What Lies Beneath: Excavating Crossrail’s tunnels | In-depth | The Engineer This is fascinating reading. Example facts from this piece: tunnel boring machines go at about 100m per week, and have a turning radius of 250m. Inpressive enough, but in places the tunnels these things are cutting are passing with 1m of existing tunnels, which, obviously, it would be very bad if they were to hit. "Lads, we've got to start turning now, so that we're in the right place in 2 weeks time. If we're out by over a foot, were going to cause major damage to something expensive. No do-overs. Everyone ready?"