Fibre optic strain sensors for structural health monitoring
Impact of fibre optic technologies by Kenichi Soga and Robert Mair
Image: On-site splicing of fibre optic cable at the James Dyson Building, Department of Engineering, University of Cambridge. Picture courtesy of Darren Carter, Morgan Sindall
The technology
The use of distributed fibre optic (FO) sensors for the monitoring of civil structures and infrastructure opens exciting new possibilities unmatched in conventional sensor systems.
Engineering design limits are often based on strain and/or stress developing in the structure. For structures interacting with soil (e.g. underground infrastructure such as foundations, tunnels or pipelines), the ground loads are distributed spatially (not point loads); therefore the state of the structure cannot be fully understood unless the complete in situ strain regime is known.
The use of a single optical fibre with a length of up to tens of kilometres of continuous sensing elements makes it possible to obtain a body of invaluable information on the strain and temperature distribution in civil infrastructure assets. CSIC has developed data interpretation methods that utilise new continuous data for engineering assessment of piles, tunnels, retaining walls, pipelines, slopes and bridges.
In collaboration with Industry Partners, CSIC is developing a FO application methodology that is applicable to civil engineering structures. It considers the whole-life cycle of FO monitoring from planning, deployment, operation and decommissioning. CSIC is also producing a FO guidance document, which will be available from ICE, publishing in late 2015.
Applications
CSIC’s fibre optic technologies have been applied to provide insights into structural performance and design, to monitor the construction process and for structural health monitoring. In the last year, applications included understanding sprayed concrete linings in Crossrail’s London Liverpool Street Station and instrumenting concrete tunnels in CERN and piles and buttress walls at the London Bridge Station redevelopment.
Impact and benefits
• the distributed FO technology for measuring strains and temperatures provides a unique dataset, which engineers did not have before
• applied to piles, FO technology will provide displacement profiles along the pile enabling the soil-pile interaction to be evaluated more accurately. Anomalies inside the piles that may influence the overall pile performance are also identified
• the material of optical fibre is silica, which lasts for decades. When the cable is embedded in structures during the manufacturing process, the sensors will remain operational for many years
• the analyser, which is external to the structure, can be evolved over time to provide continual improvements in the quality of the data gathered
• the potential impact of this technology for civil infrastructure design, construction and maintenance is significant, providing asset owners and engineers with new data that has not been accessible until now
New data sets present new challenges – we must now decide how data will be interpreted and used for smarter design, construction and maintenance of civil engineering infrastructure.