In the early 1980s, the city of Boston faced a serious problem. Already known for historically bad traffic congestion, projections showed that by 2010, Interstate 93 - known as the Central Artery - would face paralyzing gridlock. I-93 is the main route into and through Boston, making the highway a quagmire of local commuters and interstate travelers.
Because the current Central Artery couldn't get any wider without leveling historic neighborhoods, it was proposed that the city move the whole thing underground.
Dismissed in the early going as an impossible task, the construction phase of the 14-year project was started in 1991. At an estimated total cost of $10.8 billion, the Central Artery/Tunnel Project is one of the most ambitious - and expensive - infrastructure projects since the early days of the United States.
The "Big Dig," as it has been dubbed, is among the largest construction projects in history. Currently, it's rivaled in cost and scope only by the Chunnel (the tunnel under the English Channel that links France and England) and a dam under construction across the Chang River (formerly called the Yangtze River) in China.
Why would the city undertake such a seemingly impossible task? When the Central Artery opened in 1959, it comfortably carried 75,000 vehicles. Today, with an estimated 190,000 vehicles traveling it, the six-lane highway sees six to eight hours of bumper-to-bumper congestion a day. It was estimated that by 2010, the old Central Artery would have had bumper-to-bumper conditions 15 to 16 hours a day.
Engineering miracles Tunneling under one of the oldest cities in the United States is not without its challenges. Not only are engineers forced to create the eight- to 10-lane highway only a few feet from the foundations of 200-year-old buildings, but the whole project has to be completed while maintaining traffic flow on the current Central Artery directly above the construction zone.
To support the weight of the old Central Artery and the 190,000 vehicles that travel it each day, pumps are used in the construction of "slurry walls." These walls, which will eventually become the walls of the tunnel itself, are built in a unique manner.
To construct a slurry wall, a 3-by-10-foot trench is dug until bedrock is reached - 120 feet below the surface in some parts of the project. As the earth is removed (either by a clamshell excavator or by a continuous milling machine that grinds away obstructions with rotating wheels), a slurry is pumped into the trench. Standard centrifugal and self-priming trash pumps are used extensively to pump bentonite, which is mixed with concrete, to create the slurry. As the concrete is later poured in, it displaces the slurry, which is then pumped away to be used elsewhere in the project.
Without the slurry walls, conventional excavators would have to be used and the highway overhead would have to be demolished. With no viable alternate route to I-93, such an endeavor would simply cripple the city and was not feasible.
By the time the tunnel is completed, more than 26,000 linear feet of slurry walls, about five miles worth, will have been poured 10 feet at a time.
Tunnel building In another part of the same project, a concrete tunnel is being built, section by section, under the Fort Point Channel in South Boston.
To make construction of the enormous concrete sections possible, workers dredged out a 1,000-by-300-foot "casting basin" in the channel, built a series of round coffer dams, and created something of a concrete-section factory where the waters of the Fort Point Channel once flowed. Pumps were used to pump out the coffer dams. After a number of tunnel sections are constructed in the basin, the coffer dams will be removed, the basin flooded and the concrete sections floated into place.
After they are floated out and precision-dropped into place, the coffer dams will be reconstructed and the channel water pumped out to let construction begin on the rest of the sections.
Jobsite maintenance Standard centrifugal pumps are used in washing the mud and bentonite off trucks that are leaving the site to keep it out of the streets. Pumps are also used extensively in dewatering applications to keep the site dry so contractors can work.
The success of this enormous project is dependent upon continued innovation and support from local contractors, rental equipment dealers and several types of pumps. Without the hard work and forward-thinking, this project would certainly eclipse its $10.8 billion budget and 14-year schedule
