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GPR used for embankments on the London Underground

METRONET ANNOUNCED last month that ground penetrating radar (GPR) has been introduced on the London Underground for the first time to inspect the ground under the track on railway cuttings and embankments on the Metropolitan Line.

Under the system introduced by public private partnership contractor Metronet Rail, radar antennae fitted to a track trolley sends radio waves into the ground. Readings from the reflected signal measures the thickness of the underlying soils.

Metronet earth structures assessment team leader Neil Esslemont told GE the new equipment supplied by Zetica gives a continuous survey of ground conditions on a railway embankment. Generally on a typical 200m rail embankment, only two holes would be bored as part of an intrusive ground investigation.

Metronet said the equipment will be particularly useful to identity settlement in the ash used as fill to build up embankments - before granite and limestone ballast became the norm. If the radar detects very thick and compacted ash, some degree of settlement is possible.

'If a significant amount of ash is found on the embankment it can be a problem, ' said Esslemont. 'It can unravel at the edges [of the embankment].'

Following the GPR survey, Metronet will target potential problem areas for intrusive ground investigation.

'It will never replace an intrusive ground investigation but it will help us target (problem areas) better, ' said Esslemont. 'Without it we can only investigate one area every 100m or so. The equipment is easily portable, which allows us to gather a lot of information very quickly. On the fi rst night we went out we covered eight miles [12.9km].'

Metronet expects to use the equipment on the Metropolitan Line next summer between Chalfont and Latimer and Chorely Wood stations following the successful trials.

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