Brillouin optical fiber distributed sensor for settlement monitoring while tunneling the metro line 3 in Cairo, Egypt V. Dewynter, S. Rougeault, S. Magne, and P. Ferdinand CEA, LIST, Laboratoire de Mesures Optiques Centre d’Etudes de Saclay, 91191 Gif-sur-Yvette cedex, France F. Vallon, L. Avallone, E. Vacher, and M. de Broissia Bouygues Travaux Publics, Challenger, 1 avenue Eugène Freyssinet Guyancourt 78061 Saint-Quentin-en-Yvelines cedex, France Ch. Canepa, and A. Poulain ACOME, Usine de Romagny, 50140 Mortain, France Abstract: Safety while tunneling is one of the main challenges for underground constructions, avoiding confinement losses, which remain an important risk for public works, leading to additional delays and high insurance costs. In such applications, usual surface instrumentations cannot be set up because of high building density in many overcrowded cities. Tunnelling deals with the challenge of requiring ground surface undisturbed. One original concept proposed in the framework of the European Tunconstruct project, consists in very early settlement detection close to the tunnel vault and before any detectable effect on the surface. The adopted solution is to set-up a sensing element inserted into a directional drilling excavated above the foreseen tunnel. The methodology is based on the well known Brillouin Optical Time Domain Reflectometry (B-OTDR) in singlemode optical fibres and a special cable design dedicated to bending measurement. Two cables, based on different industrial manufacturing processes, have been developed taking into account the strain sensitivity required, the flexibility and the robustness for borehole installation, a low power attenuation and storage on a drum. Industrial prototypes have been manufactured and validated with tests in open air where settlement profiles geometry can be accurately controlled. Demonstration on job site took place on The Greater Cairo Metro Line 3 (CML3) at the beginning of 2009. Keywords: Optical Fibre Sensor, underground infrastructure, tunnelling monitoring, Brillouin measurement, BOTDR, strain, temperature, curvature measurement 1: INTRODUCTION Urban tunnelling deals with the challenge of requiring ground surface undisturbed. This means that ground subsidence must be avoided. This kind of risk exists for all the tunnelling techniques: explosives, cutter machine or Tunnelling Boring Machine (TBM). The Tunconstruct project deals with TBM tunnelling and the traditional method to secure the tunnelling is to maintain the soil pressure during and after the tunnel construction. Many settlement devices, mainly based on vertical boreholes are already available to achieve this goal and the simplest device consists in a rod anchored at a given depth. The monitoring is realized by following the vertical displacement of the emerging rod with an optical prism. The reading must be done from a distant location to take into account any subsidence of the ground level at the rod place. Vertical boreholes can also be instrumented at different depths, and settlement gauges are installed along the boreholes to detect deformations at several levels. However, these devices are not easy to use in urban environment because they need frequent corrections as they cannot be installed at the vertical of
20th International Conference on Optical Fibre Sensors, edited by Julian Jones, Brian Culshaw, Wolfgang Ecke, José Miguel López-Higuera, Reinhardt Willsch, Proc. of SPIE Vol. 7503, 75035M © 2009 SPIE · CCC code: 0277-786X/09/$18 · doi: 10.1117/12.835376 Proc. of SPIE Vol. 7503 75035M-1
the TBM axis due to existing construction. Also, instead of continuous measurement, they provide a point measurement and many boreholes are required. To perform continuous and real time monitoring, the original approach used consists in very early settlement detection the nearest to the tunnel vault, before any effect occurs on the surface, and based on instrumented horizontal directional drilling [1]. The borehole trajectory is done above the future tunnel axis and parallel to it. The solution we are working on consists in introducing a curvature sensing cable based on the Brillouin optical fiber measurement technology [2] between the tunnel and the surface (typically 2 m above the vault) and along the total length of the future tunnel. Any occurring settlement will be detected by a curvature change on the cable. The settlement can then be calculated by a double integration of the curvature.
2: CABLE DESIGN AND MANUFACTURING The knowledge and experience feedback from tunneling show that two critical phemomena may appear in job site, in comparison with standard settlement of few millimeters. The first one deals with a too large soil settlements. In this case, the sensing cable will measure the same shape of vertical displacement as for standard situation, but with higher amplitude. The second abnormal situation is an initiation of a face collapse mechanism due to a too low face pressure. Tunnel calculations are classically based on the convergence-confinement method approaches also called soil/tunnel interaction methods where the loading of lining is calculated from the distance to the tunnel face. Numerical simulations of soil settlement profiles have been carried out to design the cable. They shown that the axial strain induced on the cable would be lower than the Brillouin set up resolution and thus is not detectable. That is why we proposed a sensing cable based on bending strain with three fibres placed at 120° with respect to each others. This design presents the advantage to be independent of the cable orientation in the ground (fig. 1). As shown on figure 2, when the cable is bent the fibres above the neutral line measure a compression whereas the fibres under the neutral line measure a traction. With this cable design it is also possible to obtain from each fibre measurement εfa, εfb, εfc, all the following independent cable components:
ε fa + ε fb + ε fc
The cable axial strain:
εa =
The cable curvature strain:
ε c max =
The cable torsion in the ground:
tg ψ =
Z
3 1 (ε fb − ε fc )2 + (ε fa − ε a )2 3 ε fb − ε fc
3 (ε fa − ε a )
L0
fibre a ψ
2π/3
fibre c X
Compression
Y
2π/3 fibre b
Trac tion
Figure 1: Scheme of the 3D cable
L
Figure 2: Bending measurement principle
During this project, two kinds of cables were manufactured using different process. Cable 1: A cable (15 mm diameter, fig. 4) based on an industrial extrusion process is convenient for front face collapse phenomenon detection on relatively short base ≤ 10 m. Cable 2: The 150 mm diameter cable, or greater, can be used to enhance sensitivity. It is convenient for large spatial scale phenomena detection as settlement in comparison with low diameter. This cable is based on the: manufacturing of a 3-holes tube by the ACOME factory, the transport of such structure on tunnel site and the introduction and, at last, the sealing of the sensitive optical fibre elements in the holes after the tube setting up in the directional drilling (fig. 5). The first step to make this structure sensitive consists in introducing optical fibre inside the three holes thanks to the air flowing method (well known telecom), and to bond them to the structure using a carefully chosen adhesive injected under pressure.
