Peter Kenyon, TunnelTalk

An American fabric technology company specializing in the manufacture of space suits for NASA introduces a commercially viable inflatable product that offers cost-effective multi-point protection against flooding for underground infrastructure. Peter Kenyon investigates the development of the Resilient Tunnel Plug, a product that is attracting considerable interest from tunnel owners in the wake of the devastation caused to New York City's underground infrastructure by the November 2012 Superstorm Sandy.

Water - the essence of life, the tunnel owner's worst nightmare.

As the effects of global warming and climate change accelerate the apparent frequency of extreme weather events around the world, the need to protect vital and expensive underground infrastructure from flooding and storm surges has never been an issue more sharply in focus.

The Resilient Tunnel Plug concept

It is not without irony that the recent inundation of transit tunnels, subway stations and tunneling projects under construction in the New York City and New Jersey metropolitan area has catapulted a new solution to the forefront of the debate about the development of a cost-effective way to protect existing and under-construction tunnels against the sort of devastating flood that may either never happen, or at least may not happen for decades.

Try telling that to the Metropolitan Transportation Authority of New York City and the New York and New Jersey Port Authority, whose subway and traffic tunnels suffered massive and crippling damage in the wake of Superstorm Sandy. The Brooklyn-Battery traffic tunnel took on 43 million gallons of water during the 13.7ft storm surge. The Queens Midtown Tunnel was inundated by more than 30 million gallons of water and many of the subway and railway service tunnels took on huge amounts of corrosive seawater. The tunnels under construction were also affected. The recently started TBM drive under New York harbor for a new water supply siphon main to Staten Island was overtopped the working shaft and completely flooded the TBM and heading with seawater. The tunnel and underground works for the 2nd Ave Subway and the East Side Access project to bring Long Island Rail Road services into Grand Central Terminal in Manhattan were also under threat.

A precise figure for damage to the transportation system of the New York/New Jersey region may never be known, but a 2010 report by the New York State Energy Research and Development Authority (NYSERDA) estimated the cost of physical damage to the area's transport infrastructure in the event of a 4ft storm surge at US$16 billion. This figure reaches US$84 billion when economic impacts such as loss of revenue are added. Superstorm Sandy, at its peak, caused a storm surge more than three times greater than the worst-case assumption made by NYSERDA.

New York's Brooklyn Battery tunnel inundation

In light of such huge rehabilitation costs, the Metropolitan Transportation Authority of New York is reportedly interested in the protection offered by a new invention and innovation developed by ILC Dover in Delaware, USA. Named the Resilient Tunnel Plug (RTP), the innovation is a device that inflates in just two minutes and to a full internal pressure within 15-20 minutes.

What makes the simple concept of an air-filled plug so attractive is that it comes with two main benefits. It is relatively cheap to manufacture and fit, when weighed against the relatively small (though nonetheless devastating) risk of a serious 100-year inundation event and there is ease and minimal disruption involved in retrofitting the device at about any location within an existing underground network.

Filled with either air or water, the RTP expands to plug the kind of broadly circular, though slightly irregular, shapes that typify working tunnels and their accompanying peripheral architecture of rail tracks, cables, junction boxes and other structures.

A 16.2ft (4.9m) plug has already undergone rigorous pressure testing to 25psi (1.7 bar) in a series of full-scale tunnel tests at West Virginia University (WVU), and now - four years after the original concept left the drawing board as part of a WVU project - the first commercial order has been received.

ILC Dover, the fabric technology company and design and commercial manufacturer of the plug, has a 30-year history of developing astronaut suits for the American space agency NASA. In recent times it has been working on the development of lightweight and resilient inflatable structures and space domes that can be deployed quickly and easily in the lunar environment. It is this kind of extreme material technology that ILC Dover has spent the last four years adapting for a more earthly application. The project is a partnership led by the Department of Homeland Security Science & Technology Directorate, the Pacific Northwest National Laboratory, West Virginia University and ILC Dover.

RTP inflated to fit typical tunnel shape

"We have just signed our first contract with an American transit authority for a system installation of the RTP in their subway tunnels," said ILP Dover Director of Engineering Dave Cadogan. "Full installation should be completed over the next 18 months."

Commercial confidentiality prevented Cadogan identifying the first customer, but he confirmed that the installation will be for "multiple tunnels".

"We currently have two other transit authorities in the US that are also very interested, and we are at the level of having conversations with them about how, contractually, installation could occur. One potential customer has some very long tunnels and has a desire for intermittent plugs in order to isolate trapped vehicles in the event of an inrush."

There has also been keen interest from both Korea and Japan, with offers of a possible commercial partnership to help market the technology at a fast-growing Far East market in a region which already has some of the world's longest under-sea crossings.

