Large-Diameter TBMs

Deep-level subway tunnels are usually built with tunnel-boring machines (TBMs), which can dig and create their own lining even under other infrastructure, such as older intersecting tunnels. But then deep-level stations require larger caverns, which are expensive to dig from the surface. Three-quarters of the cost of Second Avenue Subway Phase 1 is the three stations. As commenters Jim and Anon256 noted a year and a half ago, to avoid this problem, such cities as Barcelona pioneered the use of large-diameter TBMs, which have enough space to accommodate tracks together with platforms by their sides. This is especially useful for construction in dense city centers, where surface disruption must be minimized and demolitions of buildings that are in the way are expensive. I claim that this is the optimal construction method for both regional rail to Lower Manhattan and the North-South Rail Link in Boston.

In Barcelona, the internal diameter of the TBM used for Line 9, 11.7 meters, is enough to have both directions of a two-track line use one tunnel. With an internal horizontal slab, trains can be stacked so that each direction gets one track and one platform at a station, which looks about 4.5 meters wide in diagrams. Between stations, there is enough space for each of the two levels to have two tracks, allowing crossovers. The only required construction outside the tunnel is access points, which can be drilled straight down for elevators or at an angle for escalators.

While the cost of Barcelona Metro Line 9 is about $170 million per kilometer, more than three times the original budget, compared with $40-60 million per kilometer for most Spanish tunneling projects, it is still much lower than the cost of comparable projects tunneling under preexisting subway systems that have stations built by blasting caverns or cut-and-cover construction. In addition, the standards are relatively easy to adapt to the standards of American mainline construction, since the Line 9 trains are powered by catenary and are only ten centimeters shorter than the LIRR’s M-7s. Mainline catenary is energized at 25 kV and requires more clearance than low-voltage rapid transit catenary, but this adds only about half a meter to the total diameter: German standards call for 27 centimeters of clearance from 25 kV.

To allow two lines to meet at cross-platform transfers, there are two possibilities, both used by narrower-diameter TBMs (or older tunneling shields). One, used by the London Underground’s tube lines, is to have two parallel circular tunnels with numerous passages drilled between them. Another, used by some subway lines in Shanghai and Tokyo as well as by the Harlem River tunnels of New York’s Lexington Avenue Line, is to overlap the two circular tunnels, using a tunneling shield with a double-O tube (DOT) design. The DOT design is more complex and would also require any access point to either obstruct the platforms or go at the platform edges, but would create a wider platform allowing easier cross-platform circulation.

In Boston, regardless of which design is used, the North-South Rail Link involves three central stations in which two tubes (one feeding the Worcester and Providence Lines, one feeding the Fairmount and Old Colony Lines) meet: South Station, Aquarium, and North Station. Each should have a cross-platform transfer, in the style of the Hong Kong MTR: at Aquarium northbound Providence and Worcester trains should face northbound Fairmount and Old Colony trains and likewise for southbound trains, whereas at South and North Stations, northbound trains should face southbound trains. This way, people transferring between two points south of the link could transfer cross-platform at South Station, and people transferring between two points north of the link could transfer cross-platform at North Station.

A large-diameter TBM has enough space not only for crossovers, but for trains to switch what levels they’re on. With a design speed of 100 km/h, a curve radius of 500 meters, and a superelevation ramp lasting 2 seconds, it takes about half a kilometer for the track on the lower level to swerve sideways so as to no longer be directly under the upper-level track, climb to the upper level while the upper-level track descends, and then swerve sideways again so that both tracks are on the correct side of the tunnel to allow a cross-platfom transfer. There is space to do this between both pairs of successive stations. The portals could be constructed where convenient on the approaches to South Station and immediately north of the Charles, and the infrastructure for pairing lines at the north end with the two tubes could be done above or below ground, based on local tradeoffs between disruption and cost.

