fiber optic sensors for underground structural health monitoring

FIBER OPTIC SENSORS FOR UNDERGROUND STRUCTURAL HEALTH MONITORING: SURVIVABILITY OF SENSORS UNDER SHOTCRETING AND DRILL-AND-BLAST IMPACTS MULEY PRAVIN SUDHAKAR1, YANG YAOWEN1, SHAM WAI LUN1 and KAR WINN2

6FKRRORI&LYLODQG(QYLURQPHQWDO(QJLQHHULQJ1DQ\DQJ7HFKQRORJLFDO8QLYHUVLW\ 1DQ\DQJ$YHQXH6LQJDSRUH (PDLOSUDYLQVP#QWXHGXVJF\Z\DQJ#QWXHGXVJVKDP#HQWXHGXVJ 2 7ULWHFK&RQVXOWV3YW/WH6LQJDSRUH (PDLOXNDUZ#WULWHFKFRPVJ Measurement of structural parameters, such as strain and temperature at thousands of locations, along a single fiber sensor, is achievable by distributed fiber optic sensors. Different from the conventional monitoring systems, this unique feature opens new possibilities for structural health monitoring (SHM) of large underground structures. However, limited research has been conducted on maximizing the possible applications in underground structures such as tunnels and caverns. To develop a SHM scheme for underground structures, different fiber optic sensors (FOSs) are investigated based on the Brillouin Optical Time Domain Reflectometry (BOTDR) technology. The main aim of this research is to develop a BOTDR based underground monitoring system that survives during drill-and-blast of construction. After developing the sensors for the system, implementation issues such as survivability of sensors under shotcreting and drill-and-blast are addressed. A water spray test has been conducted in the laboratory to check the survivability of the sensors against shotcreting pressure. On the basis of these results, the sensors were then installed in a tunnel for trial. The trial test demonstrated that these customized sensors survived the pressure of shotcreting and drill-and-blast impact. The results showed that the distributed fiber optic sensing technique as a powerful monitoring tool has great potential in underground applications. .H\ZRUGV: Fiber optic sensor, BOTDR, structural health monitoring, underground structures, survivability.

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Numerous countries around the world now include, among their infrastructure, a number of underground structures such as tunnels, caverns and other structures, which require inspection from time to time to ascertain that they are still safe and capable of withstanding various environmental effects. Such inspections and associated non-destructive testing procedures can reveal progressive damage, and allow appropriate repair measures to be taken before the damage deteriorates to the extent of making the structure unserviceable. For these large structures, with high initial construction costs, it is now recognized that monitoring programs are desirable right from the outset in order to detect any signs of damage as early as possible, and allow appropriate interventions to be taken. Programs of this nature, if properly implemented, can extend the useful life of the structure quite considerably, with the utility value gained more than justifying the costs of the monitoring itself. However, this thinking is more widespread, and much research on the issues of monitoring, damage detection and long-term performance of structures is going on worldwide, where high-rise buildings and long-span bridges are abundant. The

Advances in Underground Space Development – Zhou, Cai & Sterling (eds) c 2013 by The Society for Rock Mechanics & Engineering Geology (Singapore). Copyright Published by Research Publishing ISBN: 978-981-07-3757-3 :: doi:10.3850/978-981-07-3757-3 RP-103-P284

