The simple engineering that will make NYC's L train continue to roll

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Since the last of the brackish waters came out of the Canarsie Tunnel following the great Sandy storm of 2012, New Yorkers are preparing for the pain. Public transport officials have long warned that the 94-year-old tunnel water damage, filled with metro equipment that was just as old, would eventually require a long, painful and deeply disturbing rehabilitation. It is the tunnel that crosses the East River and carries many of the 400,000 daily users of the L metro into popular areas of Brooklyn, such as Williamsburg and Bushwick, to Manhattan.

The operation was scheduled for April 2019, while the L-train section that was carrying New Yorkers through Manhattan and Brooklyn was to be closed for a 15-month repair job. In front of what they have officially called "L-pocalypse", local authorities have come up with many projects aimed at strengthening bus service, encouraging cycling, creating new ferry routes and all what they could think of to prevent all these commuters from driving. and making the already bad traffic completely catastrophic.

These plans (as well as the wildest proposed by concerned citizens) became much less necessary on Thursday morning, when Governor Andrew Cuomo convened a surprise press conference to proclaim that no, train L will not close completely and, yes, it will remain to be fixed for the future.

The new plan for the next few years is to keep the train open and operate normally during the week, while making repairs at night and on weekends (the details are unclear). The Metropolitan Transportation Authority's board of directors, which manages the subway, has not yet adopted the new plan proposed by a commission made up of half a dozen university-based engineers. Columbia and Cornell, whom Cuomo met last month, two years after the decision. done to close the line. The agency issued a press release on Thursday afternoon in which she had "accepted the recommendations."

It is clear that politics is at work, but it is unlikely that New Yorkers will be interested, as long as the subway continues to run. And if so, it will be thanks to two metro engineering infrastructure elements: power benches and cable shelving.

Let's start with benchwalls. If the train stopped in the tunnel and you had to go out, it is the concrete that runs along each wall and looks like big benches on which you will walk. Facilitating emergency exits is one of their main functions. Without them, you must jump off the train, land and risk hitting the third rail. The bench walls also retain most of the products essential to the proper functioning of the metro, including power and communication cables. When the workers were building the line, which went into service in 1924, putting the cables into concrete was the best way to protect them from things like starving rats and water damage.

During the last century, these protective walls began to deteriorate, a process accelerated by the floods caused by Hurricane Sandy. Explaining its complete shutdown plan in 2016, the MTA said the tunnel bank walls "need to be replaced to protect the structural integrity of the two tubes [east and west] who transport trains through the tunnel. "

Replacing these elements involves scrambling concrete, removing rubble, replacing interior wiring, laying new concrete and drying it. It's a job you can not do at night or on weekends, as each section lasts for several days. And you can not run trains without leaving a gateway to lead people to safety in an emergency.

The new plan is to give these benches a bit of downshifting. They will still be used for emergency exits, but they will not hold the cables. Instead, the L-train will use a "cable-wiring" system, in which new power and communications lines will be hung and connected to the sides of the tunnel, above the protective walls. It turns out that their protective sheath has progressed since the era of prohibition. "We have made tremendous progress in materials," says Peter Kinget, president of Columbia's electrical engineering department, who served on the committee1. If the liner catches fire, it does not emit harmful smoke. It is impervious to vermin and H2O, thus avoiding the need for concrete shielding. The workers will also support the sections of the bench that collapse with the fiber-reinforced polymer, Cuomo says, leaving the old inactive cables buried inside.

This decoupling of benchmark tasks is a big problem because it makes the job much easier to perform. You can reduce the service at night and on weekends (by running the trains in one of the twin tubes of the tunnel) and let the workers slide underground, installing the racks and new cables segment by segment. During normal hours, the train operates as it usually does, drawing energy from cables already in the walls of the seat. Once the work is complete, the MTA will switch the trains on the new set of cables.

According to Cuomo, cable holders were used for the new London, Hong Kong and Saudi capital Riyadh metro lines. This would be his first use in the United States and the first time it would be used to repair an existing line.

"It's a smart solution," said Matt Cunningham, a civil engineer and global infrastructure director for Canadian engineering firm IBI Group. It's cheaper and easier than replacing all table walls filled with cables, and it's a proven method. "It's going to work."

Which raises the unanswered question of why this idea is surfacing now. Why not before the MTA decides to shut down completely, then spend two years preparing it? This makes Cuomo the politician who has avoided the L-pocalypse spitting traffic – but one can also wonder why he did not come to the rescue earlier. (He's been a New York governor since 2011.) At his press conference, he presented this solution as a new solution, which is true if you compare it to the techniques used to build the metro in the previous century, but not so much. you take a slight, narrower view. "It's not new technology that's just coming in," says Cunningham.

Of course, limiting service during the night and the weekend to remedy this situation will always be a source of suffering, and the MTA has a terrible record of mismanagement of this type of operation. We must therefore doubt any promise concerning deadlines or costs. "You do not get a root canal on five teeth, you have a root canal on three teeth," said Allan Rutter of the Texas A & M's Transportation Institute. "There will be pain."

In infrastructure such as dental surgery, you must accept some drilling and discomfort. But less is definitely more.

1Story was updated on Saturday, January 5th at 12:50 ET to correctly mark Peter Kinget's position.


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