FEATURE SENSORS AND DETECTORS
is expected to be employed in more structures to support the construction and management of city infrastructure, although, according to de Battista, it is more likely to be useful within structures such as tunnels and bridges, where there is a higher level of doubt concerning the structural performance over time.
‘The more uncertainty
there is in a structure, the more need there is for this kind of instrumentation,’ noted de Battista. ‘This is one of the reasons why a lot of our projects have been in underground structures – for example foundations and tunnels – because there is a lot more uncertainty there.’
Fibre optic sensors integrated into four Crossrail sites provided new insights into the behaviour of the shaft linings
The fibre optic sensors integrated in both the railway bridges and the new Dyson building use two technologies: Brillouin phase distributed strain sensing, and fibre Bragg gratings with point sensors. Fibre Bragg gratings allow engineers to obtain point measurements of strain, whereas the Brillouin distributed technology enables continuous measurement of strain along the layout of the
fibre optic cable. Because these technologies are embedded within the structure, they can be used to create a map of strain of the bridge, building or tunnel in question. ‘This [map] is the same as what is produced when an engineer designs a building with a finite element software package – they predict the strength of the structure of the building, and they create a colour coded map,’ explained de Battista. ‘But we are actually doing this in reverse – we can identify what is happening inside the building in real time, and present the findings in the same way.’ To take a measurement, the
end of fibre is plugged into a fibre optic analyser, which sends pulses of light into one end of the fibre at a rate of a few thousand pulses per second. For each pulse, the analyser measures the amount of back scattered light returning to it. By measuring the time it takes for the back-
scattered light to reach the analyser, the location of the back-scattered reflection can be determined. ‘Essentially, what the analyser is reading is the Brillouin frequency of each specific location along
the fibre – let’s say at five-centimetre intervals,’ said de Battista.
‘If at any point the fibre has stretched a little bit, then we will be able to see that the Brillouin frequency of the back-scatter coming from that particular location has shifted, because the Brillouin frequency is proportional to the strain and temperature within the fibre,’ he said. The data obtained from approaches such as these can help engineers make informed decisions about a structure’s design, construction and maintenance, based on reliable evidence. For example, the fibre optic sensors integrated into four Crossrail sites by the CSIC provided new insights into the behaviour of the shaft linings and retaining walls during construction. According to the CSIC, ‘rationalisation of design approaches for shafts and retaining walls, based on actual observed performance, will be of direct economic benefit to city infrastructure, leading to more efficient designs, reduced amounts of material and faster construction.’
The CSIC led the development and installation of the fibre optic sensors at four Crossrail locations, working with the UK’s Knowledge Transfer Partnership (KTP), and which was funded by both Crossrail and Innovate UK.
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