Case Study: Wandsworth Bridge (February, 2019)
Richmond and Wandsworth Council commissioned Zetica to investigate Wandsworth Bridge, London, UK for any evidence of water penetration and retention between the carriageway surface and the bridge deck.
Wandsworth bridge is a steel cantilever bridge, designed by Sir Thomas Peirson Frank and completed in 1940. Its unusual colour scheme was designed to camouflage it against aerial bombing.
Our solution combined a multichannel, stepped-frequency dense array and a 4-channel impulse ground penetrating radar (GPR) system. These complimentary technologies allowed us to harness their strengths to bring about the required data with a high level of accuracy and certainty in our interpretations.
Our interpretation methodology was based on the principle that areas where water is trapped at an interface between two pavement layers will result in a higher amplitude reflection compared to areas where the interface is dry and the two materials are well bonded.
The impulse system would be used to interpret the pavement and bridge construction layers and to map the relative amplitude of observed layer interfaces. The stepped-frequency array would be used interpret and map anomalous responses in three dimensions, including buried utility services and other detectable below ground features.
The quality of the data across the Site was excellent. Both GPR systems achieved signal penetration to the inferred base of the bridge structure. The asphalt-concrete interface was clearly visible as a continuous interface at a depth of around 100mm, while a less continuous layer interface at ~50mm represented an inter-asphalt interface.
Reflection amplitude variations were mapped at both material interfaces. Areas of localised elevated amplitude were mapped and interpreted to be be caused by water saturation or delamination at the relevant interface
The stepped-frequency dense array data identified a large amount of anomalies, structural features and buried utilities. A selection of examples are shown in the images opposite, including (top left) a buried utility running parallel to the road, (top right) a manhole chamber, (bottom left and inset) a surface crack and (bottom right) an expansion joint.
The locations of all detected anomalies, structural features and buried utilities were extracted and added to a CAD model which accompanied our technical report which our client described as clear to understand.