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Name: Jason
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: 2001


Question:
How do engineers solve the problem of bridges that have their support towers in moving water like a creek or river?



Replies:
There are a variety of solutions used for bridge foundations, some very high-tech while others date back to Roman times. The one that might be selected would depend on the depth of the water, the speed of the water, and the geological conditions. I am not sure whether you are interested in the types of foundations or how they are build, so I will describe a little of both.

One method is referred to as pile foundations. In this case, the foundation consists of long poles, referred to as piles, made of steel, reinforced concrete, or wood. These piles are hammered into the soft soils beneath the bridge until the end of the pile contacts a hard layer or the pile is driven so deep that the friction between the pile and the soil will support the load. Piles are frequently driven in "groups" that may consist of dozens of individual piles. A large bridge may have hundreds of individual piles.

An alternative to piles is drilled shafts. In this type of foundation a big auger, which looks like a drill bit, is used to cut a deep hole. A steel reinforcing basket is lowered into the hole and the hole is then filled with concrete (which will harden underwater, by the way). To keep the hole from collapsing while it is being driven, either a steel casing is placed in the hole as it is driven or the hole is filled with a dense clay-water slurry which the steel and concrete can displace.

The other type of foundation is referred to as a spread footing. This is often used when solid rock or very hard soils are at a shallow depth. In this case, a solid block of concrete will be cast directly on the prepared rock, once a suitable surface has been created.

If the body of water is not too big, the pile and drilled shaft foundations can be constructed from the shore using large cranes. Otherwise, special barges are used. In other cases, structures called cofferdams are used. Cofferdams are basically big steel tubes or boxes that are placed in the water and then pumped out. Workers then go inside the cofferdam to prepare and construct the foundation.

One effect of fast moving water is that the foundations may be undermined. Engineers refer to this as "scour". To combat scour, piles of large stone or other items may be placed around the foundations to deflect and diffuse the water currents. However, after a flood or other unusual event, bridges sometimes still must be closed until the foundations can be inspected using divers or underwater cameras. Even under normal circumstances, bridges must periodically be carefully inspected above and below the water to ensure that they remain safe.

Prior to building a bridge, the subgrade conditions are always carefully investigated using probings, sonar, and borings. For large, complex projects the cost of the foundation investigation alone can be millions of dollars, which may seem like a lot until you consider that a major bridge project can be hundreds of millions of dollars. For example, the new Cooper River bridge in Charleston, SC, which will be the longest cable-stayed suspension bridge in the world, is expected to cost $631 million.

Civil engineers who specialize in investigating and designing foundations and any other construction involving earth materials are referred to as geotechnical engineers. In addition to foundations, geotechnical engineers are involved with designing retaining walls, landfills, earth dams, tunnels, pavements, embankments, and many other unnoticed structures which make modern life possible. I point this out because the structural engineers tend to get all the glory while we geotechnical guys (and gals) labor obscurely with the invisible parts that make the structures stand up. So, I appreciate your question.

Andy Johnson



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