More on Station Costs

Talking to Marco Chitti about the history of Italian construction always fills me with hope. He’s been gathering data about metro construction in Milan and Rome, and told Elif, Eric, and me about the issue of building through constrained areas. Historic city centers are constrained because tunneling can damage buildings – the first two lines in Milan, built in the 1950s and 60s at abnormally low costs, caused some damage to buildings, since they involved cut-and-cover under streets only 12-15 meters wide. The good news is that tunneling with a tunnel boring machine is fine now. Stations remain an enormous challenge – but the conversation did fill me with optimism about future construction in cities that were not global imperial capitals 2,000 years ago.

TBM technology

Tunnel-boring machines have advanced to the point of being archeology-safe. Italian heritage protection laws from the 2000s forbid any risk to historic buildings and historic sites, but TBM technology at this point allows preserving artifacts. It involves injecting a gel ahead of the cutting edge, which is not supposed to be a cost-raiser.

The result is that tunneling is cheap. This is not a matter of low wages – in fact, Marco cites higher wages for Italian skilled workers who staff TBMs, up to €4,500 a month net, which rises to about €9,000 gross with social contributions. These are based on a nationwide scale that only weakly varies with location, which helps explain why Naples costs are not low despite the region’s low incomes.

Station construction

Station construction costs vary immensely by location. In Rome, on the same project, stations in a suburban part of the city might be €60-70 million. This does not mean construction is trivially easy: Rome’s suburbs still often host historic sites, having been home to patrician villas in Antiquity, and in fact the word suburb dates to that era. However, it’s relatively safe, and I don’t think Line C ran into such sites.

Then in the most constrained parts of the city, things are different. The extension plans for Line C deeper into city center have station costs in the €400-600 million range. This is not what things cost everything within Rome, or even everywhere within the densely-built parts of the city. But the Line C extension passes through the most historic sites. An already-under construction segment will go to the Colosseum, and a planned extension deeper into city center is to go to Piazza Venezia, at the Wedding Cake, and it is that station that is projected to cost €600 million.

The reason for the high cost is that it is not possible to do archeology- and building-safe cut-and-cover. Piazza Venezia doesn’t quite have enough room for a cut-and-cover dig of a full-length station. It is fed by a wide street, the Via Fori Imperali, and I asked Marco why not build cut-and-cover there, but he pointed out that the street goes through the historic Forum. It is in fact elevated over the ruins; any cut-and-cover there would endanger the Forum, and is not acceptable.

Without cut-and-cover, the only alternative is to mine the stations. Rome investigated the option of large-diameter TBMs on the Barcelona L9 model and found it infeasible, since the tunnels are so big they might themselves cause some building damage. Once the stations are mined from a small shaft, their costs explode. Second Avenue Subway built stations using the same method, and had similar per-station construction costs.

The good news

Mined station construction is in practically all cases not necessary. New Yorkers talk about the city’s high built-up density as a reason why costs are high. But in terms of actual stuff in the way of a tunnel, there’s less in New York than in Rome or Istanbul, which has even lower construction costs.

In fact, there is a line in Rome that is rather similar in urban geography to Second Avenue Subway: the Line B1 branch. It runs under a 27 meter wide street flanked by modern buildings that are about 9 stories tall above ground but also have underground parking, Italy having such a car culture that the middle class expects to own cars even in Rome. The cost: €527 million for 3.9 km, in 2010-15.

Moreover, the hard rock in New York should make it easier to build stations while maintaining building safety. Manhattan’s schist is brittle and therefore requires concrete lining, unlike the more uniform gneiss of Stockholm, famously forming natural arches that are pretty to look at from within the tunnels. However, it is still better soil for construction than the sand of Berlin’s U5 extension, to be opened next month, or the alluvial soil of Amsterdam.

The explanation Marco gives concerning station construction is physical and not institutional. This means it should transplant well into another setting – which it does!

