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American Morning
At site of World Trade Center Disaster, Workers Have Hit Rock Bottom
Aired January 15, 2002 - 07:40 ET
THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.
THIS IS A RUSH TRANSCRIPT. THIS COPY MAY NOT BE IN ITS FINAL FORM AND MAY BE UPDATED.
ANDERSON COOPER, CNN ANCHOR: At the site of the World Trade Center disaster workers have hit rock bottom. After four months of recovery and removal at ground zero, engineers are now facing a different challenge: preventing the damaged structure around the site that holds back the Hudson River -- called the bathtub -- from collapsing. It's also called the slurry wall.
Dan Hahn is a structural engineer who helped build the Trade Center more than 30 years ago. Now he's again lending his expertise and he joins me now. Thanks very much for being with us this morning.
DAN HAHN, STRUCTURAL ENGINEER: Thank you.
COOPER: What is the slurry wall or the bathtub, as they call it?
HAHN: Well the bathtub basically is similar to your bathtub in your own home, except that it keeps the water and the mud out of the space, rather than the water inside.
COOPER: We've got a picture, I think, of it. Maybe that would help if we could show the first picture of the site. This is?
HAHN: That is the slurry wall, or the bathtub wall.
COOPER: All right.
HAHN: And what you see there are the anchors that tie the wall down to rock. This is a picture of when the bathtub was originally built, or the slurry wall was originally built. That tube that you see in the middle of the picture is the path tunnel that was -- that had to come through the slurry wall at that time.
COOPER: Now how far down does the wall go?
HAHN: The wall goes down 70 feet.
COOPER: OK.
HAHN: And...
COOPER: Is this the bottom of the wall?
HAHN: A little bit lower, but not much more. COOPER: Now why do they call it the bathtub?
HAHN: Well, I guess really, at the time, trying to describe it to people who don't follow engineering very, very closely. The idea was to try and describe something like a box that would keep the water out. Now...
COOPER: So it's basically -- it's like a 16-acre bathtub that literally keeps water and mud and everything else...
HAHN: Yes. Mud and everything out, that's right.
COOPER: Now is the wall in danger of collapsing?
HAHN: No. It never has been in danger of collapsing, in my opinion. And I said that very soon after the September 11th attack.
COOPER: But there is -- there is water starting to collect in the bottom of the site?
HAHN: Yeah, there is water today, and it's been basically rainwater these days. Now, initially when we first looked at this back in September, there was water in the site too, and the water was coming through the path tube over into New Jersey. And people were concerned about it in the port authority and they asked me to design some concrete plugs to plug up the tube over in New Jersey, which we did.
But we measured the water at that time, and we concluded that the source of water was not the Hudson River or ground water or anything like that. We saw the source of water as being the firemen's hoses; we saw the source of water as some normal leakage that occurred in the basement. And then, again, we were able to correlate rainfall. A couple days of later we would see an increase in flow.
COOPER: We're going to roll some construction tape, and I think you see some of the water that you're talking about. Now how -- this is -- this is I think from the original site.
HAHN: That's correct.
COOPER: So the site is 16 acres?
HAHN: Total site is 16 acres, that's right.
COOPER: Now how are they trying to -- that's the slurry wall when it was originally done?
HAHN: That's correct.
COOPER: How are they now trying to sure (ph) it up?
HAHN: OK, initially, when the -- after the World Trade Center was built, there's six floors below the sidewalk level. Those floors braced all the walls; it braced from one side to the other.
COOPER: You can pause it right here. These are the braces?
HAHN: These are the tiebacks that were done during construction to hold the wall up in the interim, so that until they got the floors in the basement installed.
COOPER: OK.
HAHN: Those floors, then, were there when the two towers came down. They were demolished in a great number of places.
COOPER: How far -- how far is it from the river to the wall?
HAHN: Oh, a couple of hundred feet. That's all.
COOPER: OK.
HAHN: Those floors were demolished. But in demolishing those floors, the debris from the towers came and actually provided the suring (ph) to the walls to prevent it from tipping over. In other areas the floors were still in tact, and they held up the wall. In a couple of areas, there were no floors and no debris. And we were concerned there; we backfilled (ph) with soil behind the wall so that would provide a counter pressure to the wall.
COOPER: Well, what my understanding is, what the workers are going through right now, it's basically a two-fold job. They're removing the debris and removing the -- the foundation of the building. But, at the same time, they have to keep the wall up, and the debris is actually keeping the walls up. So it's sort of -- it's a tricky operation.
HAHN: Well, the operation right now is that you remove about 10 feet of debris, then you install some of those tiebacks that you saw in this picture a little while ago. They're brand new tiebacks; they're being installed right now. About 250 have been installed since the start of construction. And there's about another 550 to go. So it is a stage construction, in which you remove some debris, you put in tiebacks, you remove some more debris, you put in some more tiebacks.
COOPER: Let's actually just pause this picture right here if we could. Now this is -- what are we looking at right here?
HAHN: That is part of the original One World Trade Center tower.
COOPER: OK. And part of this -- this is part of the underground structure?
HAHN: That's correct.
COOPER: So this is actually keeping up part of this wall.
HAHN: That's correct. Right now it's...
COOPER: So as they remove this debris they have to sure (ph) up the (UNINTELLIGIBLE). HAHN: That's correct. That's correct.
COOPER: OK. Now we've already reported how quickly this is going. Does the speed of this recovery effort really surprise you?
HAHN: I think it's pleasantly surprising. I think when people gave me estimates several months ago there was just no frame of reference on how long this would take, because this disaster was monumental, if you will. And, yes, Oklahoma was a disaster and there was a lot of debris to remove there, but not compared to this. So I think it's going faster than people thought, and I think that's a good thing, really.
COOPER: What is it like for you? You worked on the site some 30 years ago; you worked in the building, I think you said, for 20 years...
HAHN: That's right.
COOPER: ... and you were down there on Friday. What is it like for you going back?
HAHN: Well, and I lost some friends and colleagues that were in the building at the time it went down. It's somewhat emotional, there's no question about that. And a little upsetting, quite frankly. But we'll get through it all. I know New Yorkers will.
COOPER: That we will. Dan Hahn, thanks very much for being with us this morning.
HAHN: You're welcome.
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