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domain of large and expensive structures has extended beyond very tall buildings and long-span bridges, to underground structures such as tunnels and caverns. The parameters that, in the most representative manner, reflect the tunnel structural behavior are convergence, strain and deformation in tunnel walls and soil, cracks and joint openings and the contact pressure between the tunnel and soil or rock. Tunnel monitoring strategies are well developed and based mainly on the conventional measurement instrument. Monitoring strategies based on the use of discrete optical fiber sensors are similar to the conventional ones: the discrete traditional sensors can be exchanged with the optical fiber sensors that bring advantages in terms of insensitivity to harsh environmental conditions (corrosion, humidity, electromagnetic fields, etc), accuracy, stability and long term performance. Distributed sensors were devised to monitor big and long structures where Extrinsic FabryPerot Interferometric and Fiber Bragg Grating sensors fail to give cent percent viability. Brillouin Optical Time Domain Reflactometer (BOTDR) based distributed FOS are generally used for monitoring purpose. KomatsuHWDO (2002) discussed the measurement principle of the strain measurement method using optical fibers (BOTDR method). Case studies of application to the deformation of telecommunications tunnels, ground subject to landslides and so on, and also an outline of an automatic measuring system were presented. Li (2004) reviewed fiber optical sensor for health monitoring in various key civil structures, including buildings, piles, bridges, pipelines, tunnels, and dams. Ohno HW DO (2001) introduced recent examples where BOTDRs have been applied to actual structures. Results of three field tests, namely, as a damage-detection system for America's Cup yachts, as an optical fiber sensor for detecting changes in river levees, and as a strain-sensing optical fiber embedded in concrete structures were also discussed. ZhangHWDO (2010) built the dike model, monitored the dike strain changes influenced by seepage and loading, and verified the feasibility that the distributed optical fiber measurement system can basically reflect the dike deformation. It can be an important direction for the development of dike strain monitoring. Recent advances in strain measurements using optical fibers by Klar and Linker (2010) allow the development of smart underground security fences that could detect the excavation of smuggling tunnels. Klar and Linker (2010) presented the first stages in the development of such a fence using BOTDR. Two fiber optic layouts were considered and evaluated in a feasibility study that included evaluation of false detection and sensitivity. %27'5WHFKQRORJ\ The technology development begins with the introduction of the optical time domain reflectometers (OTDR). In this procedure, an optic pulse is launched in the fiber and a photo detector measures the amount of light that is backscattered as the pulse propagates down the fiber. The time information is then transformed to distance information if the speed of light is known. The Raman scattering technique is first used for sensing applications in the 1980s, while Brillouin scattering is introduced at a later date as a way to improve the range of OTDRs and then for temperature and/or strain monitoring applications. Since, this article is focus on the BOTDR technique, the OTDR and Raman scattering technique will not be further elaborate. Nikles et al. (1997) described that the Brillouin scattering occurs because of an interaction between the propagating optical signal and thermally excited acoustic waves in the gigahertz range present in silica fiber, giving rise to frequency-shifted components. The functional process of BOTDR is described in Figure 1.


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Figure 1. Schematic diagram of distributed BOTDR temperature and strain sensing system

It can be seen as the diffraction of light on a dynamic grating generated by an acoustic wave (an acoustic wave is actually a pressure wave that introduced a modulation of the index of refraction through the elasto-optic effect). We can obtain the Brillouin gain through the Lorenzian spectral profile. HeimanHWDO (1979) expresses the exponential decay of the acoustic waves results in a gain, gB(v), given by, (1)

which ¨vB is the full-with at half maximum (FWHM) and go, the Brillouin gain coefficient. The acoustic velocity is directly related to the density of the medium that is temperature and strain dependent. Since the grating propagates at the acoustic velocity in the fiber, the diffracted light experiences a Doppler shift (Figure 2), as a result, the named Brillouin frequency shift carries the information about the local temperature and strain of the fiber. The Brillouin shift, ȞB, which undergoes a Doppler frequency shift is depends on the acoustic velocity and is given by, (2)

where 9a is the acoustic velocity within the fiber, n is the refractive index and Ȝ, the vacuum wavelength of the incident light wave.

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Figure 2 Time and frequency analysis of BOTDR Doppler shift

