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human
How are the pillars of bridges constructed (bridges over water specifically) ?

Bridges built over water are supported by these huge pillars. I have always wondered:
 
1) Are they built outside (on land) and then placed in the water. If the water is pretty deep, the pillar must be massive quite a burden to lift and position correctly

2) How do they ensure the pillar is steady on the underwater floor. If the soil beneath is loose and gravely, how do they ensure the pillar gets a good solid foundation which will not just slip away on the soft soil below?
gpt
1) They are usually built in place, not on land and moved into place. Although this is starting to happen more with smaller bridges over water using pre-cast concrete elements.

When building in place they will place what is called a "cofferdam" around the area where the pier will go. This is a wall made of steel sheet piling that is embedded into the soil. Once the wall is built fully around (circle or semi-circle for stress reasons) they pump the water out.

There are other ways of doing this, but the above is probably the most common.

2) The pillar (or pier as it is usually called) is built on a large foundation that usually encapsulates a group of pre-cast driven piles or drilled shafts (essentially a bunch of concrete columns driven into the soil). These are embedded to a depth that will provide stability for the pier above.

Usually the length is determined based on the loads of the bridge alone, and then is increased a certain dimension based on how much scour is expected due to the flow of the water and the soil type.

Piers that can be expected to be struck by barges or ships will either have a deeper foundation, or they will have some sort of barrier protection that will deflect seafaring vessels from impacting the piers. The larger the type of vessel that can be expected in the channel, the higher the likelihood that these barrier protections will be required rather than increasing the length of the foundations.