Thread by @jimdwyernyt

1. Here are some details about the L train tunnel project and how it changed. It’s long. Some of it is geeky. It is an attempt to explain the small pieces that allowed the big shift in the project.

2. The main reason the tunnel was going to close for 15 months was to do a job that, it now seems, really doesn’t need to be done: demolish 30,126 feet of concrete — nearly six miles — and then build it all back new.

3. The concrete is called the bench wall, or duct bench. Right now it carries cables. To remove it you need many people working with small power tools, and spacesuit-environmental protections. It's a monstrous, slow job. The bench is the honeycomb part of this drawing.

4. We don’t need bench walls, so tearing six miles of them down and building them back is a waste. Moreover, most of the bench walls are fine — at least 60%. The new plan would leave most of it where it is, and demolish only small portions that pose a danger..

5. Important: These benches are IN the tunnel, but they are NOT the tunnel itself. They are fixtures. “The bench wall is not serving a structural purpose,” said Andrew Smyth, Columbia engineering prof.

6. The tunnel, age 100, is structurally sound. Tests taken after the flood found that the tunnel lining can support 5000 pounds per square inch, strength comparable to modern bridges. “It's pretty amazing,” says Jerry Jannetti of WSP, the MTA's consultants on this since 2013.

7. When the tunnel was built a century ago, the purpose of the bench wall was to protect power cables from fire. Technology has changed.

8. Remember the Swissair flight that crashed off Novas Scotia in 1998? Disasters lead to reform. A fire apparently started in an entertainment system and spread along flammable power lines. New standards require fire resistant sheathing of cables on airliners.

9. Similar standards apply for cables in train tunnels. Now they don't need concrete ducts. They can be slung in racks. This approach is widely used. A fire protection code applies to rail tunnels.

10. Cables are hung in racks in the No. 7 extension, for instance.

11. So, hello racks, goodbye duct bench. But parts of the bench might fall on the track. They have to be removed or buttressed. To figure out how much is a hazard, ultrasound testing was done last week, to be followed up by field inspection, says WSP’s Jannetti.

13. The pieces that are not-dangerous but not in prime condition will be wrapped in a material called fiber resin polymer material — a lightweight substance that strengthen deteriorating structures. It has strength like steel but not the weight. It lasts decades, says Smyth.

14. This stuff has been used in California since the 1994 earthquake to buttress bridge columns. It is used in marine applications, on piers, for instance. “Usually it’s being used to carry real loads that are critical to life safety - to keep a structure standing,” Smyth said.

15. In NY, this material is in use on the Kosciuszko Bridge; the Meeker Avenue viaduct beams; and the RFK Bridge, Jannetti said.

16. It has also been used in the No. 7 line extension, NYCT President Andy Byford says.

17. It buttresses elements of the 168th Street and 181st Street stations on the No. 1 line, per Jannetti.

18. “It’s incredibly conservative to use it on the bench wall,” Smyth said. In this context, conservative means "taking no chances."

19. Moreover, Smyth said, it will be attached in two different ways simultaneously: 1. with adhesive and 2. with mechanical fixtures.
(Redundancy.)

20. More redundancy? Fiber optic monitors — used in many modern structures — will be installed to signal if there are changes to the abandoned concrete. But that might fail, right?

21. In addition, LIDAR -- a laser surveillance system that can be attached to the front of trains -- will also be able to measure any movements in the concrete.

22. Perhaps fatal flaws will yet emerge. I might well have overlooked some weak spots or not kept up with the technical language. But the notion that this is a band-aid solution seems untrue at its core. These are all serious engineers looking for a responsible solution.

23. This was a collaboration between the academic engineers and the original engineering team —“people that have scoured the inside of the tunnel, agonized over data,” Jannetti said.

24. Working groups are dividing up the project to move this concept into the fine grain of actual engineering.

25. “The big lesson here — and this is probably all infrastructure projects,” says Jannetti, “is that at some point having an independent team come in and do a fresh look probably isn’t a bad idea.”

26. He continued: “You might see independent reviews happen more in New York. You might see this nationally.”