What’s the best way to plug a giant hole? Why with a giant plug, of course. That’s the thinking of the Department of Homeland Security’s (DHS) Science and Technology Directorate (S&T), which has created just such a giant plug to contain flooding or dangerous gases in mass transit tunnels. Measuring roughly 32 feet (9.7 m) long and with a diameter of 16 feet (4.9 m), the giant plug is an enormous inflatable cylinder that can be filled with air or water in minutes to quickly seal off a section of tunnel in the event of an accident, natural disaster or terrorist attack.

Developed through S&T’s Resilient Tunnel Project in partnership with Pacific Northwest National Laboratory, West Virginia University (WVU) and ILC Dover, the plug is tunnel-shaped with rounded capsule-like ends and holds 35,000 gallons (132,489 liters). When deflated, the plug is designed to pack into a small storage space in the tunnel where it can be remotely triggered for immediate inflation by the tunnel’s command center.

The plug’s circumference is sufficiently larger than that of the tunnel to ensure a tight seal even when dealing with the various pipes, vents, lights, and subway tracks that line transit tunnels. The material used to make the plug needed to be pliable enough to conform to the irregular shape of a tunnel, while still being strong enough to hold back a full tunnel’s worth of water. An initial full scale prototype with a single-layer design failed during pressurized testing, with the team ultimately settling on a plug with three separate layers.

The outer layer that gives the plug its shape and strength is made up of a thick webbing of a liquid-crystal polymer called Vectran. A second layer of non-webbed Vectran and final layer of polyurethane seal the inflation medium – be it water or air – inside. The plug's designers say relying on commercially available materials for the plug’s construction helped keep development costs down and make them more affordable for mass transit operators.

Dave Cadogan, Director of Engineering at ILC Dover, which is the longtime maker of NASA space suits, said, “"We used the same design and manufacturing techniques we use in space suits and inflatable space habitats. The webbings and underlying layers form a tough barrier that is strong and resilient to damage."

In January 2012, the giant plug was successfully tested in Morgantown, West Virginia, in a specially-built test tunnel that was configured like a tunnel in a major metropolitan city. The plug was initially inflated with low-pressure air – which could be enough to restrain explosive or dangerous gases in real world applications – before the air was replaced with water to achieve the plug’s design pressure. When the closed end of the test tunnel was then flooded to mimic the pressure of an actual tunnel flood occurring well below sea-level, the seal held.

While there’s no word of when they’ll be packed into actual subway walls, the designers say the simplicity of the giant plug makes it much less expensive and disruptive than retrofitting a transit system with retractable, watertight doors.

Source: US DHS, ILC Dover

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