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3: CAIRO DEMONSTRATION 3.1: Site description The Greater Cairo Metro Line 3 will extend from Imbaba station to Cairo International Airport and Phase 1 is extending from Attaba station to Abbasia station with a total length of about 4.292 km. The test area (fig. 3) was located at the extremity of Abbasia station, right after punching through the diaphragm wall of the shaft. The testing equipment has been set-up in two 4-meter depth perpendicular trenches. One longitudinal trench was parallel to the TBM axis beginning 2 m away from the outer skin of station’s diaphragm wall. It was equipped with 20 m long of 15 mm diameter cable (fig. 4), and 12 m long of 140 mm diameter cable (fig. 5). One transversal trench perpendicular to the TBM axis and crossed the longitudinal trench 17 m away from the outer skin of diaphragm wall. It was equipped with 18 m long of 15 mm diameter cable and 12 m long of 400 mm diameter cable (fig. 6). The manufacturing of a so high diameter cable was justified to validate the feasibility of the technology is case of very low settlements (normal situation). Classical settlement gauges (T1 to T14) based on optical prism have been installed for comparison.
Sensing cable 15
T1
T3
T6
TBM Axis
T2
T4
T5
T7
0 T1
TBM Axis
T8 T 9
Sensing cable
140
Sewer Ø=1200 depth 3.35m - not used
15
T1 1
Sensing cable
T1
HDPE pipe
200
2
Sewer Ø=300 depth 3.8m - in used
BYSet system 3 T1 T1 4
Sensing cable
400
HDPE pipe
200
Ch. 24
Figure 3 : Instrumentation on the test area
Figure 4: Sensing cable (φ 15 mm)
Figure 5: Sensing cable (φ 140 mm)
Figure 6: Large diameter sensing cable (φ 400 mm)
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3.2: Results Measurements were stored continuously from 25th to 29th of January 2009, when the TBM drilled from the metric point 0 m to the metric point 32 m. We were able to calculate the curvature from the bending strain data with the 400 mm cable and also to calculate the settlement in the transversal trench (fig. 7). The maximal settlement value of 11 mm, obtained along the tunnel axis is in good agreement with the classical reading from the prism (T10) between 8 mm and 10 mm. Moreover, axial strain measured by the two cables in the same transversal trench give interesting information about the transversal convergence evolution versus time, due to drilling operations. A compression strain of about 1000 µm/m has been measured. Moreover, cable measurements in the longitudinal trench provide an interesting correlation between axial traction (at the front face) followed by an axial compression (5 m behind the front face).
Figure 7: Settlement in transversal trench
4: CONCLUSION The objective to continuously measure and control settlement during tunnelling works has been reached with industrial equipments: cable and tubes with embedded optical fibres and industrial Brillouin reflectometer. Right now, both technical solutions can be used to secure tunnel sites under construction. For early collapse detection, the industrial sensitive optical fibre cables up to 30 mm in diameter are available on long lengths (several km). For more accurate settlement detection, a higher diameter cable may be needed. The solution based on such special cable instrumented with optical fibre sensing elements on-site, has been validated and high diameter tubes including holes are also available from Acome.
Author contact:
[email protected], http://www-list.cea.fr/ Acknowledgements: The authors would like to thank the financial support from the EC 6th Framework Project TUNCONSTRUCT (Technology Innovation in Underground Construction), Contract Number: NMP2-CT-2005-011817.
References: 1:
2: 3:
V. Dewynter, S. Magne, S. Rougeault, N. Le Loc’h, P. Ferdinand, M. Aubrit, M. De Broissia, F. Vallon, L. Avallone, C. Leple, A. Poulain, “Underground tunneling monitoring based on sensitive optical fibre cable and Brillouin reflectometry”, 3rd Int. Conf. on Structural Health Monitoring of Intelligent Infrastructures, Nov. 13-16, 2007, Vancouver, British Columbia, Canada. M. Niklès, “ La diffusion Brillouin dans les fibres optiques : Etude et application aux capteurs distribués“, (Thèse, n° 1674, Ecole Polytechnique Fédérale de Lausanne, 1997). M. Niklès, “Brillouin gain spectrum characterization in single-mode optical fibres”, Journal of Lightwave Technology, Vol. 15, n°10, Oct. 1997, pp.1842-1851.
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