That man is the richest whose pleasures are the cheapest. - Henry David Thoreau
Question #87482 posted on 08/01/2016 11:44 a.m.

Dear 100 Hour Board,

So to my understanding there are two forms of making a subway tunnel: cut-and-cover and tunnel boring machine. Most things I have read have said that even when a tunnel boring machine is used, cut-and-cover is used to make the station itself. Is there any way to make a station using a tunnel boring machine? (Or any other way other than cut-and-cover?) I would be very surprised if cut-and-cover was used for the Washington Park MAX station, for example, but I can't find anything to prove it one way or the other.

-I <3 Trains.


Dear Train-<3-er,

Those are the two main methods, and the two most practical for long distances (which most subway tunnels are). The main principle here is that money is nearly always the governing factor for design and construction methods. Cut and cover is cheap at shallow depths and when nothing important is being disrupted. Tunnel boring is expensive, but sometimes there are structures or right-of-ways above the tunnels that are impossible to remove or relocate. Tunnel-boring machines usually don't create tunnels big enough to house a station, and some excavation is required anyway for elevator, stairway, and ventilation shafts so they are not often used for stations. There are other methods of underground construction, and many were developed by the mining industry. The idea is that you have to temporarily support whatever you are tunneling under until it can be permanently reinforced. Tunnel-boring machines provide both the temporary support and the material removal. Cut-and-cover just removes the need for the temporary support. Excavation gets more expensive the farther down it goes, and with how deep the Washington Park Station is, I would be surprised if it was excavated as well. I wasn't able to find anything substantial, but I also doubt that it was completely excavated. To see examples of methods other than tunnel boring and cut-and-cover, check out these pictures of a similar project in New York (especially photos 6, 11, 13, and 19). Here is a non-exhaustive list of ways to build a tunnel, and here is one construction picture of the station itself. I love this kind of stuff so don't hesitate to email me with questions at skipper[at]theboard[dot]byu[dot]edu.

The Skipper