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Re: New MTA statement on L-train project

Posted by Stephen Bauman on Sat Jan 12 08:02:45 2019, in response to Re: New MTA statement on L-train project, posted by Joe V on Fri Jan 11 19:03:35 2019.

The tunnels have a alkaline-silica reaction (sea water is reacting with the cement in the shoulder walls) causing it to expand.

You need to differentiate between the tunnel liner and the duct bank. The tunnel liner, which surrounds the cast iron tunnel rings, was inspected and is intact. It's only the cement that comprises the duct bank that's damaged.

Neither the previous plan nor the current plan calls for replacing the tunnel liner, which isn't damaged. There are plans to patch any cracks in the liner, that's part of both plans work statement. The cracks did not result from Sandy damage but from normal wear and tear and lack of maintenance.

The duct bank does not provide any structural support for the tunnel. It merely encases the cables. One of its purposes was to provide fire protection by confining any electrical fire within the duct bank. That function can be provided by wrapping the cables in fire retardant material. That's part of the new plan.

Its other purpose was to provide a catwalk for passengers to evacuate the tunnel. That purpose has been pretty much compromised because of obstacles that have been placed in the tunnel since its opening. The proposed plan keeps one of the catwalks free of obstacles by locating the cables on the opposite side.

The duct bank's danger comes from its crumbling cement. The problem is twofold. The crumbling cement can fall on the tracks and cause a derailment or similar mishap. The crumbling cement could not support weight to function as a catwalk. The fiberglass encapsulates the duct bank and contains any crumbling cement within it. This prevents any duct bank pieces from falling on the track. The fiberglass also acts as an exoskeleton to support the weight of any evacuees.

Rebuilding the duct bank did not provide any additional protection against a Sandy repeat. If salt water were to accumulate within the tunnel, a similar result should be expected. The salt water would penetrate the duct bank and corrode the cables. Locating the cables on the wall provides a bit more protection. The cables would be located higher up than they had they been located within the duct bank. More salt water accumulation within the tunnel would be required to damage the cables.

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