How do you replace a 50-year-old highway system without stopping the traffic? Building went to Boston to find out.
The biggest construction project ever carried out in North America is under way in the centre of Boston. Nicknamed the Big Dig, the $14bn (£9.8bn) project involves dismantling an elevated two-mile section of Interstate Highway 93, which slices the downtown area in two, and burying it in a 10-lane tunnel. Three gigantic intersections are being built, as well as several bridges and an underwater link to the airport across the bay. The project, built to withstand an earthquake measuring nine on the Richter scale, employs 4000 people and work valued at $3m (£2m) is being finished each day. Started in 1991, the project is not expected to be completed until 2005.

Officially called the Central Artery/Tunnel project, the Big Dig aims to end the gridlock that brings the city to a standstill for hours each day. Interstate 93 was built in the 1950s to handle 75 000 cars a day; it now carries 190 000. The new tunnel will have a capacity of 245 000.

The whole project, a joint venture by construction giant Bechtel and engineer Parsons Brinckerhoff, is being carried out without interrupting the flow of traffic through the city. The old highways have been shored up with a temporary steel frame while excavation goes on directly below them. "The whole project is the equivalent of giving a guy triple-bypass heart surgery – while he's running the Boston marathon," says Jack Quinlan, the Big Dig's director of public affairs.

Since work started, the cost of the project has ballooned from an original $10.8bn (£7.6bn) owing to problems encountered when tunnelling through the Boston subsoil. One key section beneath a railway line was delayed when engineers found oozing mud instead of solid clay. Some 1600 PVC pipes were sunk into the soil and pumped through with calcium chloride brine – chilled to -30ºC – to freeze the ground, making it solid enough to allow a stable tunnel to be excavated. Then a prefabricated 100 m long section of tunnel was pushed beneath the train tracks using hydraulic rams at a rate of about 700 mm a day.

Prefabricated sections were also used on a stretch beneath the Forth Point Channel, constructed behind a cofferdam in the city's port. When complete, the sections, weighing up to 48 000 tonnes each, are made watertight and the dams removed, allowing them to be floated into position. They are then sunk onto piles driven into the bed of the channel, using lasers and global positioning satellites to place the sections to an accuracy of 6 mm.

The Big Dig is unique not just for its size. In a country where gridlock is the norm but where large scale public investment is viewed with suspicion, Boston is the only major city taking a long view on its transportation problems. Yet there is something peculiarly American about its approach. The whole project is geared to allow car use to grow exponentially, while investment in public transport alternatives is minimal. Traffic is expected to reach its 245 000 vehicle capacity as early as 2010. The four-lane airport tunnel, which opened in 1995, is already deemed too small.

Burying the highway will create a 12.1 ha ribbon of open space through the middle of the city, potentially allowing the downtown area to be reunited with its waterfront. But as yet there are no firm plans for the space and no money to pay for anything more than grassing over the tunnel. Critics fear this will create a crime-ridden wasteland in the heart of the city.

"It's a very laborious process to get people to talk in urban terms," says Hubert Murray, an expatriate English planner who is drawing up proposals for the site at the behest of local businesses. "They all live in the suburbs and they don't really trust the city. The inner city is associated with murder and mayhem."

Bostonians have spent the past decade grumbling about the disruption to their city and the spiralling costs – which local taxes are having to cover. "Projects in my district aren't getting funded because of the overspend," says Sue Tucker, state senator for the Essex area of Boston. But the wave of investment pouring into the city on the back of the promised infrastructure improvement – with regeneration projects springing up along the length of the new artery – is beginning to turn public opinion around.

The greatest achievement of the Big Dig has been getting the project built at all, given America's rampant NIMBY culture. Its secret was to buy off opponents: one-third of the $14bn (£9.8bn) cost has been spent on "mitigation" – perks and compensation for businesses and communities affected by the works. These have ranged from a new multistorey car park for residents no longer able to park their cars beneath the elevated highway, to a new Holocaust memorial and a row of gingko trees in Chinatown.

The whole of Boston is desperate for the project to end – including the construction team. "We've had every problem Teddy Roosevelt had with the Panama Canal, except yellow fever," says Quinlan wryly.

Big Dig facts

  • Bechtel/Parson Brinckerhoff has achieved an extraordinary safety record on the job. In September this year it completed a total of 2 347 695 hours (more than two years) worked without a single day lost to work-related injury.
  • The new roads will be monitored by the world’s biggest surveillance system, comprising 500 cameras and 20 000 sensory devices measuring everything from emissions and noise levels to vehicle height and speed.
  • 10 million m3 of soil will be excavated, the equivalent of 13 premier league football stadiums filled to the brim. Most of it is being transported to the former rubbish dump at Spectacle Island in Boston Harbour, where it will form a new park.
  • The total amount of concrete poured is 2 900 000 m3, and enough reinforcing steel was used to wrap a one-inch bar around the world.