tbms are cheaper if you must go deep, but deep is a negative for subways since people must get down which takes time (faster is always better).
It is claimed that cut and cover is toa much surface disruption - but cut and cover is much faster and so the disruption to any one place can be quick enough. Better to manage disruption than eliminate it.
when a tbm breaks your project is delayed while you wait for parts and then fix it in place.
Tbms do go under everything else, but experience proves you can find things and go around them quick enough (sometimes there is a delay as somethingeof archeological interest is found - but evperience shows this is still cheaper than a TBM.
Yes TBMs are cool and useful but the simple shoud not be over looked
Coolo, you are building tunnels in a city that doesn't exist yet.
Start here - https://en.wikipedia.org/wiki/Opportunity_cost
Do you have any interesting examples of modern day cut and cover that are not part of a TBM run?
> TBMs are cool and useful
They are what is needed to move humanity forward.
You have trillion $ centers that using 3D you can add massive amounts of access to. This is not like reclaiming land from water, which is ~stealing, this is value adding through topography, it's creation.
What projects in developed countries have used cut and cover recently? In trying to find out, I see that HS2 under west London and the Canada line under Vancouver chose tunnels over cut and cover because it was cheaper.
Canada Line was mostly Cut-and-Cover - only the bits below downtown and crossing below the water were bored, the bulk of the underground was done cut and cover for cost and speed to make sure it opened for the 2010 olympics.
It was not a popular choice - not really announced before the project was approved, and local businesses along the route took a big hit.
Vancouver's current Broadway Line Extension is being done with TBMs to avoid the impact that the cut and cover canada line segment construction had.
I know in Sweden the Västlänken project partially used cut-and-cover at least for the part going southwards..
Case in point – the Karlsruhe tram tunnel (listed in that dataset as simply "Tunnel Boring Machine") used a tunnel boring machine for the main east-west tunnel, but a combination of NATM and cut-and-cover for the north-south branch. The stations and the associated road tunnel project were all cut-and-cover, too.
Not really a new project, but parts of the subway in Stockholm are cut and cover. One of those tunnels (from the 1930s) has been leaking in water for some years and is up for a total overhaul, so basically digging up everyting and doing a new cover.
The section is 8 m wide and 925 m long, projected timeline is 6-7 years starting this fall. It will be a massive project, as one of the busiest streets in Stockholm is directly on top of it.
(better translate from the german version, as it has more information and examples)
Here's an old example of an umbrella bridge over Oxford Circus during the construction of the Victoria Line. There's a longer video out there of the construction of the Victoria Line that covers this in more detail.
Some of the stations on the Broadway Subway Skytrain extension in Vancouver use a similar approach, where half the road is closed and a road deck is built, then traffic is shifted over to the new road deck while excavation takes place from the side. There's some great views of this while riding the bus.
Edit: If the road deck is left in place permanently then it is a permanent elevated road deck built over a cut and cover tunnel. I can see how some people might consider this the "cover," but that is atypical in the industry and not what people are usually talking about when they say cut and cover. I'll concede that this approach sometimes happens, but I wouldn't call it "often" like GP does and I'd also note that even under this scheme the final surface/cover (e.g., the roadway) is completed after the underground excavation and structure are finished, meaning that the cut still precedes the cover.
The most common terms for this in the industry are "lid" or "cap." As in, you put the lid on, or cap, the excavation or cut and cover tunnel.
Note that I said street and not highway. Some highways (limit access) are good for longer trips, but your subway would be better off using a side street parallel to the highway since while most people use the highway your subway doesn't need to get to any point on the highway while points on the side street - while of limited interest have at least some interest. City transit design is a complex subject that whole books are written about (sadly the people who are in charge don't read them)
Older cities without existing grids can have older metro lines following streets (a limitation of the time they were built, by cut and cover) but newer ones generally don't follow streets. They provide more direct routes, and new crossings over rivers or to islands.
Look at Prague or especially Copenhagen for example.
https://openrailwaymap.org//mobile.php?style=standard&lat=55...
But not necessarily in a form that provides a suitable track alignment for rapid transit.
<gestures at the contents of an average HN comment section whenever the subject is public infrastructure>
So, while it's doable, it's not exactly "Just dig a hole, build the tunnel and cover, done." It's a major disruption to shops along the street.
A problem with TBMs is it can be difficult/costly to secure the tunnel sufficiently against water ingress. If you mess up that one the cost of your tunnel can easily double or more.
Here in Norway we recently built a railway tunnel with TBMs (Blixtunnelen) and it's having problems with water ingress. A fairly mild problem relative to how bad these can get, but it's enough that the tunnel constantly has to be closed for repairs to the railway infrastructure due to water drips.
That over 90% of projects are using TBMs strongly suggests they're usually the better option.
Tunneling engineers do what the customer says. They will tell you what the options are and what they cost then let the customer choose the evpensive option if they want. Their job isn't to choose between options it is to eliminate the impossible (unsafe) ones and let the customer choose the pros andecons of the rest.
There isn't some industry saying "yes we could cut and cover here, but we prefer the slower more expensive option of a TBM"!
