The Two Methods of Subway Construction

Massive Tunnel Boring Machine Helps LA Extend Light Rail System
Massive Tunnel Boring Machine Helps LA Extend Light Rail System. David McNew/Getty Images News/Getty Images

Subway construction can use two different methods: "cut and cover" and "deep bore."

Cut and Cover Method of Building a Subway

Older subway systems, such as those found in Toronto and New York, were built with a method known as "cut and cover". In "cut and cover" tunneling, the pavement of the street is removed, a hole for the subway and stations is dug, and then the street is restored. The "cut and cover" method is much cheaper than "deep bore" but the alignment is restricted to the street grid.

"Cut and cover" also results in the stations that are much closer to the surface (as little as twenty feet below the surface), which significantly reduces passenger access time. On the other hand, "cut and cover" results in serious disruption to traffic along the street for a significant amount of time; this disruption usually results in negative effects, especially for store owners along the corridor.

Deep Bore Method of Building a Subway

In "deep bore" tunneling, boring machines are inserted into a hole dug at a convenient spot along the proposed line and then proceed through the earth little by little, up to eighty feet per day, until they have carved out space along the entire corridor. These boring machines are huge. The ​world's largest is fifty feet in diameter. Boring machines generally can excavate only in one fixed shape, which is generally circular. Because these machines do not have to follow the existing street grid, they allow for much greater flexibility in route design.

In addition, there is no disruption to life along the surface. Except at the machine insertion points, you would not even know a subway was being built. In exchange for these advantages are two major disadvantages. One is financial: "deep bore" construction costs significantly more than "cut and cover"; the underground stations alone can cost $150 million.

Because of a large number of variables that make up the cost of subway construction, it is extremely difficult to quantify the cost differential between the two methods. The second is access: passenger access to "deep bore" stations is significantly more difficult than "cut and cover" stations, making the subway much less useful for relatively short trips.

Often, the nature of the soil conditions and existing underground construction dictate one of the above strategies. In terms of the soil conditions, the height of the water table and softness or hardness of rock may mandate tunneling at a particular depth. In terms of existing underground construction, the presence of a large number of tunnels, basements, utility lines, and pipes may make "cut and cover" construction impossible to attempt.

How Subway Construction Method Is Decided

The nature of a particular metropolitan area's rapid transit growth strategy can also suggest one or the other methods. Because the initial cost of constructing and lowering the tunnel boring machine into the ground is so great, it seems as though the "deep bore" method is conducive to the one-line-at-a-time-but-continual-expansion approach. Building several "deep bore" lines simultaneously requires several of the expensive machines, and a boring machine is a very expensive capital investment to leave idle.

On the other hand, the "cut and cover" method seems like it would fit well with a major expansion plan involving several lines, as it is relatively easy to do and at least some of the political impacts may be ameliorated if the disruption can be limited in time but not in scope.

Because of the negative community sentiment that often accompanies "cut and cover" construction, almost all new subway construction is done using the "deep bore" method. One exception was Vancouver B.C.'s recently opened Canada Line and proves to be an excellent example of problems caused by the disruptive nature of the "cut and cover" method. One merchant has already won a lawsuit for C$600,000 - since overturned on appeal - due to damages caused by construction disruption, and 41 additional plaintiffs filed suit last year to recover damages.

Interestingly, the amount of money they wish to receive is equal to the savings realized by building the line using the "cut and cover" method instead of the "deep bore". 

It is likely that uproar over the temporary disruptions that accompany "cut and cover" construction will mean that almost all subway construction in the future, at least in the United States and Canada, will be of the "deep bore" variety, with the exception that soil conditions may mandate "cut and cover" construction. This result is too bad, as the cheaper nature of "cut and cover" construction could allow more proposed lines to be grade separated, which would allow for higher speeds and probably higher ridership. "Cut and cover" construction would also allow for more stations, which would make it easier to stop operating bus service along the rail corridor instead of operating duplicative bus service the hours could be redeployed to routes intersecting the rail line and make it easier for people who do not live within walking distance of a station to access the line.