In Berlin, the city-center U5 extension, including U55, is in today’s money around €240 million/km. The stations look like cut-and-cover to me, and if they’re not then it comes from severe NIMBYism since the line goes under the very wide Unter den Linden, but one of the stations is basically under the river and another is under U6 and involves moving the U6 station, and the sandy soil is genuinely bad to tunnel through. Suburban extensions in Berlin, with easy cut-and-cover stations, are consistently in the €100-150 million/km range, which is barely higher than the non-Forum Italian range. So Berlin looks fine, and just needs to invest resources into U- and S-Bahn extensions and not into extending the A 100 motorway.

Can New York have what Italy has?

Almost certainly! Second Avenue is not an old or narrow street by Italian standards. Nor are any of the other streets slated for subway construction in New York, such as Nostrand, Utica, and even 125th. Importing construction techniques from Italy and Germany should be feasible. There may be problems with local politics – New Yorkers absolutely hate admitting that another city may be better than theirs in any way, and this makes learning harder. But it is not impossible, and so far there do not seem to be any physical or economic obstacles to doing so.


  1. Hugh B

    125th=easy? Yes, it’s a perfect circumferential, largely because of the many radial stations you intersect, which you have to build under. Where are you putting the stations if not under older stations, requiring expensive mining?

    • Alon Levy

      At a right angle to older stations, like the Parisian transfer stations. So one station would cut-and-cover between Lex and Park, one between Fifth and Lenox (or Lenox and Powell maybe), one between Douglass and St. Nick (or St. Nick and Columbus), one under Broadway under the steel arches. Soil’s not great (w00t, underground river), but I still don’t think it’s as bad as the Unter den Linden shitshow and at U5 cost Lex-Broadway is $700 million.

      • yuuka

        Based on the supplemental FEIS, 125/Lex on the SAS is already between Lex and Park and perpendicular to the 456 station, though for some reason they made it 20 feet deeper and are mining instead of cut and cover.

        But since there’s not enough space for a full-length B Division platform, part of the platform still has to sit under either the 456 station or Metro-North anyway.

        • Matthew Hutton

          Surely they could cut and cover most of it and then mine the remaining part?

          • Onux

            You could, but depending on the depth of the existing line you may not save a lot of money. The earth likes to re-close deep narrow openings, and so the expense of piles and tie backpacks becomes significant. A rule of thumb is the cost of excavation is exponential: if 10’ costs $1M, 20’ costs $4M, 30’ costs $9M, etc. (this super rough estimate applies to urban excavation with retaining walls, open pit mines go much deeper at less cost). If you add the cost of mining (high overhead) to a deep cut, the cost advantage over cut and cover only diminishes.

            Note in almost all cases cut and cover is much cheaper to mining stations (thus SAS Phase I should have been cut and cover), but transverse lines in Manhattan where every station must pass under one or more existing stations may be the place where cut and cover won’t help. The economical solution there is probably the Barcelona method of putting the station inside an over large TBM bore.

          • Alon Levy

            In general boring costs are linear in bore diameter. See e.g. this comparison:

            For crosstown Manhattan lines, I think it depends? On 125th Street, which is not in the CBD and which is crossed by three shallow cut-and-cover tunnels, it’s probably best to do cut-and-cover and underpin the stations, or maybe do normal bores under the stations and then cut-and-cover the stations.

          • Onux

            TBMs are also generally price insensitive to depth, since everything is moving horizontally not vertically, and to a lesser extent can bore closer to (i.e. shallower)/avoid underpinning costs since Earth Pressure Balance and slurry shield TBMs can support the ground during excavation with the tunnel ring supporting it behind. Both are conducive to tunnels crossing other lines to lower cost and minimize vertical circulation.

            You are right, with the 456 and Metro North being the only only difficult station site on 125th, it might be best to cut and cover the line and just accept underpinning as an unavoidable cost of the line.