In Lower Manhattan, the problem is capacity. The system would involve a line from Atlantic Terminal to Jersey City or Hoboken intersecting a line from Grand Central to Staten Island. There is room for only one station, and some configurations, notably any in which the New Jersey end is at Exchange Place, require a cruciform station, without cross-platform transfers. Moreover, this station is at a site with much more intensive development than Downtown Boston, and close attention must be paid to capacity. This is why I bring up DOTs in the first place: London-style passages may not allow sufficient circulation of transferring passengers. The platforms would be obstructed with many escalators between the upper and lower levels since there is no room for Hong Kong’s three-station cross-platform transfers, and peak demand for egress to both street level and intersecting subways is also likely to be very high.

The optimal solution seems to be to have no real Lower Manhattan station beyond the platforms and access points. Most ticket-vending machines should be placed at street level next to the escalator and elevator banks, and the blocks above the station should be pedestrianized to allow for access from the middle of the street, avoiding the need for a mezzanine. The width and pedestrian volume of Lower Manhattan streets are such that it would be at good human scale.

The remaining capacity issue is sufficient space for escalators. There are four tracks in total, each of which is inbound from some direction, and at the peak there could be a 12-car, 300-meter long train with 2,000 passengers every 2 minutes per track. If all passengers are discharged and the trains leave the station empty in the morning peak, then the required capacity is 240,000 people per hour. This is in fact quite unlikely, even though there is only one Lower Manhattan stop: many Staten Islanders work in Brooklyn or Midtown, people from points north of Grand Central are more likely to get off the train at Grand Central than to stay on until Lower Manhattan, and there is a substantial volume of commuters between Brooklyn and points west or north of Manhattan, who would benefit the most from through running.

Factsheets by Kone and ThyssenKrupp suggest each meter-wide escalator has a practical capacity of 6,000-7,000 passengers per hour. If we assume half of a full train capacity’s worth of passengers get off at the station, not including passengers who transfer, then we need 120,000 passengers per hour, i.e. seventeen to twenty escalators. This can be done quite easily with two parallel circular bores, at the cost of restricted capacity for connecting passengers. With a DOT design with 8-meter wide platforms, it’s still possible to have an escalator bank at each end of each platform; the large separation between the upper and lower levels, about 6 meters, allows independent escalators at the end, though not anywhere else. The widest standard escalator is a meter wide at the step and requires a 1.6-meter wide pit (see above ThyssenKrupp link as well as brochures by Kone and Otis), enough for a three-and-one or three-and-two escalator bank at each end, giving twelve peak-direction escalators. Eight additional escalator banks in a one-and-one configuration (or perhaps four in a two-and-one configuration, which is a wider platform obstruction) can be placed roughly evenly along the upper-level platform, along with elevator shafts, escalators that only connect the two platforms, and access points to intersecting subway lines.

The advantage in both New York and Boston is that there’s no need to construct a station beyond those shafts and bores. The station mezzanine in this configuration is a street, most likely Broadway in Lower Manhattan and (according to prior North-South Rail Link plans) the greenway above the Central Artery tunnel in Boston. The station retail is ordinary street retail. Fare control is roving inspectors riding the trains or patrolling the platforms. It’s still a multi-billion dollar undertaking due to all the underwater access tunnels, but the cost per kilometer could be held down to normal first-world levels even while crossing the difficult infrastructure of Lower Manhattan and Downtown Boston.


  1. Eric

    Or you could put a little building over your outdoor mezannine and make it into a complete station with whatever faregates or other stuff you think is necessary.

  2. Eric

    Any security issues with long tunnels that are only partly occupied by a train track? I could imagine large numbers of homeless people hanging out there…

  3. Ant6n

    The North-South tunnel in Berlin has overhead rails instead of caternary allowing lower clearances. (I think the city tunnel in Vienna has those as well)

  4. Joey

    Tangentially related: There seems to be a lack of consensus on when it is necessary/not necessary to have two parallel segments of the tunnel be physically separated. I think the justification for physically separating two parts of the tunnel (usually opposing travel directions) is fire safety, but for instance, the Bologna-Florence high speed railway (primarily tunneled) is build with only a single bore and access points every 5 km (a short segment which lacks access points has a parallel service tunnel). Are there good justifications for the necessity/lack of necessity for physical separation?

    • Alon Levy

      A lot of HSR tunnels use twin bores to increase the amount of free air arond each train, and avoid very large relative passing speeds in tunnels. Japan does single bores but its trains are much more tightly sealed than European HSR trains because of all the legacy tunnels from the 1960s and before.

