Metronet Rail, responsible for the maintenance and renewal of two thirds of the Tube, has introduced ground penetrating radar to investigate embankments and cuttings.

The technology was first used on the London Underground in two trials on 10 June and 5 August when specialist rail geophysical company Zetica Ltd undertook surveys for Metronet between Chalfont & Latimer and Chesham stations on the Metropolitan line.

The radar antennae are fitted to a track trolley and send radio waves into the ballast. Readings from the reflected signal provide indications as to the thickness of the ballast and the underlying soils. Areas where the ballast or underlying materials are unusually thick may indicate historical settlement of the track.

Although Metronet is using the readings initially in connection with stability assessment of earth structures such as embankments, the technology also has the capability to indicate areas where the condition of the ballast or drainage is poor, affecting track condition. This means the survey can also be used to guide maintenance regimes for ballasted track and drainage. 

Ground penetrating radar (GPR) technology has been used extensively by Network Rail and rail companies internationally. The introduction of GPR to the Tube network reinforces Metronet's commitment to bringing new and innovative solutions to the Underground.

This is further supported by the creation of Metronet's dedicated innov8 team which aims to deliver creative solutions to new and old problems, develop better ways of working and implement new technology and engineering solutions.
Metronet Earth Structures Assessment Team Leader Neil Esslemont said: “GPR technology enables us to obtain an accurate assessment of the composition of the track formation on the Tube network by safe, non-intrusive methods.
“The equipment is easily portable, which allows us to gather a lot of information very quickly, in this case around 4 miles of track in one night shift.”