Cities all over the world rely on giant tunnels for transporting goods and people. You may know these tunnels as ‘subways’. These mass transit systems are great for travel and commerce but terrible for containing floods. Engineers worry that flooding or the release of dangerous gases in a subway tunnel could spread unstoppably throughout an entire city.
To contain such a flood, researchers from the Science and Technology directorate at the Department of Homeland Security have developed a ‘tunnel plug’. This enormous inflatable cylinder can be stored at strategic locations within the tunnel. Should the tunnel need to be sealed off, the plug is moved into place and filled with either air or water until it’s large enough to seal the hole.
Designing such a plug was quite an engineering feat. For one thing, the empty bag had to be small enough to store out of the way and light enough to quickly maneuver into place. Even more importantly, the expanded plug had to be tight enough to prevent any leakage, yet flexible enough to fit around the many pipes, tracks and ledges that make up the circumference of a subway hole. And of course, the plug had to be able to withstand the pressure of a tunnel full of water barreling towards it.
After several attempts, a successful prototype was developed. The engineers were able to inflate the plug inside a test tunnel where it sealed off the equivalent of a large tunnel flood.
The plug uses high strength fabrics to withstand the pressures of a flooded tunnel.
Photo by ILC Dover. 
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