Bridge construction over waterways, how?

I am originally from Memphis, which is on the Mississippi River. Here, we have two bridges, one built in the 1920’s-30’s (maybe) and other built in the early 1970’s. What I do not understand at all is how the foundation and the pillars are placed underwater. I have talked to welders who have gone deep underwater in the Mississippi River, would not the rapid water sweep anyone away?

How are underwater bridges built?

Google for “caissons” first link brought up
have fun

Consider the Brooklyn Bridge construction (as an example):

The towers were constructed using a caisson method (section here) which is rarely used today.

The men have to work in a compressed air chamber below the water, digging out the soil and removing it through shafts to the surface. The progress was painfully slow. Blocks were added on the surface, and as the weight of the structure increased, the building slowly sunk (at a rate of about 10-12 inches a week). This had to be rigorously controlled to ensure it ‘settled’ uniformly.

In order to reach solid bedrock to ensure the foundations would be stable the men at Brooklyn had to dig down about 160ft, IIRC. At this depth the pressure of the compressed air meant the workers suffered from the bends and other afflictions (caisson disease) very quickly, and could only would for an hour a t a time. Many died from misunderstood symptoms. It was a very difficult and dangerous procedure.

IANA Civil/Structural Engineer, but I have seen modern structures on water made on piles.

You just take a Barge and pile a column ( just like you drive a nail) to the bottom and then grout it.

If you wish to have a look at modern, cutting edge caisson design, look no further than the recent works at Akashi Kaikyo Bridge in Japan.

(home page here]

I AM a bridge engineer and most bridge foundation construction in the US uses drilled shafts. (for both over water situations and for most of what I design, highway overpasses) You’d only use a caisson on a really big bridge in deep water.

Drilled shafts are easy to place from barges without anyone ever getting wet. You simply drill a big hole to a predetermined depth, place a steel casing, and fill it with concrete slurry to form the bottom of the drilled shaft. Concrete is such a great building material because it doesn’t need to dry to harden, it hardes through a chemical process. So the chafts set up even under water.

Here’s a link with a neat drawing (warning - pdf file): Lake of the Ozarks

And here’s a link with construction photos: DS Const

If you want more info, search for “drilled shaft construction” and you’ll find lots o’stuff…

There are several channels on cable that show programs almost weekly about building bridges and other large structures. Check Discovery, History, TLC channels.