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Central Artery/Tunnel Project (Big Dig)
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Vital Statistics:
Location: Boston, Massachusetts, USA
Completion Date: 2004
Cost: more than $10 billion
Length: 18,480 feet (3.5 miles)
Purpose: Roadway
Setting: Soft ground
Materials: Steel, concrete
Engineer(s): Bechtel, Parsons Brinckerhoff, Quaide Douglas

Some call the Central Artery/Tunnel Project in Boston, Massachusetts, the "largest, most complex and technologically challenging highway project in American history." Others consider it one of the most expensive engineering projects of all time. Locals simply call it the "Big Dig." By the time it's finished in 2004, the tunnel will be eight lanes wide, 3.5 miles long, and completely buried beneath a major highway and dozens of glass-and-steel skyscrapers in Bostonís bustling financial district. What does it take to dig a tunnel like this? A lot of hard work and a handful of engineering tricks.

Central Artery/Tunnel Project (Big Dig)
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Today, engineers use special excavating equipment, called "clamshell excavators," that work well in confined spaces like downtown Boston. These special machines carve narrow trenches -- about three feet wide and up to 120 feet deep -- down to bedrock. In Boston, engineers are pumping liquid slurry (clay mixed with water) into the trenches to keep the surrounding dirt from caving in. Huge reinforcing steel beams are lowered into the soupy trenches, and concrete is pumped into the mix. Concrete is heavier than slurry, so it displaces the clay-water mix. The side-by-side concrete-and-steel panels form the walls of the tunnel, which will allow workers to remove more than three miles of dirt beneath the city.

As if tunneling beneath a city isnít hard enough, the soil beneath Boston is actually landfill -- itís very loose and soggy. Engineers had to devise a few tricks to keep the soggy soil from collapsing. Their solution: freezing the soil! Engineers pump very cold saltwater through a web of pipes beneath the city streets. The cold pipes draw heat out of the soil little by little. Once frozen, the soil can be excavated without sinking. Engineers also inject glue, or grout, into pores in the ground to make the soil stronger and less spongy during tunnel construction.

Here's how this tunnel stacks up against some of the longest tunnels in the world.
(total length, in feet)

Chart showing the relative size of the longest tunnels in the world
Central Artery/Tunnel Project (Big Dig) 18,480' (3.5 miles)

Fast Facts:
  • The project will excavate a total of 15 million cubic yards of dirt, enough to fill Foxboro Stadium -- where the New England Patriots football and Revolution soccer teams play -- 15 times.
  • Reinforcing steel used in the project would make a one-inch steel bar long enough to wrap once around the Earth at the equator.
  • Moving all the dirt in the tunnel will take more than 541,000 truckloads. If all those trucks were lined up end to end, they'd stretch 4,612 miles. That's the same distance from Boston, Massachusetts, to Brasilia, the capital of Brazil.
  • The tunnel will emerge next to the FleetCenter, home of the Boston Bruins hockey team, and will cross the Charles River under the widest cable-stayed bridge in the world, the Charles River Bridge.