          • yuuka

            To be fair, this is because of their insistence on having a transfer mezzanine at level -4 between the 456 and SAS (the 456 platforms being stacked, two on level -2 and two on -3)

            If they forced all transfers through the existing 456 mezzanine and simply expanded that for SAS, it would probably have justified cut and cover with the shallower mezzanine level, help make the station less deep, and also remove the technical complexity of punching up into the lower 456 platform level.

  2. df1982

    Fills not *feels you with hope.

    The case of Italy is a good example of cost excesses successfully being reeled in, but whether it can be easily replicated is another question. It happened in the context of a state mobilisation against Mafia corruption, which was far more widespread, flagrant and criminally violent than the problems currently faced in the Anglosphere.

    Not only that but the two biggest parties in Italy both collapsed in the early 1990s (the PCI due to the effects of 1989, and the Christian Democrats due to Tangentopoli). Of course, the political void this left behind later led to Berlusconismo, but I’m guessing that technocrats managed to wrest control of chunks of the state apparatus in the meantime. It’s hard to see something similar happening in the US, UK, Australia, etc.

    • Alon Levy

      Fuck. Thanks for the correction, I just updated. This typo makes me feel more camaraderie with Matt Yglesias than anything else, up to and including agreeing with his Rana Plaza take.

      And yes, the fact that what mani pulite fought was actual criminal corruption and not the legal kickbacks of the American system – create jobs in a state and the state’s US senator will manipulate the contract in your favor – is a big boost. On the other hand, the US benefits from having much less Berlusconi risk, because it already got a Berlusconi. This isn’t something that I think is replicable in Britain, where there’s no corruption as far as I can tell, just boneheaded civil service reforms that imitate the US and not Euro-land, going back even before Cummings’ tenure as prime minister – Alex Harrowell who sometimes comments here blames a trend that started under Major, which sounds right.

  3. tompw

    “New Yorkers absolutely hate admitting that another city may be better than theirs in any way, and this makes learning harder”
    Such a shame they lost Andy Byford 😦

  4. michaelrjames

    Without cut-and-cover, the only alternative is to mine the stations. Rome investigated the option of large-diameter TBMs on the Barcelona L9 model and found it infeasible, since the tunnels are so big they might themselves cause some building damage.

    It’s quite new (link below) but I wonder if there is a role for the TBMs that can vary the diameter (from 9.9m to 11.6m). Most of the tunnel would be the smaller diameter which is enough for two side-by-side trains while the wide section would become like Barcelona L9: one train above the other with their adjacent platforms. No mining required.
    Largest hard rock TBM built for US
    By Leila Steed, 09 January 2020

    • Tom M

      Interesting, I note that the article describes the TBM decreasing bore diameter during the boring operation. I wonder if such a machine is able to increase bore diameter as well?

      • Tom M

        I should have included this in my previous comment. To be clear, my assumption is to be of use for a multiple station underground line, there would be multiple points during the boring operation where the TBM would need to both decrease and increase the bore diameter.

    • Eric2

      9.9 to 11.6 sounds like a minimal difference in diameter – probably better to do everything at 11.6 and avoid the technical issues of varying diameter.

      • michaelrjames

        That was just the case in that single example. It would seem one could achieve the ≈9m to ≈12m range for the side-by-side versus double-deck arrangement. The point is to have the ability to build the narrow tunnel where constraints exist of the type Alon discussed, yet to be able to do tunneled stations, all with minimal disturbance of the surface or heritage structures along the route.

    • Onux

      The article says they will bore the large diameter section then take out “removable pieces” to bore the smaller section. This suggests a one-time step down process, not a TBM that can dynamically adjust size.

      Furthermore, this is a main beam TBM designed for hard rock, where the tunnel supports itself and the TBM can be pulled back to remove those pieces. This procedure would not work for EPB or slurry TBM’s that require a shield the diameter of the tunnel. Removing pieces of the shield to adjust its diameter would cause tunnel collapse.

