Home 7 Line Extension A tale of two tunnel boring machines

A tale of two tunnel boring machines

by Benjamin Kabak

Actually, because the 7 line extension had the pleasure of two tunnel boring machines digging at once, this post should probably be called “A tale of three TBMs,” but the literary reference just isn’t the same. Anyway, I digress.

This evening, I found myself walking south from 46th St. on 10th Ave. heading to meet some friends in the West Village. As I walked past the 41st St. area, I did an informal survey of the area and counted at least five buildings either brand new or under construction as well as countless other developments that were just a handful of years old. These were the buildings housing the residents who would stand to benefit with a station at 10th Ave. and 41st St. Maybe REBNY can deliver for them after all.

Little did I know that, a few hours earlier, New York City officials had gathered underground near that very same spot to celebrate a milestone. The second of the two tunnel boring machines working its way from 11th Ave. and 34th St. to 41st St. west of 8th Ave. had broken through the tunnel wall. With the other TBM reaching its destination in mid-June, the boring for the 7 line extension is complete, and now the finishing work can begin.

Shortly after 4 p.m. this afternoon, the TBM reached its destination, and Mayor Michael Bloomberg, MC-ing the ceremony because the city’s billions are funding the project, toasted the progress. “It will bring us one step closer to the moment about three years from now when the number 7 train will stop at Times Square and then continue on to the new 34th Street station on the Far West Side,” he said. “And that moment will be the culmination of all of our efforts to realize the full potential of the Far West Side and revitalize an area of the city with new business and new residents and parks and open spaces.” Despite these lofty goals, the second station for this $2.1 billion expansion remains in limbo.

For the MTA, yesterday’s ceremony marked the end of a 13-month period of digging, and the MTA is ahead of the schedule. The boring, which was supposed to last 2-3 years, took six months due to cooperation between the MTA and Port Authority. With only the stations to construct and the ventilation infrastructure to build, the MTA and City are racing the clock. Will the 7 extension be open before Mayor Bloomberg’s third term is up?

A shot of Second Ave. at 90th St. where a drill probe punctured the street. Photo courtesy of The Launch Box.

Meanwhile, across town, the boring underneath Second Avenue has faced its own set of challenges. Progress, as we know, has not be swift, and a little over a week ago, crews ran into a bump in the road with a drill probe popped through the streets of Second Ave. Ben Heckscher over at The Launch Box wrote an extensive post on the incident. Here’s his retelling of the vital bits:

A drill that was connected to the Second Avenue subway TBM went off course at about 3 a.m. [on July 8] and broke through the surface of 2nd Avenue at East 90th Street.

No injuries were reported and luckily the drill did not pierce the 30-inch gas main, the 36-inch water main or any of Con Edison’s cables below the surface of Second Avenue in this area. If the drill had pierced a gas main, the consequences clearly could have been tremendous.

During the TBM mining operation, which is taking place about 60 feet below street level, the sandhogs perform a process known as “probing” using a rock drill to determine ground conditions and water inflows ahead of the TBM. Apparently one of the probes that they drilled went in an unintended direction and ended up bursting out into the open air at East 90th Street.

I spoke to Kevin Oritz at the MTA shortly after the incident, and Ortiz told me that the authority had asked the contractos to cease probe drilling while the authority investigates the accident. “We don’t anticipate this to have an impact on completing of the first run of the TBM before the end of the year,” he told me.

Still, there are real and legitimate concerns at play. South of the Launch Box and north of the 86th St. station, the MTA has not relocated utilities, and city records do not paint an accurate portrait of the goings-on underground. As Heckscher pointed out, had the drill probe ruptured the gas line, the subsequent explosion would have been catastrophic. The Upper East Side is lucky indeed that the probe drill missed these vital arteries.

And so slowly onward churn the TBMs underneath Second Ave. As the 7 line extension moves ahead on schedule, the SAS project lumbers forward with an unknown timeframe and drilling incidents that stymie progress.

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pea-jay July 16, 2010 - 1:09 am

how is it that these two projects are faring so differently?

Scott E July 16, 2010 - 7:48 am

In this particular case, I think it’s just a matter of dumb luck. The same contractor is digging the tunnels for both (which makes me wonder why a mission-complete #7 TBM isn’t moved to Second Ave, but I digress).

How long are these probe drill bits? I would imagine that, for a TBM to work, it must be far enough beneath the surface, deep enough in the bedrock, to prevent a collapse. This patch in the road looks like a simple pothole-filling, which as we all know, doesn’t last. I also wonder if this was operator-error or a machine malfunction.

Alon Levy July 16, 2010 - 8:38 am

Because of the lower population density along the 7 extension’s route, the TBM there can afford to be less careful about where it’s digging. At least, that’s what one of PB’s engineers told me.

rhywun July 16, 2010 - 3:25 am

Was wondering that myself. I can only guess that the city-funded project is probably (somewhat!) less hindered by the mammoth bureaucracy that accompanies any sort of building here.

bob July 16, 2010 - 11:38 am

No. Both are being built by MTACC. The same bureaucracy. What is being forgotten is that the 7 line started construction earlier, so they have worked out the kinks. Once 2nd Ave works through their issues (which are more complex due to where they are working) the pace will pick up.

But I don’t understand – if the probe is supposed to be going horizontal, how does it angle up for a great enough distance to hit the surface without being noticed?

Redbird July 16, 2010 - 1:17 pm


This type of occurrence is not unheard of when drilling through rock (both with small drills like this or larger steel piles for foundations).

The drill likely hit a fault line between softer and harder rock and started riding up along the interface w/the harder and softer rock. If the contractor was drilling 100′ of probe, it is conceivable that the drill steel began turning up with the first 20-40 feet of the run and then they drilled 60′ up before noticing–they probably thought they were going straight.

rhywun July 17, 2010 - 6:41 am

I was thinking more of the funding than the building. As in, the 7 line extension seems to have fewer layers of bureaucracy because the city is funding it rather than Albany. But point taken about the starting dates.

Andrew Sidrane July 16, 2010 - 9:54 am

I thought they were using two different types of TBM because the ground is different under 2nd ave than the ground under 11th.

Anon July 16, 2010 - 10:50 am

Speaking of tunnels, this tunnel just closed according to the Director/Producer http://www.whatsbehindthewall.com/

Joe July 16, 2010 - 11:04 am

You mean, the Atlantic Ave tunnel closed for tours? (not to get off topic, but I just visited it recently)

Anon July 16, 2010 - 11:31 am

“Mark Grande
July 16, 2010 at 8:31am
Subject: The Tunnel Has Closed
Due to circumstances beyond our control, The Tunnel film has been canceled. If you happen to see a a tv special on the Tunnel in the next year or two we had nothing to do with it. Stay away.”

Bob’s tunnel tours appear to be open

Clarke July 16, 2010 - 3:02 pm

Wikipedia says that the 7 subway extension will have service tracks that extend to near 23rd Street. If this is true, then does this mean that tunnel boring is NOT actually complete and the MTA just needed to get some good press?

Redbird July 16, 2010 - 3:04 pm

No, the TBM’s started at that end of the job and worked their way north, then east to 41st Street and 8th Ave. They are finished will now be partially disassembled then backed through the tunnels to their launching point on the far West Side.

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