      • Joey

        Okay, but what about for not-high-speed applications? London’s Crossrail, NYC’s SAS, SF’s Central Subway, etc are all being built with two bores. But for instance the Atocha-Chamartin standard gauge tunnel (accommodating high speed trains but not at high speeds) was recently finished with a single bore carrying both tracks. Two bores allows you to have a center island platform at stations, but given the significant added cost of two bores rather than one, I have trouble believing this is the only reason why there are two tunnels in many of these projects.

        • ssmithobserver

          I wouldn’t look to NYC, London and SF for examples of subway tunneling done right.

        • Richard Mlynarik

          The Dutch HSL-Zuid single-bore Groene Hart tunnel is interesting. 300kmh design speed. (The tunnel itself being a ecological/”NIMBY” structure rather then being strictly required by topography.)

          The design-build contract winner actually did some design (unlike, say, the rigged US “DB” contracts) — the original design reference being for a pair of tunnels with connecting evacuation passages. The built tunnel is a single 7250mm outside diameter 6650 inside radius bore with a central 450mm width verticall firewall separating it into two 42.5m^2 halves (nice dimensioned cross-section here (PDF page 164)) with emergency cross passages ever 150m.

          I did once know but can’t now remember the employer of engineers of this “split” design — ARUP? Bechtel? Regardless, it’s novel and of some interest.

    • Max Wyss

      For HSR, I think the trend is definitely towards single-track bores, just for the reasons Alon states. In fact, you might look at the diameters of the German Neubaustrecken where trains can meet at speeds of around 300 km/h. This makes smaller bores for single track tunnels less expensive (particularly for long tunnels). And the advantage is that if one bore is not usable (for whatever reason), the other can still be used.

      • letsgola

        Conversely, if you are pretty deep and in competent rock (like NYC) would that favor a large diameter bore? The disadvantage of a large diameter bore in NYC would seem to be the need to remove a lot of rock, which would drive up the cost, but I don’t have a good feel for how much that would impact cost relative to two smaller bores w/ more excavation at stations.

  5. Max Wyss

    The already mentioned power rail (for example produced by Furrer&Frey), reduce the distance by another 20 cm or so, because there is no need for a suspension wire. I think it has now been approved for speeds up to 120 km/h.

    • Alon Levy

      In New York, it might be easier to just bag the catenary and go third rail. Of the four lines this is meant to connect to, one is not even electrified and the other three use third rail (one with an underrunning shoe, two with a regular overrunning one). If there’s not enough clearance in the Metro-North tunnel into Grand Central to install catenary then there’s no real reason to use high-voltage catenary. The only real reasons to have catenary are that in electrifying the Erie lines it’s easier to electrify with catenary if the grade crossings aren’t separated, and that the rolling stock on the New Haven Line and in the electrified parts of New Jersey uses catenary.

      However, in Boston, there’s an advantage to electrifying with 25 kV catenary, which is that there’s no legacy third rail. There are also way more grade crossings than are remaining in New York.

      • Joey

        You might be able to add a bit more clearance in the Park Avenue Tunnels and GCT by removing the ballasted track and replacing it with thinner slab. If this proved impractical or ineffective, how would you deal with the issue of two incompatible third rail standards?

        • Alon Levy

          Changing the third rail is easier. Also, the M7s can have either kind of third rail, though they can only change from one kind to another in a maintenance shop; the modifications to allow them to change at a station or on the fly should not be too expensive by the standards of building 25 km of tunnel.

          • Joey

            Changing at a station implies that the station has neither type of third rail though (meaning that it has catenary), or that it has to happen on one side of the train at a time between two stations. Is that doable?

          • Mirco

            @Joey: You could do the change one side at a time, accross two stations, and switch the side opposite to the third rail in each of the stations.

  6. jim

    I’d add that wide bore tunnels should be used for subway construction as well. Phases 3 and 4 of the Second Ave Subway have nine stations, so they’ll cost three times as much as Phase 1 using narrow bore tunnels and blasted station caverns. Even if wide bore tunneling is more costly than narrow bore tunneling, avoiding blasting nine station caverns has to come out cheaper.

