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A Feasibility Study on the Application ofFiber-Optic Distributed Sensors for StrainMeasurement in the Taiwan Strait TunnelProjectBIN SHI a , HONGZHONG XU a , BIN CHEN a , DAN ZHANG a , YONGDING a , HELIANG CUI a & JUNQI GAO aa ACEI, Department of Earth Sciences, Nanjing University, Nanjing,ChinaPublished online: 21 Jun 2010.

To cite this article: BIN SHI , HONGZHONG XU , BIN CHEN , DAN ZHANG , YONG DING , HELIANG CUI &JUNQI GAO (2003) A Feasibility Study on the Application of Fiber-Optic Distributed Sensors for StrainMeasurement in the Taiwan Strait Tunnel Project, Marine Georesources & Geotechnology, 21:3-4,333-343, DOI: 10.1080/713773406

To link to this article: http://dx.doi.org/10.1080/713773406

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A Feasibility Study on the Application of Fiber-Optic

Distributed Sensors for Strain Measurement in

the Taiwan Strait Tunnel Project

BIN SHIHONGZHONG XUBIN CHENDAN ZHANGYONG DINGHELIANG CUIJUNQI GAO

ACEI, Department of Earth SciencesNanjing UniversityNanjing, China

Taiwan Strait Tunnel (TST) will be an extra-long tunnel or tunnel-bridge complex,running over 150 km of seafloor geologic body with complicated topographic andgeologic units. It is therefore necessary to measure and monitor the strain dis-tribution along the TST. In this article, the Brillouin optical time-domain reflect-ometer (BOTDR), a newly developed strain measurement and monitoringtechnology, is introduced, and the feasibility of its application in the strain mon-itoring for TST is analyzed through the monitoring achievement with BOTDRof a tunnel located in Nanjing City, China. The results indicate that the BOTDRhas many unique merits such as distributed measurement, long-distance, real-time,and resistibility for strain monitoring application in a tunnel such as TST. Finally,a preliminary scheme for BOTDR application in TST monitoring is proposed.

Keywords BOTDR, fiber optic distributed sensor, monitoring, strain measure-ment, TST

Taiwan Strait Tunnel (TST) project was formally proposed in the Symposium onTaiwan Strait Tunnel Project, held in Xiamen, China, November 25–27, 1998. AsProf. Fang wrote in his report: TST is a highly interdisciplinary and comprehensiveproject under extreme adverse environmental conditions (Fang 2000). So it isnecessary to gather researchers from various fields to verify the feasibility of the TSTproject. From a technical aspect, the strain measurement and security monitoringduring and after building TST is a very important integral part of the entire technical

Marine Georesources and Geotechnology, 21: 333–343, 2003

Copyright # Taylor & Francis Inc.

ISSN: 1064-119X print=1521-0618 online

DOI: 10.1080=10641190390266525

Received 1 September 2003; accepted 15 October 2003.The authors gratefully acknowledge project support from the National Science Fund for

Distinguished Young Scholars of China (40225006).Address correspondence to Bin Shi, ACEI, Department of Earth Sciences, Nanjing Uni-

versity, Nanjing 210093, China. E-mail: shibin@nju.edu.cn

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system. TST, the longest tunnel in the world, will extend 150Km, and runs over avariety of complicated topographic and geologic units under the seafloor, includingseismic and tectonic zones. Thus, a very high degree of engineering measurementtechnology is needed. Obviously, some of the traditional and conventional mea-surement and monitoring techniques such as the point-mode resistant chip methodwill have not meet the needs of such a long tunnel system.

Recently, the Brillouin Optical Time Domain Reflectometer (BOTDR) has beenrecognized as a powerful distributed fiber-optic sensor with its real-time monitoring,long measurable distance, high measurement accuracy and high durability. TheBOTDR has begun to be applied in deformation monitoring and health diagnosis ofvarious infrastructure engineering such as in tunnels, dikes, bridges, and subways(Bao et al. 2001; Haruyoshi et al. 1997; Liu and Zheng 1999; Ohno et al. 2001; Qiang1999; Wu et al. 2000; Yang et al. 2000).

