Bridges

[thepheebster]

How are the bridges on the Thames built? How do they make it stay on the bottom, coz surely they can't dive down under the water with cement because the cement would get all washed away by the water. Do they drain the water out first or what?

[jenstar]

Strange as it may sound, one of the wonderful properties of concrete is that it sets just as well underwater as on dry land. So it can simply be secured in place with wooden boards which are removed later - not to keep the water out, just to hold it in place until it's set.

[Mortartube]

In some fast flowing rivers a tube can be sunk into the river bed and the water is then pumped out. Supports and concrete are then placed inside the tube.For bigger areas steel sheet piling is driven into the river bed and the water is pumped out and then the supports are built inside. These is known as a coffer dams or caissons. 

[Pinotage]

Coffer dams were used for the contruction of the new foot bridges at the Charing Cross bridge