How do they construct and build bridges in swamps and marsh?

Batman would know.

Pilings into the bedrock, much like the highway connecting the Florida keys.

I don't know about bridges, but in order to build a simple house in a swampy

area like New Orleans, they must first put down something called "pilings".

It's poles into the ground as long as a telephone pole. So they are connected

to each other, and it forms a framework that can stay afloat on top of the

muddy mucky land. Sometimes, when a big truck goes by on a street in

New Orleans, you can feel the ground shake and sway, because it's like

gelatin.

Ocassionally, a house can sink or tilt over time, and it looks a bit like a

ship wreck.

They use piling. Ether resistance piling that are driven deep enough to reach a load bearing strata or until the depth of the piling provides sufficient load bearing capability due to friction between the pile and the soil.

The overall term for what everyone is describing is "deep foundations". Go deep enough, and sooner or later you'll find adequate support. Most foundations I've dealt with in poor soil rely on driven piles. If you can drive a pile to rock, then your bridge sits on rock, it can't sink through rock. But even without rock, you get friction along the sides of a pile - a longer pile has more friction along its length. You can see this yourself, sharpen a stick and try to push it into the ground, the deeper you go, the harder it is to continue going deeper. Enough piles with enough length and you could support a bridge on thick gravy.

The actual "how they construct it" side is more complicated, there are a wide variety of means and methods, probably too much to go into here. In general, poorer site conditions mean more money for things like temporary works, or more innovative ideas. One of my favorites (that doesn't get used all that often) is to prop up a component on a barge that is filled with water. Float it into place, then pump out the water so the barge floats higher, raising the component to where you want it.

they usually drive support piers or construct footings anchored on bedrock or in competent soil (sediment) that can handle the load. It is complicated to explain, but all construction pretty well requires linking to a (relatively) stable and unmovable base if the construction is to last.

Different soils respond to loads (weight or pressure) in different ways, and the force of the overlying weight has to be transferred in a way that will not exceed the soil's ability to resist movement. What needs to be done to get to that condition depends on the soil and the load.

They anchor the supporting columns of the bridges to bedrock below the mud. They pile drive a concrete column into the ground until it reaches a hard layer of rock. then they build the column on top of that anchored concrete column. Or they can build a suspension bridge with the two ends anchored to dry land and then use suspension cables to hold the bridge up in the air.

Depending on the location and size of a swamp or marsh sometimes it is drained first. Then pilings are pounded into the bedrock.

they have to pound steel pilings THRU the swamp and INTO the bedrock under it ...............