    It’s not even clear that wide bore tunneling, in New York, would be more expensive than narrow bore tunneling. The reason that tunneling in New York is more expensive than anywhere else is the sandhog manning required by work rules. But the manning requirement doesn’t appear to be linked to tunnel diameter, so as long as the wide bore tunnel doesn’t take more than twice the time to bore than a narrow bore tunnel, labor costs for one wide bore tunnel should be less than labor costs for two narrow bore tunnels. Yes, it generates four time the muck which needs to be carried away and there’s higher launch costs, etc., but labor costs appear to drive tunneling costs in New York.

    Yonah Freemark a while ago suggested that the Second Ave. Subway, after Phase 2, should be continued across 125th St. Again, a wide bore tunnel, with platforms within the tunnel, would be ideal for this continuation. Stations at St. Nicholas and Lenox Aves, as well as at 3rd Ave would only need access shafts into the fare control zones of the existing stations (A,B,C,D at St. Nicholas, 2,3 at Lenox and 4,5,6 at 3rd Ave). Essentially, those stations would come free. The cost of the extension would be little more than the cost of tunneling.

    • Eric

      “The reason that tunneling in New York is more expensive than anywhere else is the sandhog manning required by work rules.”

      Does that really make a significant difference? Let’s say a unionized worker earns $100k/year and the TBM operates for a year. If 100 workers are needed to operate it, that comes out to $10 million labor costs. That doesn’t come close to the cost difference between NYC and Spain, which is measured in the billions for any significant project.

  7. JohnDMuller

    Nice, fiscally responsible plan.

    Now maybe we could add a 4 or 5 story cast iron scale model (x 10 or whatever) of a classic steam locomotive spanning Broadway, or some major intersection to serve as the head-house. It would also serve as a tourist trap, shopping mall and/or a political ego legacy landmark signature piece for those of us who like a little flair with our frugality.

    As to the intersection between the two routes at the lower Manhattan station, if it seems inadvisable to make whatever curves it would take to create side by side tunnels, a tall skinny X crossing might allow sufficient extra connecting tunnels to be drilled somewhat away from the center to avoid requiring all transfers to occur at the axis.

    I fear that this barebones design might not survive complexifiers citing issues such as unbalanced service demands (or whatever). For example, with the three Metro North domains each wanting their trains to go to Wall Street instead of having to transfer, there will probably be more candidate trips to Staten Island than would be necessary. That would suggest extra tunnelling to be able to terminate some runs downtown and/or a connection to the Brooklyn route (and appropriate consideration of the various train powering systems).

    Of course, you could just run all the trains thraough to SI anyway. Talk about induced demand – reactivating the full SIRT, you’ve got three lines on that end too. Even if the trains just terminated at St. George, the cost of running extra trains under the harbor might easily be cheaper than the alternatives. Of course, other real or imagined/political issues may create other add-ons or diversions (Governors Island or Statue of Liberty anyone?).

    The midtown end of your tunnel has some interesting aspects as well, but I’ll save them for another post.

    • LetsGoLA

      NFPA 130 is yet another way that the US saddles rail operations with requirements that it would never dream of applying to highways. Is the NFPA going to say that only one car should be allowed at a time in the ventilation zones on the Big Dig? No. Are they going to require a “viable extraction capability to remove non-incident [cars] in the same time frame as the activation of the ventilation response”? No. Yet a car fire in a highway tunnel is a far more likely event. Rail tunnels, like highway tunnels, are designed with emergency egress points. Seems to me like it’s solving a problem that doesn’t exist.

  8. Ant6n

    Btw, if station construction in terms of platforms is cheap, but it’s hard to move so many people per hour out of the station, one could also consider building two stations in lower Manhattan, and distribute the load, while reducing connection/walking distances for people who work there.

    • Alon Levy

      Yes, that is true. Given a faregate-free system, the advantage of connecting to a specific subway station (i.e. Fulton) is reduced, so might as well build stations at Wall Street and City Hall, which could also give Staten Island-Brooklyn riders a cross-platform transfer.