Supported by the ‘985’ Project of Nanjing University and a key project of theEducation Ministry of China, the BOTDR fiber-optic monitoring laboratory forinfrastructure system monitoring, the first one in China was set up in 2000 atNanjing University. Led by the first author, the research group has made a series oftrials regarding the applications of BOTDR in the tunnel strain measurement andmonitoring, and has obtained some significant results. In 2001 and 2002, BOTDRhas successfully been applied in the strain measurement and monitoring of twotunnels, Nanjing Gulou tunnel and Xuanwuhu lake tunnel. The former was built sixyears ago; the latter is an ongoing tunnel construction. The measurement andmonitoring results demonstrate that the distributed BOTDR monitoring schemeadopted in tunnel projects is feasible and effective to monitor the deformation dis-tribution. The function and measurement accuracy of the BOTDR can meet theneeds of the tunnel measurement and monitoring.

In this article, the basic principle of BOTDR and some research results on thetunnel measurement and monitoring are presented. A preliminary scheme for thestrain measurement and monitoring for TST with BOTDR is drafted.

Basic Principle of BOTDR

The detection principle of BOTDR is briefly outlined as follows: a continuous light,emitted from the DFB-LD laser light source, can be separated into a probe lightoutput to the optical fiber to be measured and a reference light for heterodynedetection. The probe light can be modulated into a pulse light by an intensitymodulator. Brillouin backscattered light takes place as the pulse light launched intothe optical fiber interacts with the acoustic phonons, and a frequency shift ofBrillouin backscattered light occurs compared with the frequency of the launchedpulse light. The frequency shift amount is in proportion to both the longitudinalstrain of the optical fiber and its temperature (Haruyoshi 1997). Figures 1 and 2show the strain dependence and temperature dependence of the Brillouin frequencyshift.

The core technique of BOTDR is Brillouin spectroscopy and Optical TimeDomain Reflectometry (ODTR) that enables BOTDR to measure strain generated inoptical fibers as distributed in the longitudinal direction. When the strain occurs inthe longitudinal direction of optical fiber, the backscattered light of Brillouinundergoes a frequency shift that is in proportional to the strain. Brillouin frequencyshift is function of strain e and can be expressed by Equation (1):

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vBðeÞ ¼ vBð0Þ þdvBðeÞde

� e; ð1Þ

where vBðeÞ is Brillouin frequency shift with strain; vBð0Þ is Brillouin frequency shiftwithout strain; dvBðeÞ=de is the proportional coefficient of strain that is about0.5GHz (=% strain) at the wavelength l¼ 1.55 mm; and e is the strain.

Pulse light is launched into one end of an optical fiber, and the Brillouinbackscattered light occurs and is detected at the same end. The distance Z from thelaunched end of the optical fiber is given by Equation (2):

Z ¼ c � T2n

; ð2Þ

where c is velocity of light in a vacuum; n is the index of refraction of an opticalfiber; and T is the time interval between launching pulse light and receiving thescattered light.

Figure 1. Strain dependence of Brillouin frequency shift change.

Figure 2. Temperature dependence of Brillouin frequency shift change.

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Figure 3 show the tridimensional diagram of BOTDR measurement principle. Itcan be seen from Figure 3 that the BOTDR is able to both measure the strain andlocate the distance where the strain occurs along the optical fiber.

BOTDR Advantages

Compared with the conventional strain monitoring techniques, the advantages ofBOTDR can be summed up as the follows:

1. Distributed. BOTDR can continuously and simultaneously measure the strainof the structure at any points distributed along the optical fiber from only oneend of an optical fiber. With a network of optical fibers, the BOTDR canperform full scale monitoring for the structure, which is very difficult orimpossible for the conventional point-mode monitoring techniques to do.

2. Long distance. Large infrastructures such as tunnels, dikes, oil pipes, subways,and large bridges often span the tens or hundreds of kilometers, which is toolong for the conventional point-mode monitoring techniques to monitor andmeasure the deformation distributed in various parts of the structure.BOTDR, however, can do that due to its long monitoring distance of over80 kilometers. On the other hand, the optical fiber in BOTDR serves as boththe sensor and the signal transmission medium, so BOTDR is able to monitorthe structure from the remote monitoring center and doesn’t need somebodyon the site to do it.

3. Real-time. BOTDR is capable of monitoring the strain abnormities along theoptical fiber in real time and of showing where, how much, and when thestrain occurred. Thus, BOTDR can be used to monitor the strain distributionof infrastructure system dynamically.

Figure 3. Tridimensional diagram of BOTDR measurement principle.

