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Comment by nkrisc | Hacker News Reader

> people are being whisked through a network of tunnels, deep below the bustling city

Also compared to London or many metro systems, Chicago’s is not deep underground at all. As a Chicagoan I was very surprised the first time I saw some of the escalators in London or Washington.

In some parts Chicago’s is almost literally just basement level with nearby buildings.

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The big innovation for the Underground was the tunneling shield. It kept the tunnel from collapsing and has places for people to dig out the face.

The reason that the Underground uses small tunnels was because expensive even then to pay people to dig out tunnels by hand.

How did they build those deep tunnels before the invention of TBMs? It must have been slow going.

Get some people down there with shovels and carts, same as any public works project earlier than ~1720.

There was already a big coal mining industry so I assume they'd have used those techniques.

>How did they build those deep tunnels before the invention of TBMs?

Same approach as TBMs just manual AF. Dig a bit, put in supporting structure. Rinse and repeat.

They've been at it since 1890 though so that helps

The scenario suitable for TBM is surprisingly limited so even nowadays many tunnels are still dig using the good’o way, mostly with explosives

??? explosives? in mud? (well, clay)?!

tunnels are bored depending on the soil, length, diameter. Most projects I have seen use TBMs and the New Austrian Tunneling Method. Explosives are quite the minority (not many tunnels are in solid rock), even the Gotthard tunnels were dug with TBMs

If you have access there is a fascinating program on BBC iPlayer about digging the Victoria Line in the sixties. It's not all manual work but a surprising amount still is.

https://www.bbc.co.uk/iplayer/episode/p00sc29t/how-they-dug-...

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