Exactly. When I get to a cut and cover station, it's often just a single flight of stairs down. It's often only 15 seconds or so to reach the track and wait for the train.
Stations along bored tunnels sometimes take as much as 5 minutes to go from street to train. One I used to regularly use had three separate multi-story escalators, in addition to long walkways between them.
So even if cut and cover disrupts businesses and car traffic, I can't help but wonder if decades of extra delays to reach the tracks from many thousands of people a day doesn't wind up outweighing that.
Is there a reason bored tunnels need to be substantially deeper? Does boring lead to structural stability issues that the installation of the tunnel structure doesn't mitigate?
I do know that tunnel boring machines can disturb the surface, and that substantial effort goes into minimizing this. (That is, minimal surface disturbance isn't automatic on account of the fact that you're boring.) I believe it sometimes has to do with how much pressure they need to apply to the tunnel face; the ideal pressure for cutting can lead to sink or swell on the surface. Presumably this is less of an issue the deeper you go. I don't know much about this stuff though. This is maybe relevant:
> This thesis summarizes and evaluates the performance of Tunnel Boring Machine (TBM) construction in Singapore's MRT network. Surface settlement induced by the tunneling process can cause damage to underground utilities and foundations and buildings and/or disrupt daily life by damaging roads and pavements, and is used in this thesis as a measure of performance.
So I did a little research on this, and it seems to be two main things.
Partly yes it seems to be a question of stability -- you're not going to bore and leave just a couple of feet of soil above, because of the way you want the arch of the tunnel to support itself, and that's not going to be strong enough. And close to the surface you've got looser soil etc. Going deeper does provide greater stability, in a way that isn't needed for cut-and-cover because that is supported by steel beams, not arches. But this doesn't generally require you to go crazy deep.
The bigger factor behind these crazy-deep tunnels seems to be pre-existing infrastructure. If you're doing cut-and-cover, you're digging out all of the infrastructure first and then replacing it -- it's highly disruptive. While if you're boring a tunnel, you need to go below everything that already exists -- sewers, water lines, deep building foundations, pre-existing subway lines that it crosses underneath, etc.
> The bigger factor behind these crazy-deep tunnels seems to be pre-existing infrastructure. If you're doing cut-and-cover, you're digging out all of the infrastructure first and then replacing it -- it's highly disruptive. While if you're boring a tunnel, you need to go below everything that already exists -- sewers, water lines, deep building foundations, pre-existing subway lines that it crosses underneath, etc.
Makes perfect sense!
And yeah, it's yet another case of path dependence: we'd build far different cities if we had the option of designing "from scratch" rather than layering retrofit over retrofit under retrofit.
In an ideal world, we'd have tunnels connecting B1/B2 of every major building in a city, and many people's trips would be elevator-transit-elevator.
Stations along bored tunnels sometimes take as much as 5 minutes to go from street to train. One I used to regularly use had three separate multi-story escalators, in addition to long walkways between them.
So even if cut and cover disrupts businesses and car traffic, I can't help but wonder if decades of extra delays to reach the tracks from many thousands of people a day doesn't wind up outweighing that.