(mostly ex-) construction technician here. There is also a "form factor". TBM's dig "round" holes. The cross section of those Stockholm tunnels (I know, I ac

(mostly ex-) construction technician here. 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