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10.07.17

Railway bridge condition monitoring

Source: RTM Jun/Jul 17

Liam J Butler and Nicholas de Battista, from the Cambridge Centre for Smart Infrastructure and Construction (CSIC) at the University of Cambridge, discuss their innovative work on the Stafford Area Improvements Programme to condition monitor two railway bridges.

Regular visual and tactile inspections are the basis for informing management decisions, establishing remaining capacity and ensuring the safety of the UK’s bridge stock. Bridge inspections represent a multi-million pound investment by both highways and railway authorities. For example, Network Rail spends over £50m annually on bridge inspections, half of which is spent on tactile/detailed inspections. Aside from their cost, bridge inspections can place operators at risk, only provide condition-based information (i.e. no quantitative link to remaining capacity) and may not capture the condition of critical elements which are either hidden or inaccessible. 

The challenge is to develop monitoring systems and data analysis tools capable of providing the information required to assess structural capability remotely and in real time, thereby significantly reducing the frequency of tactile inspections of bridges. A secondary challenge is being able to assess in real time the effect of increased or abnormal loads on the structure which could inform points of intervention and reduce the need for additional repair or strengthening. 

Integrated monitoring through the installation of sensors during an asset’s construction can enable both a proactive and reactive approach to bridge asset management. 

Fibre optic sensor technology

For the past six years, the CSIC at the University of Cambridge has been developing fibre optic sensing technology for monitoring civil infrastructure. This technology includes both distributed fibre optic sensing (DFOS) and fibre Bragg gratings (FBG). 

With DFOS, a standard optical fibre is transformed into a sensor which can be interrogated with a specialised optical spectrum analyser to provide a measure of strain and/or temperature along the whole length of the fibre. DFOS cables can be embedded or attached to a structure to track changes in strain over time within the structure at any point along the cables. 

On the other hand, FBG sensors are point sensors which mimic traditional strain gauges. However, unlike strain gauges, FBGs use light signals and it is possible to have several FBGs in series on a single fibre optic cable. Similar to DFOS, FBGs can be embedded in or attached to the surface of a structure to provide highly accurate dynamic measurements of strain. 

Fibre optic sensors provide several advantages over traditional instrumentation. A single optical fibre can provide tens (in the case of FBGs) or thousands (in the case of DFOS) of sensing points, all from a single fibre optic cable. Since the optical fibres only carry light signals, rather than electricity, the sensors are immune to electromagnetic interference and are intrinsically safe. Additionally, the optical fibres are composed of glass which is inert and therefore will not corrode or deteriorate over time, thus providing sensing capability throughout the lifetime of a structure. 

Self-sensing railway bridge prototypes 

Through CSIC, a research project was undertaken to install fibre optic sensors in rail infrastructure. Completed in April 2016, two new bridges were instrumented with both DFOS and FBG sensors during construction, providing the structures with ‘self-sensing’ abilities. These self-sensing bridge prototypes form part of the Stafford Area Improvements Programme, a £250m award-winning rail redevelopment project on the West Coast Main Line near Stafford. 

The first bridge, Underbridge 11, is a prestressed concrete girder bridge with infill concrete deck. The second, Intersection Bridge 5, is a Network Rail type ‘E’ half-through bridge with a concrete composite deck. The bridges were instrumented and monitored during the various construction stages, with the fibre optic monitoring systems capable of providing information required for real-time capability assessment.

The primary focus of this investigation was to use sensor data as a means of tracking the entire load history, right from the beginning of the construction of the bridges and throughout their service lives. Data from sensors are currently being acquired and processed on a monthly basis. The processed data so far have revealed relatively low strain under typical day-to-day passenger trains, as compared with the maximum expected train loading used in the design of the structures. 

Another objective of this project was to track the load path from the axles, through the rail, into the sleepers, through the ballast and into the bridge superstructure. Therefore, FBG sensors were also installed during the manufacturing process of several prestressed concrete sleepers. These were then installed and integrated into the overall structural monitoring system for each bridge. The in-service data provided by the self-sensing sleepers has not only allowed for the tracking of loads through the track bed, but has also provided axle, bogie and train-specific information such as train speed, direction, axle spacing, etc. In addition, data collected during the sleeper manufacturing process has led to a better understanding of material behaviour, fabrication techniques and the effect of environmental conditions on sleeper performance. 

It is anticipated that these bridges will continue to serve as important demonstrators for the use of fibre optic sensing technology in rail infrastructure. Given the overall potential of this technology to enable data-driven approaches to design, construction and maintenance, it is foreseeable that future infrastructure projects, such as HS2 and Crossrail 2, may adopt fibre optic sensing technologies as standard permanent instrumentation.

FOR MORE INFORMATION

W: www-smartinfrastructure.eng.cam.ac.uk

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