This a question for a curious mind.
How can columns or pillars be built underwater?
Where is their foundation laid, and how is it even possible to do such a thing?
Today we are going to make a sincere attempt at answering these questions.

Columns

Construction over waters is done by employing 3 techniques broadly.

These techniques are governed by the pertaining conditions on site.
e.g. Type of Sand underwater, Type of Water, Weather conditions of the area and Depth of water.

Also Read: What is horizontal drilling and how does it work?

In case of shallow waters, a temporary foundation is made on which piers (a type of pillar) is constructed to support the upper structure. No rocket science is employed here and things are in general, Simple.

What if the water is deep, very deep and the soil conditions are not supportive?
In that case, Engineers employee different techniques to raise the piers.

Here comes the Cofferdam Technique:

Technically Cofferdam is a structure that retains water and soil in a particular area. No matter how deep the water is cofferdam can be built.  Now in a cofferdam, an underwater bounded room type structure is constructed which is enclosed with soil and water. Then the water and excess soil are pumped out to make the area dry. It requires constant pumping out of the water and other undesirable materials.

Columns

Cofferdam walls are generally made up of sheet piles supported by wailers, internal braces, and cross braces. Cofferdams are temporary structures and are removed after the piers are constructed to support the bridge. Owing to their temporary nature cofferdams construction materials are available in template form.

Cofferdams are of three types :