      A 20-meter deep 300-meter long station with escalators at the ends of the platforms has a separation of nearly 400 meters between the access points at each side. So a nominal Fulton Street station would have escalator portals going as far south as Zucotti Park and as far north as the southern edge of City Hall Park, and a nominal City Hall station would have escalator portals as far south as Fulton Street.

      • JohnDMuller

        As you point out, things are pretty close together down there; would there be enough room between stations to flip the tracks in one of the tunnels or would the 2nd station be offset vertically and just present one cross-platform transfer? With 2 deep stations down there, escalators could surface almost anywhere in Lower Manhattan. I pity the engineers trying to figure out where it is safe to put them.

        Another station stop to consider might be at 34th St., for the sake of the Empire State building and to accomodate an unlikely, but potentially useful transfer to the NEC.

  9. Andre Lot

    Proof-of-payment large-scale systems probably wouldn’t work in US. It is not feasible to assume otherwise, considering:

    – the lower reliance on long-term subscription plans
    – more confrontational interactions with law enforcement
    – less programs available for very low-income groups who have economic incentives to doge fares (students, for instance)
    – lack of compulsory ID-carry for the citizenry, which makes it easy to dodge enforcement by refusing to identify yourself, which could require not an administrative check but law enforcement procedures, which cost some money

    • Alon Levy

      What you say may be true of urban rail, but regional rail relies heavily on season passes, which are even more discounted than in Europe. German monthly passes are in the 32-36 single fares range; Northeastern commuter rail monthly passes are in the 27-28 range. Light rail systems such as Portland’s and Calgary’s use POP without problems even with less favorable season passes, and in Los Angeles and Vancouver, Cubic needed to lobby to get POP replaced with expensive faregates and proprietary smartcards.

    • Nathanael

      Compulsory ID carry cannot be implemented safely in a country with as much police corruption as the US.

      In a country with reasonably well-behaved police, the compulsory ID laws are not enforced. If you’re out in front of your house, the police will let you go back into your house to get your ID. If you’ve lost your ID, the police will be reasonable about it.

      With our utterly deranged police, we can assume that they would use a “compulsory ID” law to will steal your ID and destroy it, then arrest you for not having ID.

    • ari

      I would bet that traveling without a ticket would be reasonable suspicion to arrest you and verify who you were. If you are trespassing somewhere and are arrested without an ID, they’re not going to write a ticket for John Smith at 123 Any Street and send you on your way. IANAL, but I don’t see them letting off non-ID, non-paying transit riders that easy either.

    • asdfsdf

      On the Zurich public transport network, monthly passes have ID photos on them, so there is no need to carry a separate ID card–your ticket is enough ti ID you.

  10. ari

    If the North-South Rail Link is ever built (ha) it seems silly to build three stations. Why? Because North Station isn’t near any jobs, and borders residential areas (albeit high density), highway ramps and (for half of it’s “catchment”) the river. While South Station borders the job-rich financial district and is mostly surrounded by offices, North Station has no major office buildings with 500m. Mass General Hospital is a bit closer, although the 1950s-era urban renewal street grid does not allow for pedestrian-friendly connections. Basically everything north of the station is undeveloped.

    Take a look at some data. At South Station, Hubway (shared) bikes stay balanced as people both begin and end trips at the station during rush hour. At North Station, there are 7 times as many rentals as returns in the morning, with a similar disparity in the evening—150 bikes leave the station in the AM hours (meaning Hubway has to constantly shove more bikes in). A similar phenomenon occurs at Penn Station in New York (but probably not Grand Central).

    In place of North Station should be a station between Haymarket and Aquarium. This would allow the link to serve all four subway lines with only two stations, and dramatically reduce the construction cost. Haymarket and Aquarium are only about 400m apart, and a 400m station, while long, would certainly be feasible, especially with escalators and mezzanine taking up some of this length connecting to the subway lines. (For comparison, the longest platform at South Station is 300m long, although it can serve a 16-car train.) This puts much of the downtown area of Boston within 500m of the station, and because so much of the area around North Station is underutilized, there is nearly nowhere populated which is within 500m of North Station and not within 500m of Haymarket. Even MGH is only about 150m further from Haymarket than North Station, although this could be mitigated by a) better pedestrian connections, b) in lousy weather, passengers could ride one more stop to South Station, and then transfer to the Red Line to be dropped at MGH’s door and c) if there were ever an extension of the Blue Line, it would end at MGH.

