A carriageway in Northumberland located on an embankment had been undergoing repeated localised subsidence and our client was commissioned to find the cause of the problem. As part of their investigation strategy they decided to commission Phase Site Investigations to carry out a ground penetrating radar (GPR) to try and determine if there were any near-surface variations that could be the cause of the problems. We carried out the survey using low and medium frequency antennae over a 300 m stretch of the carriageway.
Tension cracks were visible in the road surface and the GPR survey identified anomalies in the vicinity of these, although the GPR responses were more extensive that the visible cracks. These were near-surface anomalies, thought to be associated with increased moisture content, and were believed to be a product of the subsidence rather than its cause.
A number of other anomalies were identified which indicated variations in the pavement structure but no anomalies suggestive of voids were detected. Several curving and dipping interfaces were identified in the GPR data which indicated where previous ground surfaces had slumped but no features were identified which would indicate the cause of this slumping. As part of their investigations the client was carrying out coring work and when one of the cores was targeted onto the area with a dipping interface it was found that over 2 m of asphalt was present! This was a quite staggering find and obviously indicated that there had been a long-term problem at the site which had been constantly patched up.
The lack of isolated features or voids in the GPR data and the long-term nature of the problem led the client to further investigate the construction of the road over a wider area. It was found that the embankment on one side of the problem area sloped sideways to the east and on the other side it sloped sideways to the west. Effectively the road was gradually moving in opposite directions centred around the problem area. The GPR survey was a vital key in determining that the problem was not localised and so helped our client determine the overall cause of the subsidence.