5 minute read
Bridges
The future of rail bridge investigations
How new technology is replacing outdated methodology – a case study of The Lyne Bridge
This 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).
HTA applied varying GPR equipment which included the Proceq GPR Live, the GSSI Structure Scan Mini and the GSSI Structure Scan Mini XT. Utilising this technology had the far more important result in accurately finding the exact tendon location and depth prior to attempting to expose said tendon.
This allowed for the works to be carried out in a timely fashion. Without using this technology, the tendons would not have been located and investigated and conventional methods would have made it near impossible to carry out the necessary testing and investigations.
Rise of the machines Due to the innovative design of the Lyne Rail bridge utilising both post tensioning construction and cable stay construction types meant the actual working window to ascertain the external condition and internal corrosive condition of the cable stays, was around two hours, during this time included a full M25 closure.
Very much aware of the limitations of current testing and investigation methodologies HTA constantly spends time on research and development of new technologies and approaches. This previously included research as to the most update methods and actions of investigating cable stays, which resulted in time spent in Tampa, Florida with IPC Limited, an American company who had developed an NDT device that tested cable stays using robotics.
After being shown the possibilities and understanding the technology HTA felt confident that this approach had far reaching benefits to the testing and investigation industry.
The outcome in respect to the Lyne Bridge, was that HTA and IPC were able to safety access a large percentage of the cable stays with high-definition cameras and NDT called Magnetic Flux Leakage. The robotic approach is carried out by clamping the equipment around the cable stay at parapet level all from a place of safety.
Once secured the robot is able to traverse the full length of the cable to its anchorage point some 20 metres above track level, safely under the control of the operative, located at the base of the cables, allowing work on the cable stays to be carried out while the track was live and reducing the risk of working at height. Fail safe systems are included to counteract any power failure, in such circumstances the equipment can return to the operator in a place of safety.
In summary, without Innovative NDT in the form of Ground Penetrating Radar and the deployment of Robotics the necessary works to complete the Lyne Bridge investigation would have been near impossible. It certainly would have taken far longer, been more destructive to the asset, very disruptive to the public and a higher risk to the engineers on site.
A track record Henderson Thomas Associates reduce Complex Management Systems, Hassle and Numerous Sub Contractors on site by using our multi-skilled rail teams with a proven track record of over 25 years working on National and London Rail alongside their sub consultants and contractors.
Providing comprehensive H&S Management Systems, HTA ensure works are carried out safely and meet the strict time constraints required when working within rail possession windows. Working on rail assets is complex in nature, plus accessing and completing such works requires careful planning.
Having RISQS Approved & ICI Certificated Staff with many years of experience ensures HTA is a company who can competently fulfil all requirements. For more information on HTA get in touch via the contact information below.
Tel: 01707 327 785 Email: testing@hendersonthomas.co.uk Visit: www.hendersonthomas.co.uk
