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HIDDEN WATCHERS

GEOSYNTHETICS

Geodetect is claimed by its developers to be the world's first intelligent geosynthetic. Ian Fraser reports.

Detecting sinkholes and natural cavities is an important part of constructing any infrastructure passing over areas where these features are known to occur.

While they can often be identified during construction, it is not easy to detect underground cavities caused by dissolution or collapse of previously unknown mine workings if they appear after building work has 'nished.

So in areas of risk it is necessary to take suitable precautions to prevent future damage to structures through settlement. A popular solution is to use layers of geosynthetic reinforcement because they represent good value for money and are easily installed.

But even with this reinforcement, ground movements may still occur. With time, these can migrate towards the surface and put structures at risk.

To combat these problems, geosynthetic manufacturer Polyfelt Group and Belgian fibre optic sensor firm ID-FOS Research joined forces to set up the Geodetect research programme, with the aim of developing a warning system based on optical technology inserted in geosynthetic reinforcement.

The idea was to produce a system that could accurately detect the first signs of settlement near the surface, which would allow remedial measures to be taken and reduce the risk of damage.

The Geodetect system uses a Polyfelt Rock Pec geocomposite comprising a non-woven geotextile with PET (polyethylene tereph lic) reinforcement strands stitched to it. The optical fibres are threaded inside the geotextile during the reinforcement strand insertion, which overcomes installation problems met when using traditional sensors.

A flexible sheath protects the optical fibres. This ensures the Geodetect system is watertight, not susceptible to lightning strikes, corrosion resistant, free of electromagnetic interference, radiation resistant; and does not present an explosion risk (as there is no live current or risk of sparks).

The optical fibres used are fibre Bragg gratings (FBGs) - diffracting elements printed in the photosensitive core of a single mode optical fibre. This grating reflects a spectral peak based on the grating spacing.

Any change in the length of the fibre - due to tension or compression - will change the grating spacing and therefore the wavelength of light reflected back.

In the case of settlement of a structure, fibres will be stretched, and once elongation reaches a set level (ie a settlement), the system sends a warning signal.

Quantitative strain measurements can be made by measuring the centre wavelength of the reflected spectral peak. And by using different wavelengths on which the gratings are reflecting, signals of various FBG sensors can be identified.

Because each sensor has its own characteristic wavelength, they can be connected in series on one optical line or in a star configuration.

By using an optical switch several hundred sensors can be measured by a relatively small low cost data collection unit.

This device, the Geodetect FBGScan, is connected to a computer (or laptop) for study of the optical fibres' spectral readings. Geodetect FBG-Scan is also available in a hand-held version which is compatible with palm-tops for intermittent monitoring. This feature makes the system ideal for analysis of structures where the level of risk does not justify continuous monitoring.

Research on the performance of the Geodetect system included laboratory tests at the University of Grenoble, full-scale site trials and numerical modelling. Its resistance to installation stresses, practical performance and the reliability and repeatability of measurements in particular were studied and validated.

A full scale field trial of the system was carried out on a stretch of railway between Mouchard and Bourg near Arbois, south east of Dijon in east France, at the end of last year.

The line runs across an area known to have natural cavities and a fault runs at 90¦ to the railway.

French railway operator SNCF decided to reinforce a 50m long, 5m wide section of the line using geosynthetics and it chose the Geodetect system to ensure both reinforcement and the detection of any potential collapse.

Design of the Geodetect system determined the best reinforcement solution to ensure the line would remain stable above any sinkhole that formed and the optical fibre network was designed to ensure that any sinkholes were accurately detected as they formed.

A warning would be triggered if surface settlement reached 6mm, trains would be slowed if the structure settled by 9mm and repairs would be needed if it settled by 21mm.

The Rock PEC 300 geosynthetic was designed to support a load of 22.5t per axle distributed on three rail sleepers and to cope with a 1.2m diameter cavity. The maximum stress that could be applied to the geotextile was 45kPa, the case for a cavity directly under the railway when a train passed.

The fire Bragg gratings network was designed to optimise the number of sensors and to ensure settlement detection by a minimum of one sensor. This meant a network of five fibres 0.85m apart incorporating 297 Bragg gratings.

To avoid disruption of traffic, work was carried out overnight on 18 and 19 October 2004. The track and ballast layer were removed and 0.5m thick layer of soil excavated over a 60m by 5.5m section.

The geosynthetic was laid and soil replaced, along with the 250mm thick ballast and finally the track.

The Geodetect system has more recently shown another potential use, on a new bypass along the RN38 at Saint Saturnin near Le Mans in north west France.

The road, being built by contractor Jean Levefre for the French Ministry of Infrastructure, has to cross a small river. The 16m span bridge abutments are 9.9m high geosynthetic reinforced-earth structures with a facing of prefabricated concrete blocks.

The retaining walls were designated as a 'reference project' for contractor and designer Betoconcept's Leromur concrete block system, which meant monitoring was needed to measure the deformations during and after construction.

Betoconcept decided to use Geodetect, because of the benefits of the system compared to conventional instrumentation systems, namely its ease of installation, the accuracy of measurements, its longevity and cost-effectiveness.

Nineteen layers of knitted geofabric with strips of Geodetect were installed at various heights within the embankment. Each strip has seven measurement gauges, adapted to the Leromur wall design. The strips were anchored into the concrete blocks.

Continuous monitoring was carried out during compaction. A maximum strain of 0.5% was observed, reducing to a stable level of a maximum of 0.28% after compaction.

Further measurements were made intermittently during construction.

Results show most of the deformation was mobilised during installation and compaction of the embankment fill. Subsequently, deformation increased only slightly, reaching a final maximum strain of 0.73% at a wall height of 6.5m. Deformation monitoring will continue during construction of the bridge deck later this year.

These first real projects have confirmed the excellent results of the research. Easy installation and the robust components mean Geo etect can be used in a variety of monitoring situations.

Ian Fraser is managing director of Polyfelt Geosynthetics (UK).

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