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The future of rail bridge investigations How new technology is replacing outdated methodology – a case study of The Lyne Bridge
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his case study covers a rail bridge which was complex in nature and without the latest NDT and the ongoing research HTA carry out would have been near impossible to investigate with the common place traditional methodology. Henderson Thomas Associates (HTA) are the leading experts in the investigation of post tensioned and cable stay bridges and the only company in the UK to robotically test a major cable stay rail bridge. The Lyne Bridge spans the M25 close to junction 12 and is both a post tensioned and cable stayed rail bridge. HTA were tasked by Arcadis Consulting (UK) Ltd, Osborne Contracting Ltd and Network Rail to carry out the following inspections and investigations. 1. Full principal inspection to all areas of the bridge to within touching distance. 2. Concrete condition testing surveys to ascertain the corrosive condition of embedded reinforcement to the various structural elements of the bridge. 3. A full phase 3 Post Tensioned Special Inspection (PTSI) to the post tensioned elements of the structure. 4. Inspection and testing to the cable stay elements of the bridge including confirmation of corrosive condition of the hidden elements of the cable stays.
Methodology/NDT Technology applied Being a major rail structure running across one of the busiest motorways in the south of England the numerous, complex inspections and investigations required in-depth planning to ensure all works would be carried out in a smooth, safe and timely fashion. Consideration had to be made to coincide complex traffic management programs on the M25 itself with Network Rail possessions to allow all works to be carried out in a safe manner while reducing mitigating risks as far as possible. Various access approaches also had to be adopted, including standard 16-metre MEWPs to access the substructure of the bridge and a 50-metre hoist to access the tall towers centrally located. In addition, HTA applied robotics to allow safe inspection and investigation of the cable stays, this was an innovative approach, adopting technology never used in the UK before this time. The technical element of the works was split into the following three main areas.
Concrete condition testing survey Utilising the most modern of NDT equipment HTA determined the depth and location of reinforcement and its relative risk of corrosion using half-cell potential and resistivity equipment. Concrete dust and core samples were also extracted and taken away for subsequent lab analysis. Due to tight time constraints of traffic management and rail possessions the simultaneous deployment of multiple HTA investigation teams was necessary. Post Tensioned Special Inspections (PTSI) HTA adopted in part their innovative three step approach to post tensioned special inspections. This involved the determination of the exact location of the tendon profiles by employing Ground Penetrating Radar technology (GPR). GPR was vitally important on this particular structure due to the depth of the tendons (circa 260mm to 300mm deep) and the highly reinforced nature of the concrete containing the post tensioned elements (tendons). Rail Professional
