Numerous metropolitan subway systems around the world include spans that pass under bodies of water. If one of those lines were to spring a leak, the resulting flooding could be catastrophic. The US Department of Homeland Security might have developed the answer — a giant, inflatable tube plug.
Earlier this year, the DHS Science and Technology Directorate, led by Dr. John Fortune, working in conjunction with teams from Pacific Northwest National Laboratory, West Virginia University (WVU) and ILC Dover, successfully demonstrated the new Resilient Tunnel Project (RTP) system.
The RTP is a large inflatable cylinder 32 feet long and 16 feet wide. It's designed to store in the tunnel's ceiling, then descend and expand to seal that section of tunnel. The plug, filled at 17 to 25 psi, is strong enough to hold back 35,000 gallons of water — enough to fill a medium-sized residential swimming pool. Local authorities would be able to remotely activate multiple plugs along the impacted line.
The RTP is constructed of three layers of material. The outermost layer, made of webbed Vectran polymer fiber, provides the overall shape of the plug. The two inner layers are made of solid Vectran and polyurethane, respectively. They are designed to provide the actual seal — not just for the flood waters on the outside, but also the air and/or water used to inflate the device.
"We used the same design and manufacturing techniques we use in space suits and inflatable space habitats," Dave Cadogan, Director of Engineering at ILC Dover, told Physorg. "The webbings and underlying layers form a tough barrier that is strong and resilient."
Since a tunnel's walls aren't perfectly smooth cylinders (they're typically veined with piping and train tracks), the plug is actually designed to expand to slightly larger than the circumference of the tube. It's built with just enough extra material to fill in the gaps, but not so much as to cause the material to fold over itself and permit fluid to leak past.
The plugs can also be deployed to prevent the spread of fires, smoke, or a bioweapon like Sarin gas. They'll be positioned near emergency exits (hopefully ), so disaster victims will be able to quickly evacuate once they reach the stopper. [Physorg - DHS - ILC Dover]