Rugged fibre optic sensors that will help detect landslides and accurately monitor soil movement under railway and bridge embankments are being developed by UK researchers.
A multi-disciplinary research team is hoping to produce highly sensitive arrays of sensors that will be able to detect the pressure of water in granular soil, but are tough enough to survive being buried in harsh environments.
As well as monitoring soil movement and pressure the sensors will provide information about its chemical makeup and stability.
Dr Stephen James of Cranfield University, who is leading the team comprising researchers from the Universities of Edinburgh and Aberdeen, said that the project’s aim is to develop a ‘multi-parameter’ sensing approach to soils.
‘Existing soil-monitoring technology can only provide data on one parameter at a time,’ said James. ‘If a borehole is dug into the soil and an inclinometer is put in, it can only measure the deformation of the soil. It doesn’t look at the pressure of water trapped within the soil, which is an important factor in soil stability. A complex interplay of physical properties determine the stability of soil.’
The sensors are based on fibre bragg-grating technology, in which a specially designed, 5mm wide structure is etched into the core of a fibre-optic cable. Light is beamed along the length of the cable and, when it encounters the structure, one particular wavelength of light is bounced back towards the source.
If the cable is stretched or heated up then the properties of the structure and the wavelength of light that it bounces back are both altered slightly. By analysing the wavelength shift the engineers will receive crucial information about the condition of the buried cable and the soil around it.
Along the length of the cable there will be a large number of the structures, each with its own determined wavelength, so the engineers will know exactly which part of the cable the wavelength comes from.
The main problem for the researchers is how to make sensors rugged enough to endure being buried beneath the ground for long periods of time, while still maintaining a high level of sensitivity. The team has experimented with a composite carbon fibre coating but it wasn’t ideal.
James said: ‘The composite coatings worked well in terms of allowing the cable to interact with the strain of the soil around it but they were no good in detecting the pressure of water or chemical sensing, so we’ll have to keep on looking for the right material for the job.’
According to James, the technology’s wide range of potential applications includes monitoring coastlines and dams to detecting avalanches and landslides.
Later this year the team is taking the technology to Iceland, where it will be used to monitor the internal movements of a glacier. Similar sensor technology is also being considered for use on civil structures such as buildings and bridges.