Podcast: Episode 2 of ‘The Next Stop Is…’

I’ll take a stab at the simple answers, Ken. First, might Phases 3&4 be TBM’d at the same time? The simple answer is that it is possible. The probable answer is that I don’t think it is what they’re planning. Among several considerations is that it depends on what depth they’re engineered to be at in the first place. Most of the utilities in the city run right under the streets – the big exception being the large supply aqueducts.

I don’t know what percent of those phases are Cut-&-Cover, and as those phases travel southward the depth of bedrock recedes. It’s close to the surface up in midtown, and it gets deeper the further south you go before it rises again down in the financial district. There’s a reason that Manhattan has two “skylines” – and it’s not as simple as capricious zoning laws. It’s much easier and cheaper to build a skyscraper foundation where bedrock is 30′ below the surface than it is to build where it is 200′ down.

But to address your question directly, the farther you tunnel from one launch box location, the farther you have to transport the tunneling spoils. That’s the reason that Phase 4 includes a barge port near Hanover. Their expectation is to launch from the south and travel north, and remove the spoils by boat from very close to the launch box. This probably means that the job was engineered with Phase 3 tunneling southward from Phase 1, and Phase 4 tunneling northward to meet up with it. But not necessarily. They could start one TBM northbound, and one southbound.

But it’s a good question to ask, because you’d be right in assuming that a great deal of the time and cost is associated with mobilization. Once a TBM is launched, it IS cheaper to keep it going than it is to stop it, and have to re-mobilize again in a different location, or with a different TBM. It’s one of the reasons I advocate for going cross-town at 125th St. all the way to Broadway, because the time to do it would be when you’re already in the ground. It would cost so much more to have to end the project at the MetroNorth Station in Harlem, and then decide to go cross-town years later. I say tunnel to the Hudson, even if it takes you until decades later to make the station connections.

But I digress.

In answer to your last question, running a wider TBM would cost a lot more – and create a tunnel with a lot of wasted space. What might be more practical would be running the same TBM’s back and forth to create four tubes – either side-to-side, or 2-over-2. Of course, it is much, much cheaper to run 4 tubes now than it is to run 2 now and 2 later. But in any configuration, 4 tubes makes the stations far more complicated and far more expensive.

I was just down under the 94th St. end of the 96th St. station a couple of weeks ago, and the station cavern was enormous compared to the actual tunneling tubes. It was reminiscent of the 1 Line up at 168th St., where there’s a mezzanine above the entire length of the station. I remember thinking that this is what it would look like if we were running TBMs big enough for two trains at a time. It’s an awful lot of material to remove without gaining any more usable track miles.