Boston’s Big Dig – one of the US’ toughest engineering challenges
11 November 2014
Author: Sean O’Neill, head of communications, National Roads Authority
First and foremost, let me state for the record that I am not an engineer. I wish I was smart enough to be one, but nature and nurture were not on my side on this particular issue. Why do I have this wish? Well, in four words: ‘The Boston Big Dig’.
Earlier in my career, I worked as a communications specialist for Bechtel Corporation on the largest urban highway renewal project in the world. I realise this is one heck of a statement to make, but it is well earned when you consider that the Boston Big Dig encompassed the removal of an active elevated interstate highway (I93) running north/south through the heart of one of America’s most historic and politically charged cities and placing it 11 storeys underground.
Additionally, the project included extending the (I90/Masspike) interstate running east/west underneath one of the busiest railway corridors on the east coast of the United States. Then it crossed above a subway line (Redline) built in 1916, located underwater in the Fort Point Channel (where the historic Boston Tea Party took place), building onwards right next to Gillette’s global manufacturing headquarters and continuing across Boston Harbour before connecting directly to Logan International Airport.
So, as you can imagine when dealing with a media-charged society that expects something so massive and challenging to be built on time and within budget, my job was cut out for me, literally, though the heart of a city.
From an engineering perspective, it was the men and women with their brilliant problem-solving skills, creative thinking and ‘never say never’ attitude that made me proud to work beside them and eventually represent them publicly as the project’s director of communications.
The basic problem in Boston was that an ageing, green-painted, steel elevated highway that opened in post-WWII America (1954) had run its life span. It well outstretched its design life and its traffic capacity. Compounding the capacity problem, traffic was forced to use the elevated highway to head east via the Callahan Tunnel, which opened in 1961, to connect onwards to Logan International Airport. This ‘Highway in the Sky’, as heralded in local media during the 1950s, also cut the city of Boston off from its historic waterfront and became a modern symbol of urban decay.
The original idea for removing the elevated highway came from ambitious local engineer working with Governor Michael Dukakis, as the secretary of transportation. His name was Fred P. Salvucci. Salvucci was a Massachusetts Institute of Technology-educated engineer and the son of Italian immigrants, whose grandmother’s house was taken when the Highway in the Sky was built. He personally committed himself to taking down that highway one day and yes, he actually did it. To his credit, Salvucci always publicly stated that it was “the men and women encompassing nearly every engineering and construction discipline that made it happen”.
To put these highway improvements in the ground 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 spanned 7.8 miles of highway, 161 lanes miles in all, about half in tunnels. All told, the CA/T placed 3.8 million cubic yards of concrete – the equivalent of 2,350 acres, one foot thick – and excavated more than 16 million cubic yards 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. It has been named the Leonard P. Zakim Bunker Hill Bridge.
The project also included four major highway interchanges to connect the new roadways with the existing regional highway system. At Logan Airport, a new interchange carries traffic between I-90 and Route 1A as well as onto the airport road system. In South Boston, a mostly underground interchange carries traffic between I-90 and the fast-developing waterfront and convention center area. At the northern limit of the project, a new interchange connects I-93 north of the Charles River to the Tobin Bridge, Storrow Drive, and the new underground highway.
At the southern end of the underground highway, the interchange between I-90 and I-93 was completely rebuilt on six levels – two subterranean – to connect with the underground Central Artery and the Turnpike extension through South Boston. The interchange carries a total of 28 routes, including high occupancy vehicle lanes, and channels traffic to and from Logan Airport to the east. A fifth interchange, at Massachusetts Avenue on I-93 has been rebuilt by the project.
The Central Artery/Tunnel (CA/T) project was public works on a scale comparable to some of the great projects of the last century – the Panama Canal, the Channel Tunnel (the Chunnel) and the Trans-Alaska Pipeline. Each of these projects presented unique challenges: the Panama Canal confronted earthslides, malaria, yellow fever and Central American jungles. The Chunnel 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 had 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 1950s and 1960s, 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.
Recognising 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 other 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 took up more than a quarter of the project’s budget.
Due to traffic improvements and substantial reductions in traffic delay, the total vehicle- hours of travel on project highways dropped 62 per cent between 1995 and 2003 and are now providing approximately $168 million annually in time and cost savings to travellers.
Residents from south and west of Boston average travel times from the I-90/I-93 interchange to Logan Airport during peak periods have decreased between 42 per cent and 74 per cent depending on direction and time of day. A 12 per cent reduction in citywide carbon monoxide levels was achieved.
Along with improving mobility in downtown Boston, the project reconnected neighbourhoods severed by the old elevated highway and improved the quality of life in the city beyond the limited confines of the new expressway. Clay and dirt from the project was used to fill and cap landfills throughout New England, including the former city dump at Spectacle Island.
From freezing the ground to stabiles historic fill (so that a tunnel could be jacked underneath live railway lines) to holding up the Federal Reserve building in downtown Boston, this was ‘Broadway for engineers’. I personally met a good few Irishmen and women working on every element of the Boston Big Dig and I am proud to say it was their hard work and education that put them on that stage.
This article is based on a presentation called ‘Boston’s Big Dig Project’, which was organised by the Thomond Region and took place on 21 October in Limerick IT, Moylish, Limerick.https://www.engineersjournal.ie/2014/11/11/boston-big-dig/https://www.engineersjournal.ie/wp-content/uploads/2014/11/Boston-2.jpghttps://www.engineersjournal.ie/wp-content/uploads/2014/11/Boston-2-300x300.jpgCivilbridges,construction,Transport Infrastructure Ireland,United States