As the Brillouin frequency shift depends on both the strain and temperature of the fiber, the sensor setup will conclude the real sensitivity of the system. Measuring distributed strains requires a specially designed sensor. A mechanical coupling between the sensor and the host structure along the whole length of the fiber has to be assured. Furthermore, knowing the frequency shift of the unstrained fiber allows an absolute strain measurement. The active stimulation of Brillouin scattering can be achieved by using two optical light waves. In addition to the optical pulse, usually called the pump, a continuous wave (CW) optical signal, the socalled probe signal, is used to probe the Brillouin frequency profile of the fiber Nikles HW DO (1997). A simulation of the Brillouin scattering process occurs when the frequency difference (or wavelength separation) of the pulse and the CW signal corresponds to the Brillouin shift (resonance condition) and provided that both optical signals are counter-propagating in the fiber. The interaction leads to a larger scattering efficiency, resulting in a mapping of the Brillouin shift along the sensing fiber. At every location, the maximum of the Brillouin gain is computed and the information transformed to temperature or strain using the appropriate calibration coefficients. Since changes in Brillouin shift are linear with strain and temperature changes as expressed in equation (3)

(3) where, İf is the longitudinal strain in the fiber and T is the temperature, a set of equations can be written to relate the behavior of two closely placed cables, which experience the same temperature and strain changes. A solution for the strain and temperature can be obtained as:

(4)

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where, Δ ν %/) is the change of Brillouin shift recorded in the lubricated cable, and Δ ν %

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the change of Brillouin shift recorded in the “strain sensor” fiber. Fε6) F76) are the coefficients of the “strain sensor” and Fε/) F7/) are the coefficients of the lubricated cable. Although, the telecommunication cables which their inner glass core is protected from the outer shell by a lubricant or gel are not suitable for strain measurements, they are ideal for temperature compensation because the inner fiber is not strained even when the outer coating is strained. Therefore, the above coefficients can be easily determined by from a calibration setup where fibers are heated and strained and by solving equation (4), the stain measurement can be determined

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Structural monitoring is one of the most important tasks for the maintenance of all underground tunnels and caverns, and usually the monitoring process lasts through the whole lifespan of the structure. Choice of monitoring system and instrumentation and the frequency of monitoring depend on the type of underground structure, surrounding soil quality, usage of the structure, tunneling method during construction and the stage of construction. The innovation is the use of distributed sensors for average strain, temperature, and integrity monitoring. A distributed sensing system, by its nature, is very suitable and efficient for the monitoring of tunnels, roads, pipes, these being structures with lengths that can achieve several kilometers. Distributed optical fiber sensors can be installed on the walls and vaults of the tunnel in longitudinal and tangential direction. Cracks and rock deformations can be detected with these sensors, as well as the local strain changes as a result of deformation. Judging on the number of parameters measured and the scale of the underground structures, distributed sensing techniques show good versatility, high cost effectiveness and great convenience. So in this paper, we will discuss the problems faced by engineers/researchers while implementing this technology. Research on the structural health monitoring is going on which will address all the issues. The progress on the testing to address the issues, will be discussed in the next section. After literature review and careful judgment it was decided to try implementing the technology to the depositories, caverns and tunnels as shown in Figure 3.

Figure 3 Method of implementation of BOTDR technology

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,PSOHPHQWDWWLRQ,VVXHV D &RUHTXDOLW\RRIWKHILEUHDQG GW\SHVRIFRDWLLQJVRQILEUH Coore of the sensoor has direct reelation to the trransfer of the strain. s If the core quality is not n good, ligght carrying caapacity of the sensor will bee affected and hence due to signal loss, thhe results wiill, too, be afffected. It was important to check core quuality for lightt carrying capacity. So beefore addressingg problem of sensor s coating, one has to findd best quality core c sensor. After rigoroous survey in th he market, we found differennt types of sinngle mode fibeer sensors cooated with diifferent types of sheathingg. These fiibers were made m to caterr mainly coommunication applications. a A very few were available whhich were capaable of capturinng strain, ouur main objectivve. Moreover the fibers avaiilable in the market were mostly coated witth a layer off Kevlar Fibre. So while tran nsferring the strrain from the structure s surfacce to core of thhe sensor, ( fiber help h free movement of theere was a posssibility of losss of some amoount of strain (Kevlar fibber core and ouuter coating ass this fiber is loosely l fitted) and we may not n be able to get g exact strrain.

