It is kept three ties ofvaults beneath the street level, beneath tidewater too, to make the tunnelling difficult.
A mile or two west of Farringdon, where the prime minister was underground to herald the tunnelling achievement, excavation continued in the heart of the West End.
More than 8,000 people have been trained at the tunnelling and underground construction academy in Ilford, north-east London, since it opened in 2011.
Also the Tunnelling and Underground Construction Academy, or Tuca, a large hangar-like building in Ilford, Essex, the majority of whose cost has been paid for by Crossrail.
To cut the rock, the tunnelling machine had to apply only about seventy-five bars of pressure to its cutting head — about a thousand pounds per square inch.
Sometimes the "tunnelling" of vision is more creative: as any artist or writer will confirm, an unmissable deadline will focus the mind like nothing else.
The tunnelling phase of the project, currently proceeding at a rate of more than 100 metres a week, is due to be completed next year.
Apart from the "tunnelling" sorters, who use handheld scanners, the most cutting-edge piece of technology on display is a biro, which is used to mark up labels so colleagues know which rail the garment is heading for.
An hour or so before that, the leader of the tunnelling organization, who will be known here as Walter, and two other students crawled through the tunnel to manage the escape.
The internal emails, released under freedom of information legislation to the building union Ucatt, were sent in September between the ATC consortium, which is running the tunnelling refit on the project, and Crossrail.
(a) Modulus of the tunnelling amplitudes.
