Dis-stress message

An innovative monitoring technology that uses fibre-optic cables to measure strain in building foundations and road embankments is to be tested on the M25.

Researchers are installing around 10km of fibre-optic cable into the embankments that support a section of the motorway in a bid to develop an early-warning system of landslides. The same technology will also be used to measure the effect on buildings during construction of London’s proposed new Crossrail rail link.

The technique uses standard optical fibre coupled with sophisticated analysis technology, resulting in what is claimed to be a new type of strain measuring system with distinct advantages over comparable monitoring techniques. The Brillouin optical time domain reflectometer (BOTDR) analyses the light reflected along the cable.

The analyser, bought by Japanese technology group Yokogawa from telecommunications company NTT, provides information on both the location and the magnitude of the strain.

When the fibre is strained by movement of the earth that surrounds it, some of the light travelling along the cable is scattered back to its source. The frequency of the back-scattered light is altered by an amount proportional to the strain at the scattering location. After analysis, this provides a ‘strain profile’ along the full length of the fibre.

Project leader Dr Kenichi Soga of Cambridge University’s engineering department says this is a significant advance.

‘The interesting aspect of this technology is that you can turn tens of kilometres of fibre-optic cable into a cheap and accurate, continuously distributed monitoring system,’ he said.

Using fibre-optic cables to measure strain is not new. Fibre Bragg Grating (FBG) is one such successful technology which uses modified points in a photosensitive fibre-optic cable to detect movement.

However, Soga said FBG technology is limited in its applications, because the sensors involved can only measure strain at a specific point in the cable, with no more than around 40 points per section of cable. Soga added that although FBG is useful for installation in beams or columns, where single point monitoring is sufficient, BOTDR is far more accurate when used in foundations that are in contact with the ground.

‘We need to understand distributed strain if we are going to develop urban infrastructure safely,’ said Soga. ‘If we just measure individual points along a cable, however close they are, there is the possibility of a slight error in calculating the strain between them. Distributed monitoring may have a slightly lower resolution, but it covers every part of the cable, which is crucial.’

The technology was developed in Japan by NTT. This is the first time it has been used in the UK, but prototype tests have taken place in Japan, where the system was used to monitor strain in the masts of superyachts and in the foundations of buildings.

In the trial along the M25, Soga’s research team is working with the Highways Agency to install fibre-optics in the motorway’s embankments.

In the past six months, the team has also worked with building companies in London, installing fibre-optics in the foundations of new buildings in Farringdon and Paddington. These two areas lie along the proposed Crossrail train link route. The tunnel for the Crossrail network will have to move between the piles for these buildings, and Soga’s fibre-optics should provide the building companies with information on any effect this might have on their foundations.