How bridges are built over water? | By Interesting Engineering

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Bridges are marvels of engineering that stand inconspicuously amongst us. We don't think of them much even when we are passing over them. Nowhere are these structures more impressive then when they are built over water which brings us to the question; how are bridges built over water? 

When the water is shallow, construction is easy. A temporary foundation is made on which piers are built to support the upper structure and the bridge is then built! It's when the water is deep that other techniques are needed. There are many methods to complete such a task in deep water but here we will explore the main three. These three methods of bridge-building are called battered piles, cofferdams, and caissons. 

Let's begin with battered piles. These are poles that are driven into the soil underneath the water. Piles are hammered into the water until they turn outward or inward at an angle. This makes the piles firm and increases their ability to carry lateral loads. Piles are inserted in the ground using pile drivers. These are mechanical devices that may be transported to a location on a floating pile driving plant. Pile drivers may also be cantilevered out over the water from piles that have been installed in advance. With the use of pile frames, pile hammers, and winches, pile drivers hammer the piles into the soil until they turn outward or inward at an angle. The piles are now ready to carry lateral loads and can provide the foundation of support for the bridge. The next step is to construct the pile caps above the piles. Once this is done the bridge is ready to be built. 

Next up are cofferdams. These are temporary enclosures made by driving sheet piling into the bed of a body of water to form a watertight fence. This is called the cofferdam. There's more to this bridge-building technique. Once the sheet piles have been inserted in the water to create a cofferdam. The water is pumped out of the enclosure. Now, the construction workers can build the bridge as if they are working on dry land. The process then becomes relatively easy. 

Finally, caissons may be used. There are two types of caissons: open and pneumatic. An open caisson is a structure that is usually shaped like a box. It is open at the top and bottom. The caisson is usually constructed on land then floated into position and sunk so that the upper edge is above water level. The caisson has a cutting bottom edge so that it sinks through the soft silt on the bed. Inside is a series of large pipes or dredging wells. These are used to dredge up the bed material. As more material is dredged up the caisson sinks and more sections are added to the shaft to keep it above water. 

Once the caisson reaches the correct depth, concrete is laid to seal the bottom and then more concrete is poured into the caisson to form a solid post. A pneumatic caisson is similar to an open caisson but it has an airtight bulkhead above the bottom edge. This is fitted with airlocks. The space between the cutting edge and the bulkhead is called the working chamber. In this space, the water is removed using air pressure. Construction workers can then enter the chamber and excavate the soil. It is important that the air pressure in the chamber be carefully monitored so the workers do not get the bends. 

But how do engineers pick which technique to use? This all depends on the condition of the site and the technology available. These are important decisions to make that only experts can fully handle. Now that you know a bit more about how bridges are built you can admire them even more. After all, all bridge-building techniques are impressive feats of Engineering.

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✍ Vocabulary Review (find the word in the text that is suitable to each below description): 

1. in a way that is not easily or quickly noticed or seen
2. having only a short distance from the top to the bottom
3. a low structure built at the edge of water, used especially for getting into and out of boats
4. A pile driven at an inclination to the vertical to provide resistance to horizontal forces
5. to hit or kick something with a lot of force
6. relating to the sides of an object or plant or to sideways movement
7. Support by a long projecting beam or girder fixed at only one end (used in bridge construction)
8. a machine that lifts heavy objects by turning a chain or rope around a tube-shaped device
9. an area surrounded by fences or walls
10. having no openings to allow water to get in
11. operated by air pressure
12. Go down below the surface of something, especially of a liquid; become submerged.
13. sand or soil that is carried along by flowing water and then dropped, especially at a bend in a river or at a river's opening
14. to remove unwanted things from the bottom of a river, lake, etc. using a boat or special device
15. a deep hole in the ground from which you can get water, oil, or gas
16. to cover a surface with a special liquid in order to protect it
17. completely closed so that no air can get in or out
18. a wall that divides the inside of a ship or aircraft
19. something difficult needing a lot of skill, strength, courage, etc. to achieve it

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