    In addition, most of the new development around North Station is residential in nature, which lends itself more to local transit than commuter rail (not that many reverse commuters). If North Station was decommissioned, the area in between the Boston Garden and the river could be developed (most of it is a parking lot, which is almost as good a use for the land as the nearby jail, with full city skyline and river views). Haymarket, on the other hand, may actually see some logical development. When it was urban renewed in the ’60s, someone must have said “here’s a node served by two of the four transit lines in the city and a bus terminal. It would be a great spot for a big parking garage.” So there’s a huge parking garage directly above the station’s headhouse, which is a fantastic place to have something which generates zero jobs. (To say nothing of the desolate City Hall Plaza across the street.) Good luck walking to work, you poor, transit-riding saps. In any case, there is talk that the hulking concrete garage will be torn down and replaced with high density, mixed-use development. Imagine what the value of that parcel would be if it connected to every commuter line as well.

    North Station would still be an important transit station (and one that was extensively rebuilt about 10 years ago, although the connection to commuter service is risible) especially with the extension of the Green Line to person-dense but transit-poor Somerville and Medford (7000 persons/km in Somerville). During events, passengers would have to walk a bit further, or take transit, but given the deplorable level of service, it’s doubtful all that many fans and concertgoers take commuter rail anyway. For customers transferring from the Orange to Green Lines, it would still be the logical transfer point, as it allows for a mostly-cross-platform transfer, while Haymarket was built as two parallel stations (which may be better suited to have connections to a future transit hub, anyway, but I still can’t figure out the concourses at Haymarket).

    So if a NSRL ever happens, it only needs two stations. I certainly think that it should also include a grade-separated loop through Cambridge for Worcester trains (at a minimum). The Providence-Worcester tube would have much greater potential capacity issues than the Fairmont-Old Colony, and looping Worcester trains through Cambridge would reduce some of the capacity constraints. But much more importantly, it would allow direct access to the ever-burgeoning Kendall Square area. The right-of-way is there and in use and would only require a few grade separations, which would be helpful to the area anyway. (The cost would be offset by a variety of factors, such as reduced shuttle costs, reduced overcrowding on the Red Line, the potential to develop properties which are currently split by the trackage but would be much more viable if contiguous; MIT already has a building built over the trackage.) Considering that there are two buildings under construction which have dug 40-foot-deep holes in the ground for garages, tunneling in the area (mostly soft fill, no bedrock) can’t be too expensive, even to get under the Red Line—no TBM required. You would have to close the Grand Junction branch for a time to dig a trench, but it was closed for several months due to an unsafe bridge this summer to no major ill effect. The City of Cambridge (better access = higher land values = higher tax revenues) and MIT (which wants to put Mass Ave in a tunnel in the long run, which could be combined with this project) could probably be leaned on to help pay for a lot of this, too.

    And this could be expanded (although at a much greater cost) with a tunnel from the BU Bridge area under Longwood and back to the Providence Line, providing regional rail access to the other major employment center in Boston which lacks it. And at a still greater cost, a second loop could run via the Fitchburg Line and Kirkland Street to Harvard Square, and then duck under the river to Allston (tons of underused real estate), Longwood and Melnea Cass Boulevard (more underused real estate and a very economically disadvantaged area which could use better access) to South Station. Oh, and then you could shut up about the urban ring (a Silver Line-level fiasco if it ever sees the light of day) because it would be served by these loops (which could run supplementary transit-level service to supplement commuter runs).

  11. Joey

    I’m curious about tunnel branching as it relates to TBMs. Bellmouths can be excavated within the tunnel itself, but what about TBM extraction? It’s not possible to extract a TBM through an already completed tunnel of the same diameter, is it? The tunnel lining would get in the way. So does that mean that every time you branch the tunnel, you need to either build an extraction pit or abandon a TBM?

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