels had occurred in the three years following the tunnel’s opening.
READ: What caused the I-35W Mississippi River bridge to collapse?
The project’s legacy
After the underground highway opened to traffic, the elevated artery was demolished to make way for open space and low-rise development. At the project’s completion in 2007, more than 45 parks and major public plazas had been created, including the seven-hectare Rose Fitzgerald Kennedy Greenway.
Critics of the Big Dig, however, continue to point to the flawed execution and huge price tag as salient lessons for future road megaprojects.
On small-scale and larger projects, engineering innovations are speeding up construction. Learn how it’s impacting project earthworks at this Engineers Australia webinar.