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4. Resistibility. Optical fiber is made of a nonmetal, quartz glass, so it resistsrusting and environmental erosion and can be used in most severe conditionssuch as humid or arid, high or low temperature. In addition, it can protectitself from electric and electromagnetic interference and avoid signal error inthe transmitting process.

5. Compatibility. Optical fiber is thin, flexible, and lightweight, so it is easy toinstall in or on the structure without degrading the structure’s strength.

6. Accuracy. BOTDR can detect as little as 30 mm strain along the opticalfiber, and its distance resolution can reach less than 1m, which enables itto meet the needs of strain measurement and health diagnosis of tunnelengineering.

A Case of BOTDR Application

Outline

In order to illustrate the feasibility of BOTDR for strain measurement of the futureTST, an application case is presented here.

The case is the Gulou tunnel, located in Nanjing City. The approximately1150m long structure runs from south to north, including a 400m approach and a750m arch. It has twin, side-by-side concrete arches each with 11.6m span and isseparated by the inner concrete wall with 1m thick between two the arches. Themaximal cover thickness of the tunnel is 12.9m, and the minimum 0.26m. Thetunnel was completed in 1996 and has been in use for six years.

The monitoring work with BOTDR for the tunnel aimed at understandingwhole and partial deformation of the tunnel and acquiring its health diagnosisconclusion. The 750m length of the western arch was taken as the monitoring zone.Optical fiber installation was completed in July, 2002, and the monitoring began onJuly 2 and lasted six months.

Project Scheme

Based on many experiments, the jacketed SM optical fiber from Corning Co, Ltd.was selected as the sensor with the following specifications: 8.3=125=900=mm(core=cladding=protective coating); weight 0.9 kg=km; maximal tension 6.6N inshort term and 3.0N in long term; minimum bending radius 3.0 cm in short termand 5.0 cm in long term; compressive strength 200N=m; conservation temperature�40 – þ80�C; working temperature: �20 – þ80�C; working wavelength 1.3–1.5 mm; refractive index error 0.36%; effective group refractive index 1.4681 (1550window).

In light of the deformation characteristics of the tunnel and 1m distance reso-lution of BOTDR, the optical fibers were installed on the surface of the concrete archwith two configurations (see Figure 4).

1. Overall Adhesion Method (OAM). That means the optical fiber is entirelyaffixed to the surface of the concrete arch as shown in Figure 4. This methodis designed to examine the entire deformation of the tunnel such as the unevensubsidence of the tunnel.

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2. Fixed-point Adhesion Method (FAM). That means the optical fiber isbonded on the fixed points on the surface of the concrete arch at a certainintervals as shown in Figure 1. This installation method is used so as to detectthe partial deformation caused by the crack zones with widths less than thedistance resolution of BOTDR.

Five lines of optical fibers were installed, four of them are affixed to the concretewall in the west side of the western arch using the OAM and the FAM mixed withO-FAM, respectively, one of them was affixed to the arch along which the opticalfiber was cinctured three times using the FAM. All of optical fibers were centralizedto connect to an optical cable after they were set, and then link to the managementroom located in the middle of the tunnel for BOTDR monitoring, (see Figures 5and 6).

According to the scheme above, a series of installation techniques was used toaffix the optical fiber on the tunnel, including fluting, smoothing, grinding,cleaning, bottoming, sticking, checking, and so on. A special epoxy resin was used

Figure 4. The configurations of OAM and FAM.

Figure 5. Plane layout of the optical fiber installation of the tunnel. Optical fiber on

western sidewall with FAM; Optical fiber on western sidewall with OAM;Optical fiber on the top; End of optical fiber; Optical fiber cable;Side wall of the tunnel.

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as adhesive, meanwhile mini OTDR was used to inspect the optical loss attenua-tion and breakpoint caused during installing. FSM-16R splicer was used to fuseoptical fiber.

Analysis of the Deformation

The deformation of the tunnel is mainly local, caused by the cracks and joints dis-tributed on the tunnel sidewall and arch. Based on the strain distribution measuredby FAM, the abnormal segments or points distributed on the measured strainspectrum were specially analyzed and their measured strain values can be convertedinto the deformation value using the following formula:

d ¼ D � e; ð3Þ

where D is the length of the strained fiber; e is the measured strain; and d is thedeformation value, in which the sign convention is that the positive sign indicatesthe tensile due to the crack opening and negative sign denotes the compressive due tothe crack closure.