      While fascinating technology, I don’t believe this has the applicability to metro construction that you think it does, by widening the bore at stations or narrowing it at obstacles then readjusting size again.

      • michaelrjames

        Hah, you may laugh. They all laughed. But who’ll have the last laugh? (Mad cackle while stroking white cat in his giant underground lair built with secret vdTBM [variable diameter TBM].)

        I may be naive but on engineering matters I take the attitude that almost anything can be done if money is not a limitation. Burj Khalifa and Beijing’s CCTV Headquarters come to mind.
        The difficulty and the huge cost of mining underground stations would appear to be the appropriate driving force here.
        As to your main objection, surely parts of all tunnels are unsupported, even if briefly, as permanent concrete segments are put into place? In fact it seems to (naive) me that the methodology actually already exists in essence and just needs some clever and motivated, most likely Swiss, engineer to put the different technologies together.

          • michaelrjames

            I have no idea what point you are trying to make.

            Incidentally that vid is, I believe, is from the tv documentary on the building of CrossRail.

          • Nilo

            You: “As to your main objection, surely parts of all tunnels are unsupported, even if briefly, as permanent concrete segments are put into place?”

            The video from CrossRail (as in CrossRail the constructor of the tunnels as you can see from the Youtube Channel) shows this is not the case. It also demonstrates nicely how you can’t spontaneously change tunnel diameter because of how non-hard rock TBMs work.

          • michaelrjames

            I don’t believe it shows that at all. Mostly because it doesn’t show the necessary detail as the cartoon shows the machine and shield but not the earth left a metre behind the cutting head. I think you have somehow imagined it. But I am open to persuasion so please give me the exact time-stamp of the video that shows what your argument claims.
            In any case here is what Wiki says, and it only makes sense: at some point, if only momentarily, the earth face must be unsupported–though it may be in the extreme cases where they inject shotcrete into/onto the soft earth while the steel support remains in place (and probably still under pressure) but it is not clear that happens much; the most common thing is for a precast concrete panel lifted into place. I’m not sure how you propose they put in place a ≈1m2 panel without exposing the earth face? I suppose a metal extension of the shield/tube could be sacrificed and left in place on the earth side of the concrete panel? Doesn’t seem to happen because this size segment of face (one or a few square metres) will be self-supporting for sufficient time.

            Open face TBMs in soft ground rely on the fact that the face of the ground being excavated will stand up with no support for a short period of time. This makes them suitable for use in rock types with a strength of up to 10MPa or so, and with low water inflows. Face sizes in excess of 10 metres can be excavated in this manner. The face is excavated using a backactor arm or cutter head to within 150mm of the edge of the shield. The shield is jacked forwards and cutters on the front of the shield cut the remaining ground to the same circular shape. Ground support is provided by use of precast concrete, or occasionally SGI (Spheroidal Graphite Iron), segments that are bolted or supported until a full ring of support has been erected. A final segment, called the key, is wedge-shaped, and expands the ring until it is tight against the circular cut of the ground left behind by cutters on the TBM shield. Many variations of this type of TBM exist.

          • Nilo

            This might be the most disingenuous, or possibly illiterate thing you’ve ever posted.

            From wikipedia: “In soft ground, there are three main types of TBMs: Earth Pressure Balance Machines (EPB), Slurry Shield (SS) and open-face type.”

            The article then goes on at some length about the differences between EPB, SS and then finally open face TBMs

            An open face TBM is distinctly different from a EPB.The link you posted says that. Do you just not read?

