You've probably heard of the Great Pyramids in ancient Egypt and the Great Wall of China. But you may not have heard of the Big Dig, which is taking place here and now and ranks as one of the largest construction projects the world has ever known.

Big Dig is the nickname for the Central Artery/Tunnel Project in Boston, Massachusetts. Now in the eleventh year of an almost fifteen-year schedule, this massive undertaking will replace much of the city's elevated highways with a network of roads that run underground for more than seven miles. Along the way, the Big Dig's architects and engineers are building major new tunnels and bridges to improve the traffic flow in and out of Boston.

The most visible of these engineering feats is the new "cable-stayed" bridge across Boston's Charles River. The construction of the bridge is 80% complete and is scheduled to open in mid-2001.

You may be familiar with traditional suspension bridges, such as the Golden Gate Bridge in San Francisco, California, or the Brooklyn Bridge in New York. The mile-long roadway of these bridges is supported by an elaborate system of vertical cables connected in the form of a parabola between two large towers.

Cable-stayed bridges are dramatically simpler, less expensive, and trimmer. "We can take the cables and run them up to the tower itself," explains Kirk Elwell, the lead field engineer for the bridge project. "You still have to have a tower, but now we're attaching the cables directly."

The traditional suspension bridge requires the building of huge and expensive concrete blocks next to its towers. These enormous weights counteract the upward pull of the many vertical cables and provide the "opposite and equal forces" necessary to keep the bridge stable. But cable-stayed bridges actually use the concrete highway running up to each tower to provide the same balancing effect.

These slimmer bridges are the product of advances in computer design. In fact, the entire bridge in Boston was designed and tested on computers before any construction began. The architects and engineers were able to adjust the computer program for high wind speeds and varying amounts of traffic. The computer design process also lets them eliminate unnecessary construction.

"What it really does is help minimize the amount of product you're building. The towers become slimmer and the bridge sections become slimmer," Elwell says. "It also cuts down on the human error part. And that happens. Let's face it. We're all human."

Still, Elwell adds, all of the high technology and advance planning cannot solve the basic problem any bridge builder faces: "The biggest problem is the installation of the foundation. Each tower is supported by shafts that go 100 feet down, and 60 of those feet go into rock. And that's all done in the blind. We never know what we're going to hit."

As trim as the bridge has become, it still boasts some impressive numbers. One of its two towers rises 300 feet, while the other soars 330 feet. The bridge is 1,457 feet long and 180 feet wide, a width that allows 10 lanes of traffic to cross in two directions.

• Check out a photo gallery of the bridge towers during their construction.

These numbers pale next to the amount of wire used to create the cables that support the bridge. Each of the bridge's 116 cables actually contains many individual bundles of wire. Each bundle is only .62 inches in diameter, but it can support 64,000 pounds of weight. Elwell says that its considerable strength creates a large margin of safety.

During construction, he notes, "We never really stress [the bundles] any more than 30%. And the traffic and other combined forces of the bridge really never take up more than 45% to 50%. So there's a lot of reserve built into the cable for earthquake resistance."

The bridge's builders are not expecting an earthquake anytime soon (the last major earthquake affecting Boston took place in 1755), but that does not stop them from taking precautions.

• The largest cable on the bridge consists of 77 bundles of wire. How much weight could the entire cable support if each bundle holds 64,000 pounds?

• How many tons of weight can this one cable support? (Remember that 1 ton = 2,000 lbs.)

• If the building of the bridge only uses 30% of the cable's capacity, how much weight will the cable be supporting?

The cable-stayed bridge across the Charles River, and the almost $100 million it will cost, represent a fraction of the multibillion dollar Big Dig. The underground highway that will surround downtown Boston by 2004 and the bridges and tunnels supporting it have inspired some lofty comparisons.

• The amount of earth removed in the Big Dig will rival that moved in the building of the Panama Canal.

• The construction phase will use 3.8 million cubic yards of cement, enough to build a 3-foot-wide sidewalk stretching across the United States, and back again 3 times.

• The project will create more than 150 acres of new parks and open space, left behind when the highway goes underground.
  

• The new road is designed to carry 245,000 cars a day, compared to the 75,000 for which the present elevated highway was designed. Estimate by how many times the capacity of the road will increase.

And the rest of the country is watching. Urban planners in Michigan, Colorado, and Texas are considering major overhauls to their city highways and are closely following the Big Dig's progress.

Learn More

• The Riverdeep Today article, "London's Bridge Is Swaying," examines the unusual problem of another new bridge.
  

• The principle of "opposite and equal forces," which keeps this new bridge stable, is a basic part of Newton's Laws of Motion. Explore this principle in the following Riverdeep activities: (Requires Logal Express. Get a free trial subscription.)
  
  

  • Middle School Gateways: Force and Newton's Third Law of Motio n
    
    

  • Physics Explorer: Newton's Laws

  
  

More Links

• Visit the home page of the Central Artery/Tunnel Project (Big Dig).

• Explore the project's other engineering marvels.
  
  
  

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