(I see NATM mentioned; there have been safety issues https://www.hse.gov.uk/pubns/natm.htm )
The HS2 cut and cover tunnels were all in greenfield: https://www.hs2.org.uk/building-hs2/tunnels/green-tunnels/ ; much of the cost there goes on planning and documentation, an under-appreciated cost. It's also questionable as to whether they were needed at all; a plain cutting with embankment open to the air would have been fine from a civil engineering point of view, or even in many places just flat track, but the tunnels were planned because people objected to a railway running through fields.
At least for Vancouver, there was absolutely an industry arguing for the slower-more-expensive TBM option on a route that followed exactly a road (the Broadway line extension) - local businesses along the route. The previous line which was mostly done by Cut-and-Cover (the Canada Line) had a very major impact on businesses along the route for years.
There are plenty of people who will acknowledge that something is cheaper _overall_ but the impact on a small group being higher can make them extremely vocal, and that has to be managed in public projects.
Anyone know what’s going on here? Intuitively you’d expect two small tunnels to be cheaper; you’re moving a lot less material.
How does the volume of material removed not directly impact machine and crew costs?
https://www.robbinstbm.com/wp-content/uploads/2017/07/Smadin...
It also adds disposal costs. The most obvious argument I can think of to support this approach is if the excess material could be sold at a profit. But that seems dubious.
That is and assumption and not self evident. There are lots of reasons one might want to use many cutters. It is a common practice in wood and metal machining, for instance[1]. Having additional cutting surfaces reduces the rate of wear, provides redundancy in case of chipping, increases the mean time between required services, allows cutters to run cooler, reduces chatter, improves surface finish, and a host of other reasons.
Seems like you could research it.
1: https://www.advancedmachinery.co.uk/content/product/8500_0_z...
Sort of CAPEX, so to speak. But what about less OPEX later, because of more flexibility, less maintenance burden, when other demands arise or conditions change?
If the tunnel is wide enough, two tracks can be built side by side, and another level of two tracks on top of it. That makes room for a total of four (potential) tracks, with the possibility to switch between them not only horizontally, but also vertically via ramps (inside that tunnel profile). It also enables single platform stations for one track on one or two of the two possible levels in such a tunnel. Without any further 'big digs', or blocking the line altogether.
Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper. You don't get to pretend that larger material removal rates are free. You don't get to pretend that the 7/4 more cutting wheels don't cost more money to replace. The 7/4 more energy to run the TBM isn't free.
Yes, the larger machine would have to have larger material removal rates just to keep up with the smaller machine. But that makes it more expensive, not less.
More expensive than one, potentially less expensive than two. Happens in manufacturing and industry all the time. Transport cost alone could be enough to account for it.
> Call me stubborn, but I need a better theory to explain why the bigger TBM would be cheaper.
Sounds like you should have a chat with the project lead.
What I’m curious about, though, is what, if anything, changed to make this single-bore approach suddenly popular. A lot of new metro lines seem to be built that way, but old (and not, like, 19th century, even most 20th century stuff) always seem to be double-bore.
I think Barcelone Line 9 is the one that popularized it. I suspect the other factor is that they figured out how to make large TBM.
Which states that due to sandy soil concrete was injected with lances. It was done by a French company.
Might be the same tunnel?
And the Erie Canal dig the sections through swampy ground in the winter. That was partially to avoid Malaria. Circa 1825
Nobody has explained to me what the boring company has actually done innovating TBM, the machines themselveds distinct from a cash injection and some energy from Musk as lead investor. TBM are flow process. It's a pipeline of inputs and outputs. It's scheduling.
The LV tunnel isn't a complete success either. It's not operating completely as promised yet is it?
The only thing they’ve done with their Vegas tunnel is to make it cheaper by making it too small to have proper emergency egress, but they can get away with it since with just their chauffeured Teslas going through it only fitting a few people each (making the system extremely low-capacity), they can’t really fit many people in at once, and it’s short enough so combined that makes it not that much of an issue.
In the newest section of the Vegas tunnel they have now only built a one-way tunnel instead of a tunnel for each way, making it cheaper again but low-capacity to a basically comedic level.
Some more discussion then: https://news.ycombinator.com/item?id=37836889
Today, rock tunneling machines have achieved tunneling rates of over 700 meters per week, and soil tunneling machines have achieved rates of over 200 meters per week, though this is dependent on the size of the TBM
Ie, that rock tunneling can be > 3x quicker than soil tunneling.
There is a half mile long tunnel in the California desert that an addled miner made with hand tools and dynamite over 38 years. Self funded and part time.
https://en.wikipedia.org/wiki/Burro_Schmidt_Tunnel
Then digging through unstable rock with water intrusion and danger of flooding the costs have to go way way up.
Probably way more expensive but you could go faster and don’t need to deal with hauling the material out. It might glassify the surrounding soil and make its own support structure too.
Get a couple kW in the same wavelength (10640nm) with a proper adjustable focus (movable diopter) and a galvo and a vacuum assist and you can just start raster-scanning your way through the rock.
[1]: https://en.wikipedia.org/wiki/Meristem#Root_apical_meristem
In the end they thrust a group of horizontal steel pipes to stabilize the soil and dug beneath it - and those pipes were 15 cm(!) below existing line 3.
Had this thought the other day and wondered if there would be any advantages over metal cutting teeth. From the article, I gather that replacing cutting teeth isn't that much of a problem nowadays so that mitigates one advantage it might have.