          • michaelrjames

            And the CrossRail construction used at least two types of TBM, one for the relatively easy boring and one for the under-river parts–obviously the most difficult bits and btw not the places you’d build a station (which is what we are talking about). The point of the bit I cited was that it concerned among the most difficult geologies, and yet it says that the earth wall is left unsupported for a period before the concrete segment (or in other cases shotcrete etc) is applied.
            Also, I don’t know what people are imagining: like that a whole 12m diameter shield is dismantled all together leaving the vast face (≈113m2) unprotected? No, obviously not. First, only the (1.5m) ‘outer ring’ would be removed/retracted leaving a 9m shield in place at all times, and second, only segments of 1-2m2 would be removed one at a time each replaced with concrete panels or stabiliser etc. Third, obviously the designers and engineers would know from the geology where such an operation could be done safely. But you know that when M4 went under the Seine at Ile de la Cite, they had to freeze the riverbed and part of the Seine itself with liquid nitrogen to allow the tunnelling below so I think modern engineers can handle the approx. 150mm supported by the edge of the shield … for the 30 minutes or whatever it takes to bung some concrete or steel in place.

            So what is your problem? If what I wrote and cited is so stupid then just say why, clearly, instead of ridiculous blank and empty statements that say nothing. I still have no idea why you linked the CR video, nor what your latest insult is really about.

  5. Mikel

    TBM technology at this point allows preserving artifacts. It involves injecting a gel ahead of the cutting edge, which is not supposed to be a cost-raiser.

    Wow, that sounds very cool! Do you have a link explaining how it works? (Italian is fine)

    Once the stations are mined from a small shaft, their costs explode.

    . I’m somewhat confused by the fact that mined stations seem to be much more expensive than cut-and-cover. Part of the criticism of SAS is about the excessive volume of its stations; have Milan and Rome been able to avoid that?

    Bilbao (richer than Rome but poorer than Milan) has been building mined stations for 30 years at (inflation-adjusted) 20-25M€ apiece, so maybe make it 50-65M€ after PPP adjustment and a linear scaling to big-city platform length. That’s more or less in line with the 70-90M€ (PPP) of the cut-and-cover stations in the upcoming L11 extension in Madrid. For what it’s worth, neither Madrid nor Bilbao need to worry about archaeology; Málaga, a much older city, decided to use cut-and-cover for its Stadtbahn and it turned out… not great.

    I’d argue that using the same method for the tunnel and the stations (like Barcelona’s all-TBM and Bilbao’s all-mined) could reduce the overhead costs, but I don’t know if there’s granular enough data to see if the effect is significant.

    • michaelrjames

      The method was used to cope for some sections of the cross-harbour tunnel of the Sydney Metro (section to CBD competed Marc 2020) because there were unstable parts that couldn’t be avoided. A recent documentary (by public broadcaster SBS, shown a a few months back) actually explained it graphically very well and showed it in action (in as much as you can see what is happening at the shield face but they did show recovering the bentonite slurry). They also showed two workmen entering the front pressure chamber and later in reverse depressurising to re-enter the world; they have to squeeze thru a smallish circular pressure cover (like a mini-bankvault or autoclave cover)–entering backwards pulling their legs up behind them–one had a touch of claustrophobia just watching them! There are teasers on YouTube of the 4-part series “Sydney’s Super Tunnel”:

      I can’t see any description of the bentonite system but it may be somewhere on the site of the TBM company Herrenknecht:
      The Phase 2 tunnels, 31 kilometers in total, are being excavated by five Herrenknecht tunnel boring machines (TBMs). Four Double Shield TBMs are excavating the sections north and south of Sydney Harbor. A Mixshield is digging its way under Sydney Harbor – the overburdens are up to 35 meters to the bottom of the harbor and above that a water depth of up to 34 meters.
      The Mixshield TBM type is optimally suited to the tunnel section under the harbor. With it, heterogeneous geologies such as in Sydney and high water pressures can be mastered safely. The support pressure is managed precisely using an automatically controlled air cushion. Two independent man locks, each with two lock chambers, are installed for safe cutting tool changes.

      Recent technical paper:

  6. john

    “Talking to Marco Chitti about the history of Italian construction always fills me with hope.”

    Marco Chitti, miracle worker

  7. Pingback: Sorry Eno, the US Really Has a Construction Cost Premium | Pedestrian Observations

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