Instrumentation of tunnels by distributed optical fiber sensors: the Fréjus Tunnel, France

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Summary

The distributed measurement done by optical fiber is a new technique that has opened many possibilities in the monitoring of deformed tunnels. In this specific case of monitoring the behavior of the Frejus tunnel (located on the France-Italy border) before and after extensive digging, Cementys used its " SensoLuxTM®" a sensor to measure the deformation of the tunnel vault. It is a cable containing four optical fibers, which makes it possible to measure the backscattering deformation Brillouin (sensitive to deformation and temperature), and Raman backscattering temperature (temperature-sensitive). The sensor is completely responsive over its entire length. Once integrated into the structure, it measures the deformation of the concrete in tension and compression, therefore determining the convergence phenomenon of the tunnel. Data can then be transmitted over long periods and distances thanks to the optical fiber, thus allowing deformation tracking in real-time.

Introduction

Auscultation of underground infrastructures is crucial to know its structural state and to be able to mitigate the potential risk of accidents during the construction or operation phase along with optimizing long-term maintenance. In tunnels, visual inspections can be difficult to achieve because of restricted access. A distanced real-time vault monitoring system detects and follows the movements of the structure.

Distributed measurement, unlike point sensors, can monitor a tunnel over its entire length. It makes it possible to precisely locate and identify potential malfunctions, with only one only optical fiber-connected.

Wide ranges of fiber optic sensors have been developed for different uses. In our article, we will introduce you to one of these major technologies: the distributed measure of pressure and temperature by Brillouin / Raman coupled optical backscattering.

What is an optical fiber?

To understand fiber optic sensors, let’s first look at what an optical fiber is. An optical fiber is a silica flexible wire that transmits light from end to end. It is extremely thin; it nearly has the same diameter as a strand of hair. Also, it is completely passive: it carries no energy other than light. The heart of the fiber can be integrated into a sheath. We can, therefore, obtain an optical cable as resistant as an electric cable.

An optical fiber is composed of two layers of silica, with different optical indices. This difference means that the light is reflected inside the heart, which ensures its transmission without a loss (Figure 1).

Figure 1. Section of a fiber optic cable

LFiber-Optic distributed measurement by reflectometry

A distributed measurement called “répartie” in French is a major advance in terms of monitoring linear structures: single fiber telecommunication can provide measurements every meter for several kilometers worth of measurements! Having no punctual sensor, the fiber itself is impacted by its thermomechanical environment and transmits information through the light directly to the measuring device. The fiber, therefore, serves as a sensor and as a means of data transmission. (Figure 2)

Figure 2. Capteur SensoLuxTM®

The measuring device analyzes backscattered light in the fiber (Figure 3) by time domain reflectometry (radar technology).

Figure 3. Measurement principle by reflectometry

A laser emits a short light pulse of about ten nanoseconds through the fiber. This transmitted flash is backscattered back to its source in the fiber, which is due to different physical phenomena (Rayleigh, Raman, and Brillouin backscatter) (Figure 4). The received signal is then analyzed in the time and frequency domains. The location of the information is thanks to the signal return time (measured for a known speed of light in the fiber).

Figure 4. Backscattering spectrum of light

Application: experimental instrumentation of a section of the Fréjus tunnel

The SensoLuxTM® sensor was installed on the Fréjus tunnel, which is the longest road tunnel in Europe, linking France and Italy across the Alps. The fiber optic sensor was placed on areas near sensitive and important work, such as the digging of a new gallery. (Figure 5)

Figure 5. Frejus Road Tunnel

The system consists of gluing the fiber optic cable to the bottom of a millimeter groove dug on the surface of the concrete, found in the coating of the reinforced concrete structure. The instrumentation was made on three rings with a distance of 5 m from each other. (Figure 6)

Figure 6. Optical fiber installation plan: the fiber (in red) is glued to each vault

The small diameter of the cable (2mm) makes it easy to integrate on the vault by structural reinforcement in grooves milled in concrete. Therefore, the fiber optic cable is protected for long-term monitoring without the maintenance of the measuring system and without drifting the data. This solution also has the advantage of perfectly integrating with the material and not being intrusive. (Figure 7)

Figure 7. Detail of a wall equipped with the SensoLuxTM® sensor embedded in a milled groove in the coating of the tunnel bottom surface.

The sensor detects local disorders such as cracks as well as general deformations such as convergence or bending phenomena. SensoLogger® measures the deformations of concrete without a blind zone and with a spatial resolution of 50 centimeters along with an accuracy of ± 10 μdef (10 " micro deformation" or micrometers of deformation per meter of reinforced concrete).

The Brillouin optical backscattering deformation measurement is a relative measure because the fiber has intrinsic mechanical stresses to the cable and its installation: the initial measurement is, therefore, necessary and constitutes the reference. The following measurements will be compared to this first one. Consequently, the initial deformation variations inflicted on the fiber during laying have no impact on the measurement.

In several studies carried out on concrete structures with thermal gradients, we have developed a decoupling methodology for phenomena measured by fiber optic sensors. This method is based on the principle of superposition of deformations during the life of the work.

Figure 8. Déformation des arches 1, 2 et 3 (µɛ)

Figure 9. Diamètre du tunnel sur un profil – amplifié 1000 fois

Civil engineering concrete structures often have as much thermal deformation as mechanical deformations. Also, the photo-elastic Brillouin effect is impacted by temperature. Overall, it can be assumed that a change of one degree Celsius in the SensoLuxTM® fiber embedded in a "free"

concrete will generate a change of deformation measured by the Brillouin effect of about 30 micro deformations. That is why we must systematically measure in the same optical cable the Raman backscattering temperature distribution on a multi-mode fiber distinct from the single-mode fiber used

for Brillouin backscattering.

Therefore, with an accuracy of ± 0.1 ° C, we can measure mechanical deformations with an accuracy of about ± 15 micro deformations. The variations of deformations measured during the digging phase of the gallery are presented by profile on a color scale. (Figure 8)

In figure 9 we observe a phenomenon of convergence that is very weak and fairly regular on the vault. We found in these digging tests that the profile of the tunnel was very slightly deformed in the instrumented areas.

The analysis of the deformations of the structure in the long term is possible thanks to the installed fibers. To interpret these measurements, the “free” part related to concrete shrinkage will have to be evaluated if one wishes to calculate the mechanical stresses that evolve over the long term.

Conclusions

Fiber-optic distributed measurement has a unique ability to instrument large-scale structures, compared to conventional gauges or extensometers; optical fiber provides comprehensive measurement over a wide area to diagnose complex deformation phenomena, in heterogeneous environments.

This technology makes it possible to represent the spatial deformation state of the tunnel in real-time, which is accessible by the work manager on a secure server.

The small size of the sensor allows it to follow the behavior of the structures from the inside of the materials (keying mortar, voussoirs, shotcrete …).

Its ability to detect and locate punctual phenomena (cracking, water inflow, fountains, etc.) is also noteworthy.

References

Lamour V., Haouas A., Dubois J.-Ph. & Poisson R. (2009) Long term monitoring of large massive concrete structures: cumulative effects of thermal gradients. 7th International Symposium on Non-Destructive Testing in Civil Engineering (NDTCE’09), Nantes, France.