Design specifications
The RTP itself is of cylindrical design that is fixed in place by friction against the tunnel wall following inflation with high air pressure. It features a three-layer fabric design comprising an outer macro-woven webbing, a secondary protective textile layer and a coated fabric pressure-retention layer, or bladder, the main purpose of which is to retain the air that inflates the whole structure (Fig 1).

The structural material is made of Vectran™ fibers which provide excellent strength at a minimum structural volume so that small packing volumes can be achieved. The material also offers flex crack resistance and resists creep rupture. This is important in achieving the design capability of remaining pressurized for several weeks at a time to allow pump-out and tunnel rehabilitation efforts.

Since the structure must be a cylindrical high-pressure system, constructed in its entirety from soft materials that can pack down very small, one of the key design elements is the webbing layer transition to the end caps. To maintain the integrity of the structure around the potentially weak area where there is less available space for the webbings (i.e. at the curvature points at the front and back of the plug), webbings are terminated at a rope interface, which then carries the load (Fig 2). At the extreme ends, where there is no room at all for webbing or rope, integrity depends on an oversized secondary layer.

The system has been tested to withstand a working pressure of 17psi (1.17 bar} and a total maximum pressure of 25psi (1.7 bar). The tunnel plug devices can be housed in discreet capsules attached to the tunnel wall profile and at multiple strategic locations below the waterline along the tunnel alignment. This affords the ability to install intermittent protection for underground structures such as stations, by establishing a kind of isolation system similar in concept to the way bulkheads along a ship's length afford protection against sinking in the event of a hull rupture (Fig 3). It also enables isolation of locations that are particularly vulnerable to water ingress, such as ventilation shafts and access points.

Fig 3. Plugs can be placed at strategic points along a tunnel's length

Development history
The RTP started as a concept at WVU, explained Cadogan. "After coming up with the idea of an inflatable product to hold back water in a tunnel, WVU took the concept to the Department of Homeland Security (DOHS), which was interested in the potential for combating the terrorist threat of sabotage inside a subaqueous tunnel, or release of poison gas in a subway network - as happened in Tokyo in March 1995

"After obtaining initial funding from the DOHS science division, a sail maker made a simple product that, during a test in Washington DC, was shown to be effective in holding back smoke, thereby demonstrating it was theoretically possible that perhaps water could be held back too," said Cadogan. "It was at this stage, four years ago, that WVU looked around for a commercial partner to develop the idea, and with ILC Dover being involved in developing inflatable space habitats for NASA it seemed like a perfect technological fit."

Two years of product design, development and testing culminated in a successful full-scale test in a mockup tunnel at WVU and at pressures of up to 25psi (1.7 bar). No plug system is completely impervious, and the leakage rate of the RTP is some 200-400 gal/min. To put this into context, New York's 300 pumping stations operate routinely to pump out 8-13 million gal/day from its subway network.

But not only is the RTP effective, it is also highly adaptable to the large variations in cross-section and shape as presented by the global tunnel industry. "The RTP is a highly scaleable product, adaptable to multiple cross-sections, diameters and configurations," said Cadogan, adding that the USA's national transportation agency, the Transit Services Authority, is now making all regional and city transport authorities aware of the tunnel plug and its potential.

As TunnelTalk reported in November 2012, tide gates proved effective in preventing damage to the Virginia Midtown highway tunnel, but retrofitting gates into other underground infrastructure would not only be expensive, it could also mean shutting down vital infrastructure during the work. It is also only suitable really at tunnel portals.

Cadogan said: "People in the industry have told us that the reason they are very excited about the RTP is because the modifications required to the tunnel itself to allow fitting are minimal. In very simplistic terms we can slide an RTP off a flatbed truck, install it, and it is then ready to go."

While the RTP, in its current evolution, is designed specifically for installation inside tunnels at strategic points below the waterline (rather than as simple plugs at tunnel portals), Cadogan has no doubts that the technology could be adapted to provide the sort of rapid-deployment solution that might have helped save New York's subway and traffic tunnels during Superstorm Sandy.

"The New York transit system is very porous," he said, "but permutations of RTP technology would undoubtedly have helped."

References

Vulnerability of cities exposed - TunnelTalk, November 2012
Post Sandy pump-out under way in New York - TunnelTalk, November 2012
Superstorm devastates New York region - TunnelTalk, November 2012
Tide gates save Midtown tunnel from floods - TunnelTalk, November 2012
New York tunneling projects weather Irene - TunnelTalk, August 2011
Bangkok examines flood prevention plans - TunnelTalk, December 2011
In search of resilient cities - TunnelTalk, December 2011

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