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% ,QVWDOODWLRQRIILEUHRSWLFVHQVRU W While fixing thee sensor on to the structure itt is required too give some prretension to thhe sensor. Thhe optimum preetension that iss required be appplied to the seensor was not known k for the available sennsors. Hencee it was neceessary to find optimum preetension for thhe sensor. Teests were coonducted to adddress this issue. The sensor may m be embed dded in the struucture for sub surface monitooring, while it needs to bee installed on thhe surface of the t structure foor surface monnitoring. Eitheer our intentionn is to do surface monitoriing or subsurfface monitorinng of the struccture, it needs to be installeed on the ucture. Fixing of the sensor on o to the struccture can be doone by a) strructure/embeddded in the stru paasting method, b) looping meethod, c) clampping method. To determine best way of fixing f the sennsor on to the structure s we haave conducted some tests. ([SHULPHQWDDO,QYHVWLJDWLRQ Q Anny sensor whiich was availaable in the maarket was not sufficient to choose c becausse it was im mportant to cheeck the qualitty of fibre forr light carryingg capacity, itss cladding andd coating maaterial, stiffnesss of the senssor coating, annd applicabilitty of the sensor to the undderground strructure monitoring. To choo ose a best sensor/cable for reeliable structuraal health moniitoring of unnderground struuctures, Muley yHWDO (2011) have h tested sennsor1-Bare fiber, sensor2-sennsor with LS SZH minimum m coating, senso or3-senor with LSZH materiaal and Kevlar coating, c sensorr4-sensor wiith glass fibree coating, sensor5-sensor esspecially desiggned for strainn measuremennt type1, sennsor6-sensor especially e desig gned for strain measurement type2, sensor77-the sensor6 reeinforced wiith stainless steel tube. Sev veral tests weree carried out rigorously r to address a differeent issues disscussed earlierr, however duee to space limittations those arre not presenteed here. Somee of these sennsors were cuustom made with w the helpp of some maanufacturer annd due to com mmercial


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agreement, specifications of the sensor will not be disclosed. Omnisens make DITeSt STA100 was used while carrying out these experiments. ,QVWDOODWLRQRIVHQVRULQDWXQQHO With the detail investigation using some parameters on the different sensors, we have arrived at a customized sensor which fulfils all specific requirements. This customized sensor is waterproof and having sufficient stiffness showed excellent results which matched with the conventional strain gauge readings. This showed the suitability of the sensor, under harsh environmental condition, for the underground application. So our next task was to test this sensor in a tunnel. After reconnaissance survey it was observed that it was not convenient to install this sensor on the surface of the tunnel but to embed it behind the shotcrete. Masri HW DO (1994) , SilvaMuñoz and Lopez-Anido (2009) and de Vries HW DO (1995) studied performance of embedded FOSs. The shotcrete consists of rich cement slurry with fine aggregates. Shotcrete is applied on the surface with a pressure of about 30 to 50 bars. The pressure with which shotcrete is applied is quite high cannot be neglected. Therefore, before going to real time application of the sensor in a tunnel we have carried out a laboratory test using water and simulating the same kind of pressure. ([SHULPHQWDO,QYHVWLJDWLRQ:DWHU6SUD\7HVW The aim of this test was to examine the influence of measurement accuracy when the BOTDR sensor is subjected to water spraying (shotcreting effect). A universal channel section was used for the testing. The BOTDR sensor was clamped, as shown in Figure 4, onto the section about 3m apart with pre-strain. Initial reading on the BOTDR analyzer was recorded. Water jet of 150bar pressure was used to spray water 1 m away from the BOTDR sensor. Water was sprayed for about 5-10 minutes on the sensor. The readings before and after spraying were recorded.

