Despite TBMs being used all the time it is genererally much cheaper to build a tunnel by cut and cover - that is just call in excovators and dig a hole then cover it. when an excavator breaks you push it aside and get another until it is fixed. Want to go faster - just hire more excavators the overall cost is the same since you are done faster.
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
You're often not just building under existing things, but through them. We now have a lot under the roads. The Victorians built the subsurface lines of the London Underground with cut and cover, but Oxford is currently suffering overrunning works to lower a road under the station, because of unknown brick arches and utilities. Even the TBMs are building beside existing tunnels and basements.
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.
> I see that [...] 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.
It also resulted in a rail line that has to slow down significantly to round screeching curves along the path of the road above.
> What projects in developed countries have used cut and cover recently?
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.
> What projects in developed countries have used cut and cover recently?
I know in Sweden the Västlänken project partially used cut-and-cover at least for the part going southwards..
The article said out of 89 current projects 80 were TBM so it isn't surprising you don't know of the exceptions. I don't off the top of my head either.
I imagine it's also a bit difficult to separate it this cleanly, as most bigger projects will probably use a mix of technologies: cut and cover where possible (if it leads to savings), TBMs or other technologies like NATM (https://en.wikipedia.org/wiki/New_Austrian_tunneling_method) for the rest. Even if TBMs are used for the tunnels, cut and cover will probably be used for things like stations, emergency access points and intermediate TBM starting points (of course, the TBM starting points might be future stations).
> I imagine it's also a bit difficult to separate it this cleanly, as most bigger projects will probably use a mix of technologies
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.
Combining is an option. However a large part of the cost of TBM in the initial get it into place and then when you are done taking it out (sometimes you just leave that expensive machine down there). Thus if you must use a TBM the farther you can go in that one dig the overall cheaper the tunnel.
My neighborhood in Seoul got a new subway line while I grew up, I believe they used cut-and-cover (it was almost at the outskirt, so they didn't have anything else underground) and it took several years.
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.
It is a major disruption for a couple years.
It's not a choice of TBM vs cut-and-cover. You also have drill-and-blast. Which method is better depends on a number of factors, including depth and the state of the soil/rock you're going through.
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.
There are many options with different compromises. Cut and Cover comes in many different versions (some of them you build the cover first and then dig under that). However I'm going to double down and suggest that despite the disadvantages everyone else has raised the overall much lower cost of cut and cover, combined with the advantages of a shallower tunnel, makes it the right answer for a lot more situations. Most people claiming to need something else for some local feature are wrong - they can work around that other objection and be better off. Note that I'm not saying the objection is invalid, just that it should be worked around.
Are you a tunneling engineer?
That over 90% of projects are using TBMs strongly suggests they're usually the better option.
They are the political option. They disrupt the surface the least and so there are less political objections even though they cost more around the world.
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.
Cut and cover only works if your subway follows streets. Yeah, I think we should do that where it makes sense but quite often you need to go under buildings or hills.
Most of the time your subway will follow streets though. Your city has built around the existing streets and people have set their lives up around the places they can get to by the existing streets. If there are two points that are near each other as a crow flies but difficult to reach (shore to an island where there is no nearby bridge) people have made sure they don't need to go there and so you rarely gain much by giving them some new ability. While if you build on the streets they can now go places they were already going via your new subway.
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)
That seems a very car-centric or American view, with cities built on a grid.
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...
Streets have existed since cities began is prehistory times. even before cars people needed to get around.
> Streets have existed since cities began is prehistory times
But not necessarily in a form that provides a suitable track alignment for rapid transit.
Thing is, you’re often building these under already-existing things. The Dublin metro project is facing significant planning challenges on the basis that thousands of homes and businesses passed can complain about speculative vibrations from the TBM; with cut and cover it would just be a complete nonstarter (I think one small section is planned to be cut and cover, but most of it will be either bored or surface rail).
That is a poor excuse - places with much worse problems have don cut and cover.
Cut and cover is basically unusable in cities because you'd have to "cut" down all the buildings you'd otherwise tunnel under.
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.
>There isn't some industry saying "yes we could cut and cover here, but we prefer the slower more expensive option of a TBM"!
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.
More information on cut and cover: https://de.m.wikipedia.org/wiki/Neue_%C3%96sterreichische_Tu...
(better translate from the german version, as it has more information and examples)
This is not cut and cover. Cut and cover is when you "cut" an excavation from the ground surface down and then "cover" it back up with dirt after constructing the structure you want below ground. You have linked to an article on NATM, which is a conventional (read: non-TBM) tunneling method.
I see; you are competely right, I am not really familiar with the the technical terms and messed up with just literally translating "cut and cover".
Cut and cover often is cover than dig - there are a lot of options
Cut and cover is what I described: excavate down, build the structure, cover it up. I am aware that there are lots of options, you can browse my comment history and see that I probably have more experience designing and constructing underground infrastructure (and tunnels in particular) than anyone else that posts on HN.
Cut and Cover is often used in transit conversations to mean any variation where you dig from the surface and not the construction method that is cut and cover. These are two different domains talking about different things that are related. Something we both need to be more aware of in conversations.
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Cut and cover is what I said. Your definition (cover then dig) makes no sense. Cover what? If you haven't dug first then there's nothing to cover.
Sometimes a temporary cover is built first to minimise disruption on the surface.
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.
This is not what the cover in cut and cover refers to. This is a temporary bridge erected over a cut and cover operation. When the excavation (the "cut") and structure are completed underneath and the backfill/concrete is placed up to ground surface (the "cover") the temporary bridge will be pulled out. The "cover" part is about covering up the buried structure after it's built to put the ground surface back to where it was.
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.
Some time ago I saw a list of all (i'm not in construcion so I only assume it was all) the different options. many people have innovated many different ways to dig while minizing surface impact. For my purposes if you eventually dig down I consider them all cut and cover - but of course if you are in construction the differences matter. For that matter even to me chose matter - but only as details that we need to argee on before starting work.
I believe the road decks on the Broadway Subway are permanent.
TBMs are for going around problems. Compared to cut and cover you can ignore entire classes of people who would be, expensive, delaying obnoxious stakeholders.
<gestures at the contents of an average HN comment section whenever the subject is public infrastructure>
> it is genererally much cheaper to build a tunnel by cut and cover
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.
I vouched for this comment, but you might want to check your general style of communication and then email dang about your apparent shadow ban.
The tunnel will be around for 100 years. Not much ophortunity cost is lost. And you lose the ophortunity cost of what ever you would spend the saved money on for ever. A longer line for example.