The converted deformations of some selected abnormal measured points duringthe period from July 4 to November 17 are shown in Table 1. It can be seen thatthere is such a tendency that the closer to the tunnel entrance the measured point is,the larger the deformation becomes. The maximum deformation value is 0.142mmat the measured point SP 27 that is the closest to the southern entrance, and theminimum is 0.094mm at SP 17 that is near the center of the tunnel. However the

Figure 6. Tridimensional layout of optical fiber installation on the arch.

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compressive deformation of all of selected measured points is very little (�0.004–�0.025), which indicates that the deformations of the cracks are not fully reversible.

The measured deformation of the tunnel is mainly attributable to the tem-perature changes after analyzing the temperature changes during monitoring. Thetemperature difference at the tunnel entrance is more than that at the central part ofthe tunnel, so that the deformation of cracks distributed in the entrance is also largerthan that of other part of the tunnel.

Tunnel Health Diagnosis

Based on the tunnel monitoring data, the preliminary health diagnoses obtained areas follows: The measured deformation of the tunnel is tiny and is attributable to

Table 1

The deformation of some cracks and joints distributed on the tunnel

Position no. Position(from south entrance)

Maximumeformation

Minimumdeformation

m mm mm

SP 13 450 0.139 �0.022

SP 15 405 0.128 �0.025

SP 17 355(near center) 0.094 �0.021

SP 21 256 0.103 �0.004

SP 23 190 0.135 �0.015

SP 25 138 0.127 �0.008

SP 27 114 0.142 �0.007

Figure 7. Basic constitution of BOTDR-based monitoring system.

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temperature changes. The current deformation incident to the tunnel is within thetolerance limit and the tunnel is under safe condition.

Summary

On the basis of the above monitoring results and analyses, the following conclusionscan be reached: the distributed BOTDR monitoring scheme adopted in this project isfeasible and effective to monitor tunnel deformation. The functions and monitoringresults of BOTDR monitoring system can meet the needs of the tunnel monitoringand health diagnosis.

BOTDR and TST Monitoring

Compared with above tunnel, TST is much longer, extending 150Km, and muchmore complicated, encountering a variety of unpredictable topographic and geologicconditions under seafloor. However, BOTDR will be fully capable of meeting the

Figure 8. Design flowchart of the BOTDR-based monitoring system.

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needs of TST monitoring, owing to the advantages stated above. As for such anextra-long tunnel or tunnel-bridge complex as TST, subsection monitoring will benecessary. TST can be divided into several subsections to measure and monitor withBOTDR. For example, if each subsection spanned 30 km, a total of five subsectionsalong TST can be monitored. Nevertheless, the length of a subsection should not beaverage, and should be determined based on topographic and geologic conditions,unit and type of the structure, construction materials and progress, and so on. Amonitoring substation should be set up at the connection of two subsections, andthen the monitored data from each monitoring substation should be transmittedsynchronously to the General Monitoring Station (GMS) via specific optical cable.Based on the monitored data transmitted from the substations, GMS should giveanalysis reports and security assessments for whole tunnel system almost on realtime, and automatically emit a warning at the occurrence of an abnormity. Thus, asfor future work on BOTDR, except for further technical improvement of BOTDRitself, developing a BOTDR-based monitoring system will be a urgent and vital taskfor future BOTDR application in TST and other tunnels. Herein a basic frameworkabout this system is presented.

Figure 7 shows the basic constitution of this monitoring system. Figure 8 is theflowchart of the system design.

Conclusions

BOTDR with distributed measurement, long-distance, real-time, and resistibility isquite applicable to strain measurement and monitoring of the TST. The TST is nowmerely in the tentative planning stage, and it will take a long time to make furtherand detailed feasibility verification. So BOTDR will also have time to be con-tinuously improved and innovated both in its non technology and in its applicationbefore the TST plan would enter substantial operation. It is fully believed thatBOTDR system will become more powerful, much longer, and more accurate toserve the TST when that dream of several generations becomes reality.

References

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Fang, H. Y. 2000. A feasibility study on Taiwan Strait Tunnel Project (a multi-purposeproject): A regionalization stage approach. Proceedings of Symposium on Taiwan StraitTunnel Project, Xiamen, China, November, 1998.

Haruyoshi, U., S. Yasushi, X. Zhi, and Li. 1997. AQ8602 Optical Fiber Strain=Loss Analyzer.Ando Technical Journal

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