As Second Ave. residents and business owners continue to bemoan and debate the impact of the ongoing construction, work underneath the avenue is continuing at a brisk pace. The MTA tells me that on Wednesday, Adi, the tunnel boring machine digging out the south tunnel, bore through a record-setting 102 feet. At the end of the day on Wednesday, the TBM had mined 2175 feet and was approaching 83rd St., nearly 10 blocks south of the launch block and 20 blocks north of its final destination.
Lately, the MTA’s pace has been far brisker than the 60 feet per-day average the contractors had anticipated, and those familiar with Manhattan’s geology tell me that this brisk pace is, unfortunately, unsustainable. The rock in the East 80s is far sturdier than the rock in the East 70s, and the TBM’s pace will probably slow to a mark closer to that average pace. Still, for a project slowed by, well, just about everything, this fast mining work is a bright spot of good news.
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Something I’m confused about.
If the rock in the East 70’s is actually SOFTER than the rock in the East 80’s, then won’t it be EASIER to tunnel through?
They have to be more careful with the supports and must make sure the tunnel is shored up properly.
Yeah, tunnelling speed isn’t actually driven by drilling speed but by “post-drilling activities”; the more difficult it is to make the tunnel structurally sound the slower it is. Solid hard rock you just drill and you’re done.
For dramatic examples of this look at the Gotthard Base Tunnel in Switzerland. The stretches of hard, one-formation rock went fast; the stretches of “soft” rock and fault lines have been going very slowly; the surface approach sections, through mixed dirt and rock, have been even slower.
And yet, one of the standard explanations I used to hear for why subway construction is so expensive in New York is that the Manhattan rock is too hard.
ben sorry to bother, but (i think) you had a link a while ago to a chart of the geology along the route of the SAS. i can’t find it. can you put it up again?
So wait, if the tunnel is nearly 1/3 done, why is 2017 an optimistic date, again? Does the finish work/station construction really take that long?
Only one tunnel is nearly 1/3 done so the drilling is only 1/6 done. It won’t wrap until the end of 2011 if it stays on schedule, and then they have to build out the tunnels themselves and do the work on the stations. It shouldn’t take that long, but apparently, it does.
Well, since the tunneling is going so fast, it looks like tunnel one will be done by Dec. They can then back the TBM out and work on Tunnel 2 and perhaps be done with Phase 1 tunneling. You guys are right, the station construction shouldn’t take so long.
On the other hand, if its somehow possible to do the station construction by 2013, the MTA should still give a date much further out in the future. Part of the complaints about these types of projects is when people hear really optimistic projections that cannot be fulfilled in the promised time frame. So I think the MTA should overestimate (instead of underestimate like it normally does) the time needed to complete the projects, and try to find ways to speed them up.
🙁
I’m confused as to where they are tunneling. I see the crews our digging up 2nd avenue for the utilities rerouting. The seem to have dug at least 12-15 feet down.
Is the tunneling machine running down the center of the avenue right under the construction crews? How deep is the tunnel?
The Lexington avenue line tunnel only seems to be maybe 20 to 30 feet below the sidewalk level. Is the 2nd avenue line much deeper?
I asked one of the engineers working on the project. She said it depends on where it is. I think what she ended up saying is that it’s 60 feet underground, but I may remember wrong.
So, the 2nd ave line is going to be 60 feet underground? That is pretty deep.