The only cut-and-cover example in our database from Paris, the Line 13 extension to Courtilles, cost 83M€/km, which is around $130 million/km in today’s money other Paris Métro extensions from the last 15 years are 50-100% more expensive, and the next tranche is even costlier, as Parisian costs are regrettably increasing.
With everything included, this should be viewed as about $200 million per km the actual median for subways in our database is about $250 million/km, but it includes expensive lines with mined stations, city center tunnels that can’t easily build cut-and-cover stations, and projects that are unusually bad.Ĭut-and-cover is generally cheaper. The typical cost of bored tunnel is much less the lines for which we have seen a breakdown in costs between tunneling and stations, which are a small fraction of our database, have tunneling costs ranging from around $50 million per km to somewhat more than $100 million per km, not counting systems, overheads, or stations. But very few cities mine stations – New York and London do, and very rarely other cities do in constrained historic centers like Rome’s. The cost of a mined station starts at $500 million and goes up. This mixed method involves opening up the street at station sites for 1.5-2 years in Paris, intermediate costs, and disruption only at sites that would benefit from the opening of a station. Mining or blasting a station is expensive, and many modern examples run up to $500 million or more, not just in high-cost New York but also in otherwise low-cost Rome. This is because, while the TBM is capable of building tunnels easily, it cannot build stations. The typical method used in the world is really a mix – the tunnels are bored, the stations are cut-and-cover. Most American, European, and East Asian cities have switched to this method in the last generation thus for example New York started to build Second Avenue Subway in the 1970s cut-and-cover, but the program since the 1990s has always been bored. This method was first invented in London for the construction of the Thames Tunnel, and has been used for all of the London Underground lines since the first two, as London lacks for wide streets for cut-and-cover work. This used to be called a tunneling shield, but the shield has been automated to the point that a small crew, only 8-12 people, are required to supervise it nowadays, and now it is called a tunnel-boring machine, or TBM. The caisson method builds a concrete structure and then lowers it into the ground, which facilitates multistory cut-and-cover structures at transfer stations.īored tunnel involves digging just one portal, or sometimes a few to speed up work, and then drilling horizontally. The Milan method sinks piles into the street early and builds retaining walls to allow for truly vertical construction, whereas traditional cut-and-cover must be sloped, which requires a wider street than the tunnel, like the Manhattan avenues or Parisian boulevards but not Milan’s Renaissance streets. These innovations include the cover-and-cut system invented in 1950s Milan (“Milan method”) and the caisson system used to build T-Centralen in Stockholm. The oldest cut-and-cover subways were dug by hand, but in the last 100 years there have been technological innovations to mechanize some of the work as well as to reduce surface disruption, which is considerable and lasts for a few years. In New York, one of the sources cited on refers to the subway as “a covered trench” rather than a real tunnel. Even more regrettably, this same thinking is common in much of the developing world, where subways tend to be bored.Ĭut-and-cover refers to a family of construction techniques all of which involve top-down tunneling. Regrettably, people don’t seem to even recognize it as a tradeoff, in which they spend more money to avoid surface disruption – some of our sources have told us that avoiding top-down cut-and-cover is an unalloyed good, a kind of modernity.
In the last generation or two there has been a shift toward bored tunnel even in places that used to build cut-and-cover, despite the fact that bored tunnel is the more expensive technique in most cases. Cut-and-cover means opening up the street top-down, building the system, and roofing it to restore surface traffic bored tunnel means opening up one portal and digging horizontally, with less surface disturbance. Subways can be built in two ways: cut-and-cover, and bored tunnel.
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