Flooding of New York's subway during Hurricane Sandy has raised interest in using a cheap inflatable plug to hold back floodwaters, a difficult engineering task involving the Pacific Northwest National Laboratory in Richland.
MORGANTOWN, W.Va. —
With a few dull thuds, the 1-ton bag of high-strength fabric tumbled from the wall of the mock subway tunnel and onto the floor. Then it began to grow.
As air flowed into it through a hose, the bundle inflated until it was crammed tight inside the 16-foot-diameter tunnel, looking like the filling in a giant concrete-and-steel cannoli.
The three-minute procedure, conducted on a chilly morning this month in an airport hangar not far from West Virginia University, was the latest test of a device that may someday help guard real tunnels during disasters — whether a terrorist strike or a storm like Hurricane Sandy, whose wind-driven surge of water overwhelmed New York City’s subway system, shutting it down for days.
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“The goal is to provide flooding protection for transportation tunnels,” said John Fortune, manager of the project for the Department of Homeland Security’s Science and Technology Directorate.
The idea is a simple one: Rather than retrofitting tunnels with metal floodgates or other expensive structures, use a relatively cheap inflatable plug to hold back floodwaters.
In theory, it would be like blowing up a balloon inside a tube. But in practice, developing a plug that is strong, durable, quick to install and foolproof to deploy is a difficult engineering task, one made even more challenging because of the pliable, relatively lightweight materials required.
“Water is heavy, there’s a lot of pressure,” said Greg Holter, an engineer with Pacific Northwest National Laboratory in Richland, who helps manage the project. “So it’s not as simple as just inflating and filling the space. The plug has to be able to withstand the pressure of the water behind it.”
The idea has been in development for more than five years — this test was the 21st — and Fortune says there are at least a few more years of testing and design work ahead. If the plugs are effective, they will be made available to transit systems around the country; at least initially, they are expected to cost about $400,000 each.
“We have frequent conversations with folks in mass-transit agencies, the true experts in the field, on how this would be deployed,” Fortune said, although he declined to name specific agencies, citing the anti-terrorism aspect of the project.
Adam Lisberg, chief spokesman for the Metropolitan Transportation Authority, which runs New York City’s transit systems, said the damage caused by the storm “is certainly going to focus our attention” on ways to protect the subways. “But it’s way too early to talk about any particular technology that’s been proposed,” he said.
Seven of the city’s 14 under-river subway tunnels were flooded during the storm, as were several major highway tunnels. Fortune said that plugs might work for highway tunnels, too, but that the larger size of those tunnels created additional technical challenges that would have to be overcome.
Work on the plug began in 2007, after Ever Barbero, a West Virginia professor whose specialty is the use of advanced materials in engineering, was contacted by a Homeland Security official looking for ways to keep a subway system from flooding if an underwater tunnel were breached — by a terrorist bomb, for example. “I didn’t know anything about this,” he said. “Then I found out what happened in Chicago.”
Barbero was referring to a 1992 episode in which an abandoned freight tunnel under the Chicago River was breached by a crew sinking bridge pilings. That led to a flood that caused close to $2 billion in damage to downtown buildings as the water spread underground.
Barbero came up with an idea and shared it with Homeland Security officials. “I said, ‘We’ll put an air bag in a tunnel,”‘ he recalled. The department was intrigued — officials were familiar with European efforts to develop plugs to seal tunnels from gas and smoke after deadly tunnel fires — and decided to finance the project. About $8 million has been spent so far.
Barbero realized that the forces exerted on the pressurized plug, and the need to rely on friction against the tunnel walls to keep it in place under the onslaught of floodwaters, meant that it had to be made from very tough materials.
Experts from ILC Dover, a Delaware company that makes high-strength soft structures like spacesuits, suggested fabric made from Vectran, a strong but lightweight yarn spun from a liquid-crystal polymer. But the first full-scale plug, made with a single layer of Vectran, failed during a pressure test in 2010.
“It ripped right down the middle, from back to front,” Fortune said.
So Barbero and ILC came up with a three-layer plug, with the outer layer consisting of woven Vectran belts. It is designed so that the tearing of one belt will not cause a catastrophic failure.
A subway tunnel is hardly a pristine environment; it is full of grease and grime — and, often, rats.
“That’s something we’ve talked about,” Fortune said. “We’ve actually put Vectran samples in tunnels, to see if rats ate it. They didn’t.”
There are also obstructions like tracks, as well an electrified third rail, pipes and safety walkways, all of which could cause gaps between the plug and the tunnel walls. Most of the obstructions can be dealt with by modifying a short section of the tunnel to accommodate the plug, which is 32 feet long when inflated. Sharp corners can be curved, flush tracks of the type used at grade crossings can be installed, the third rail can be discontinued for a stretch, and pipes can be made to swing against the ceiling.
Placement of the plugs would vary depending on a transit agency’s particular needs, Fortune said. Because water pressure increases with depth, shallower placement would make it easier for a plug to hold back floodwaters. And while a plug at either end of an underwater tube would protect against a tunnel breach, Hurricane Sandy showed that flooding can occur in many other ways — through ventilation grates and station entrances, for example. So in a system like New York’s, there may be other strategic places to install plugs.
Barbero’s initial idea was to put plugs on rail cars so that they could be sent to any location, as needed.
“That sounded really far-fetched” to Homeland Security officials, he recalled. He came up with a more permanent installation: The plug sits, folded up, inside a long container snug against a tunnel wall, out of the way of passing trains.
Pumps and other equipment are even farther out of the way. On a signal, the container opens up like a parachute bag, and the plug unfolds and inflates.