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Bridge Supports
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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Update: June 2012
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