The Inflatable, 35,000-Gallon Subway Plug

Department of Homeland Security Science and Technology Directorate

April 2, 2012 4:04 PM Text Size: A . A . A

What is the Resilient Tunnel Project?

The project started in 2007. The idea behind the tunnel plug was to be able to section off regions in a subway system so you can contain flooding and prevent a widespread event. Flooding is the most difficult, but there are also concerns about fires and toxic gas releases, all of which could be addressed by a plug.

What other ideas have people considered?

Previously, transit agencies looked at using rigid floodgates that could be moved in to block off the tunnel when needed. But these were bulky, hard to install, and expensive, so we wanted to come up with something like a balloon or big bag of air that we could build at a lower cost and section off tunnel as needed.

Why did you choose the "inflatable plug" design?

The concept of a plug was something less costly and quickly deployable. We could put it in a soft bag container, like a huge parachute, and mount it to the tunnel walls. We also wanted to use what we knew?one of our partners, ILC Dover, had done a lot of work for NASA and had a lot of experience in high tech textile design and manufacturing. The technology we?re using in the tunnel plug is actually tech they?ve used in space habitats and space suits; the Mars Rover landing bags were made from the same fabric as the tunnel plug. So even though the design and the application are novel, working with high strength textiles was something we had a lot of experience in.

What were some of the biggest design challenges?

Our first idea was to build a single layer tunnel plug out of the strongest fabric we could find, but we found that tears would propagate the fabric, causing it to fail. So we went back to the drawing board, and the current design has a three-layer construction.

The outer layer is made from Vectran fabric; it looks like a basket or woven tablecloth, a webbing design for additional strength. The two additional layers inside are a second layer of unwoven Vectran and a flatter, inner layer that holds the water or air used to inflate the plug.

Letting the plug fall into place is also a very important part of the design. If you fold this thing up compactly and then deploy it, it probably won?t go into tunnel the right way and there will be gaps. The packaging is very important, so our project team is coming up with different ways to fold it and put zip ties in place (like a fishing rag) so when a certain pressure is reached, the zip ties pop and different portions of the bag will inflate. This makes for a staged inflation and ensures that you fill the entire tunnel.

Credit: E.M. Sosa from West Virginia University

What?s the inflation mechanism?how does it blow itself up?

It depends on what you use to fill it, but the way you get there isn?t as important as ensuring it?s properly filled. There are multiple paths?water, high-pressure pumps, high capacity air compressors, fans, compressed air generators ? but if this thing doesn?t get positioned in the tunnel correctly, it won?t work. The most effective way to ensure this is to have it properly packed, tied, and inflated with air first. The initial inflation should be an air fill; whether you want to maintain it with continued air flow or fill it with water is up to the operator.

Can it be remotely inflated, and how long does it take?

Yes, the idea is to have remote inflation but the time depends on the water or air source used to inflate it. The range is somewhere around 30 minutes, usually quicker, and that?s inflation to full pressure.

How small is it when it?s not inflated?

Our target was to be within 20 inches of the wall, but that varies. One of the things we?re still working on is how to design the container so it?s optimal for each tunnel system. The transit people want a cloth-like bag to hold it as compactly as possible, but we?re still working this part. If I had to, I?d say it?s roughly the size of a really long parachute.

Where will they be placed? And how many per transit line?

This will be a decision based on transportation authorities, because you have to take into account different tunnel configurations. Trying to prevent the spread of flooding in system, you have to think, "Where would water flow? Where are other tunnels?" and figure out the best way to segment it off and maximize the value of the plug.

Where is there the biggest potential for something to go wrong?

Initially we thought we would go with a hard container, made of steel or something. But then we realized, ?What if it didn?t open?? So we came up with the solution to have a soft bag, with cutting devices to cut the lace and allow the plug to fall out. We tried to think about minimizing the things that could go wrong in the design.

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