Phase go under the bridge

Phase SI were asked to carry out a ground penetrating radar (GPR) survey of a masonry arch bridge in Scotland to obtain information on the bridge structure and fill material.

The bridge was in need of strengthening work but information on the structure of the bridge was limited. Before deciding on what type of strengthening work was needed the client needed more information on the bridge abutments, the thickness the masonry and what type of fill material was present. This information is often obtained by coring and other intrusive work but the bridge is a listed structure and was also on a main trunk road and so the client needed to keep any intrusive work to a minimum. They decided to carry out a GPR survey to obtain as much information as possible about the bridge and whether there were any voids or other anomalies present in the bridge structure, which would hopefully minimise the amount of coring work that would be needed. This was where Phase SI came in.

We carried out a GPR survey using a range of different antennae. The carriageway was surveyed using low, mid and high frequency antennae, giving a range of depth penetrations and resolutions. The two outer arches could be accessed by foot when the river was not in flood and scaffolding was erected along one side of the bridge and under the central arch. This meant we were able to obtain horizontal and vertical scans along the side of the bridge and we were also able to survey along and across the arches. Mid and high frequency antennae were used for these scans.

The GPR data provided information on the thickness of the masonry arches and also detected variations in the wall thickness and anomalies in the fill material. Variations in the carriageway were also detected but it was thought that this was due to repairs in the asphalt. It was not possible to determine the shape of the abutments from the GPR survey but the information obtained meant that the follow on coring work could be targeted to specific areas.