Tunnel vision

London Underground is to trial a digital imaging system that monitors the movement of train tunnel walls.

A full prototype of the technology was first used last spring by engineers on the Channel tunnel Rail Link (CTRL) to monitor the effect the new tunnel's construction might be having on the stability of the North London rail line above it.

According to Keith Bowers, who worked at the time for consultant Rail Link Engineering but is now employed by London Underground, his new employer is set to use the technology towards the end of the year.

Conventional surveying techniques such as robotic total stations or manual precise levelling are often used in projects such as the CTRL to measure the displacement of walls and embankments during tunnel boring.

While these can provide an extremely high level of precision, there are occasions when it is not always practical to use them. This can be due to frequently passing trains, an electrified track or where it is too dangerous to allow surveyors easy access to the site.