Lake Pontchartrain Causeway piling questions

I watched the TV show, and I’ve read the Wiki entry, but a few mysteries remain.

What is in the pilings that hold up the bridge? They are large reinforced concrete hollow cylinders. I suppose if they are open at the bottom, they would fill up with lake water. If they fill up with lake water, then you have two (inside and outside) wetted surfaces subject to deterioration from moisture. That seems undesirable. I assume they are not filled with concrete, that would be quite a bit of concrete. Would they be filled with sand from the lake bottom? Maybe the engineers that built it do not care? If they were filled with air (below the water line) it seems there would be some buoyancy effects that over a long term would be ungood.
Also, the bridge has been damaged a few times, and could be clobbered again in the future. If a barge knocks out a piling, can another piling be put in that exact same spot? If you can’t put a piling back in that exact same spot, do you have to make some custom girders to put the bridge back together that allow for the different dimensions of where you can put the pilings?

I think these questions would be applicable to the Chesapeake Bay Bridge too . . .

[li]Wiki article[/li][li]Lake Pontchartrain Causeway web site[/li][li]Pile Restoration Details (PDF warning)[/li][li]More info[/li][/ul]

By the way, engineers do not build things, they design things.