Anyone have insight into different tunnel-building technologies? In particular, when is a boring machine a more appropriate solution than drilling-and-blasting?
In this case, the tunnel is very long and deep, magnifying the unknown territory risk. Also, the bedrock in most of Scandinavia (and, I presume, Stockholm) is both very shallow and mostly granite, which is extremely hard and thus not a good fit for a TBM.
Source: My childhood in Helsinki was punctuated by the periodic sound of blasting rock as they cut out what was supposed to be the new central station. Overall, Helsinki is basically granite Swiss cheese due to parking garages, pedestrian tunnels, basements, bomb shelters, military tunnels, the metro etc and AFAIK all of it has been drilled and blasted, not TBM'd.
There is also a "form factor".
TBM's dig "round" holes.
The cross section of those Stockholm tunnels (I know, I actually worked on a few tenders for them) is very wide and flat.
Generically speaking for a TBM to be competitive you need to have a "minimal" length of tunnel, a TBM has huge - really huge - mounting/assembling (and later disassembling) costs and quite a lot of space at both ends of the tunnel.
Specifically, both (they are two tenders, E302 and E308) tunnels have not a "direct access", there are two "service tunnels" (of a much smaller cross section, rather "steep" and non-linear) that allow access to the main tunnels from the outside, and the outside area available is anyway small, so a TBM could not have been used anyway, even if the cross section wasn't so "flat".
Back to generically speaking, the rules of thumb (approximate, only to give you an idea) for choosing NOT a TBM are:
1) anything shorter than 5-7 Km
2) anything larger than diameter 12 m or not round or simil-round
3) anything in terrains that are too "hard" or too "soft" [1]
To give you a rough approximation, a (single bore) tunnel made with traditional excavation can have a production of 100-120 m/month, a TBM can usually dig around 400 m/month, so it is much faster.
There are also "mixed mode" approaches, where you (quickly) bore a smaller "pilot hole" in the center of the tunnel by means of a small TBM, usually 3.8 or 4 meters in diameter, so you have a 1:1 representations of the terrains and can (from the small bore) perform any kind of consolidation/impermeabilization etc., then you enlarge the hole by traditional blast and drill.
The much reduced amount of explosives needed for enlarging the hole and a great semplification in the ventilation plant , besides the possibility to always work on "known" terrain make this a very good approach in my experience, and the blast and drill enlargement of the pilot hole is much faster than "full face" excavation, the same rough estimate is around 150-180 m/month.
[1] or more generally where large variations in the nature of terrain is expected, a TBM for "soft" is different from one for "hard" and viceversa
Drill and blast are the most common by far, especially in mining industry where they are lots of twists and turns, and no-one cares about noise etc.
> Additionally, the original drill, which was said to drill 100 meters per week, broke down after drilling only 18 m (59 ft). The rock was too soft, so the machine could not use it to pull itself forward. The contractor tried to drill traditionally, but had to spend a lot of effort on sealing the water leaks. The contractor went bankrupt and a new contractor, Skanska, was contracted. The new contractor had similar trouble but a better contract that gave compensation for troublesome rock conditions.
TBMs can also bore in soil or sand.
Aside from disturbance and raw feasibility (not sure you can D&B underseas) the main split point is going to be the length of the tunnel. TBMs mean large up-front fixed cost but have a lower price per distance and higher time efficiency, so D&B is more suitable for shorter tunnels (how short will also depend on the diameter you need).
That aside, TBMs are awesome, I got to visit one as a kid (the operator gave tours of the TBM during the few weeks of work prep' before digging actually started in earnest) and it was a great experience, they're impressive bits of machinery & technology.
There is also the setup- carting in the pieces of the TBM and getting them into place, etc.
Because a TBM itself is a large capital investment. A small TBM (~7m diameter) is ~10m€, a large TBM probably reach 100m€. This is money you have to pay upfront before the TBM even reaches the dig.
The capital investment of D&B is a (few) jumbos which are cheaper (even large and expensive jumbos are unlikely to breach 2m€ brand new), more flexible (they're also used in mining operations) and less dependent of the manufacturer — thus easier to rent or buy used.
> can't you resell it afterwards, or rent it instead of buying?
The "back end" (gantries) can be sold back to the builder and refurbished, the "front end" (shield and cutter head) is thrown away (some are used as monuments) or recycled: https://www.herrenknecht.com/en/services/global-services/tbm...
That's still expensive as hell, and you've still sunk the money in until the builder has examined and bought back the gantries after the project has ended.
For long tunnels, it isn’t uncommon to have two TBMs that meet in the middle and are abandoned there (they bore a side tunnel long enough to contain the TBM, and the side tunnel is closed behind them)
For example, see https://untappedcities.com/2017/02/09/the-200-ton-tunnel-bor...:
”Burial of such a large machine was unprecedented in New York City, but is common practice abroad, reported the New York Times in 2011. The Spanish contracting company working on the project deemed it would be more cost effective to leave Seli in place. It was estimated that removal would cost millions and cause delays in existing construction projects. Dr. Michael Horodniceanu, the president of MTA Capital Construction, estimated at the time it would cost $9 million to remove Seli, which had cost somewhere between $6 to $8 million new.”
With Seattle's design/build contract, the construction company owned the TBM. To them it's just a tool whose cost is part of the construction package.