Non-intrusive survey bridges information gap!

Phase undertook a ground penetrating radar (GPR) survey of a masonry arch bridge to determine the thickness of the arch and information on the position and depth of the bridge abutments. The bridge in question was a listed masonry arch structure. The bridge was in need of strengthening but because it was listed the client needed to keep the intrusive investigation to a minimum.

It was recognised that it might be difficult to obtain some of the information required as it was not possible to access the side of the bridge, the abutments were located at depth and the material above them was believed to consist of sands and gravels.

Phase recommended carrying out a GPR survey from the road surface above the bridge utilising two different antennae. A low frequency antenna would obtain good depth penetration, up to 5 m in good survey conditions, but would not resolve near-surface features. A mid frequency antenna has much better near-surface resolution but its depth penetration is usually restricted to around 2 m.

The GPR survey was successful at determining information on the near-surface features, such as the depth of the pavement and thickness of the masonry arch. However, the sands and gravels were saturated at the time of the survey and this meant that the depth penetration was significantly reduced and hence information on the abutments was less conclusive. It was possible to determine the position of the abutments and to make an estimate on the depth to the top of them but it was not possible to determine their profile or extents. Phase were able to remotely survey the bridge profile using a reflectorless total station which allowed the GPR data to be accurately presented and could also be used by the client for their later design drawings.

The possible limitations of the GPR had been explained to the client before the survey commenced and so they were aware that it might not be possible to achieve all of the survey objectives. However the information that was provided is still of great benefit as it will allow the client to target their intrusive work better than would have otherwise been the case, producing overall costs savings and minimising the disruption to the users of the bridge.