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2E EVHUYDWLRQV: No physical damage fouund after testt, however, when comparring the meeasurement takken before the spray and afteer the spray, thhere are some noticeable n variiations in thee measuremennt. Figure 5 sho ows that there is effect on thhe BOTDR caable just after spraying. s Buut readings takken after 2 hou urs, it was fouund that the reelative effect was w almost nuullified as shown in figure 6. 6 Overall testiing showed that BOTDR sennsor will be abble to withstannd shotcreting pressure. Thherefore it was our next step to t try and instaall this sensor on o the wall of the t tunnel and embed it beehind the shotcrrete. ([SHULPHQWD WDO,QYHVWLJDWLR RQ7XQQHO7HVVW Drilling andd blasting is a common c tunneeling process during d construcction phase. Iff BOTDR opptic fiber sensinng is chosen as a the main moonitoring system m for the undeerground struccture, one should realize thhat the materiaal of the opticaal fiber used as a well as the packaging p techhnique is exxtremely criticaal (Inaudi and Glisic (2006), Inaudi (2002)), LengHWDO (2005), Leng (22005)) in ordder to withstannd the rough underground u coondition. Moreeover, the seleected optical fiibre must bee resistant to water, crude oil and other chemical fluid (Skontorp and Cammas (2001)). Seelection of instaallation method is of great im mportant as weell, in order to achieve high reliability r annd accuracy in the t monitoring g results and daata analysis (R RiveraHWDO (20002)). In order to study s the respo onse to drill-annd-blast in a tuunnel, a sectionn near the blassting face abbout 15m, wass chosen (see Figure 7). BOTDR B sensorr was then innstalled using specially fabbricated clampps. The distan nce between tw wo consecutivee clamps was approximately a 2 to 3m.


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After installation of BOTDR sensor on the tunnel surface, shotcrete was applied which ensured the sensor embedded firmly behind the shotcrete. Figure 8 shows the installed sensor section before and after shotcrete.

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Readings from the Figure 9 shows that the BOTDR sensor survived the drill-and-blast impact. With the successive measurements, the strain in the crown has increased which means there is certain deformation of the crown.

&RQFOXVLRQ Testing the performance of the fiber optic monitoring system in the long run presents some challenges that make this type of testing tedious and difficult. The structural health monitoring system should be chosen based on the monitoring parameters. From the current research the following points can be concluded Feasibility of using FOS depends heavily on the durability of these fibers under adverse loading condition. Quality of the fiber core is very important as it carries the probe. The optical fiber chosen should be able to carry the light signal efficiently. The type of fiber protection and number of layers of fiber protection play a pivotal role in strain transfer from structure to the fiber. Optic fiber selected should have all layers of protection firmly bound together such that there is minimal relative movement of one layer with respect to the other. Water-spray test confirmed that the sensor is waterproof and can withstand shotcrete pressure of 50bar. Tunnel trial test shows that the sensors survived during drill-and-blast, which boosts the confidence on using the sensors in application. The damage of sensor may be caused by the spalling of shotcrete. To prevent the spalling of shotcrete, one way is to install the sensors as close to the tunnel surface as possible. The other way is to avoid using thick or bulky shotcrete to cover the sensors. There should be some minimum time gap between shotcreting (on sensors) and blasting. If the hardening time for the shotcrete is not sufficient, spalling may occur during blasting. The experimental tests conducted provide useful information to address problems related to durability and reliability of FOS system, which is useful to put forward guidelines for comprehensive long term underground structural health monitoring. 5HIHUHQFHV 1 De Vries, M., M. Nasta, V. Bhatia, T. Tran, J. Greene, R. O. Claus and S. Masri (1995). "Performance of embedded short-gage-length optical fiber sensors in a fatigue-loaded reinforced concrete specimen." 6PDUW0DWHULDOVDQG6WUXFWXUHV 4: A107-A113. 2 Heiman, D., D. S. Hamilton and R. W. Hellwarth (1979). "Brillouin Scattering measurements on optical glasses." Phys. Rev. B 19(19): 6538. 3 Inaudi, D. (2002). "Application of fiber optic sensors to structural monitoring". (XURSHDQ :RUNVKRS RQ 6PDUW 6WUXFWXUHV LQ (QJLQHHULQJ DQG 7HFKQRORJ\ , 3UHVTXLOHGH*LHQV)UDQFH3URF2I63,(.4763: 31-38. 4 Inaudi, D. and B. Glisic (2006). "Distributed fiber optic strain and temperature sensing for structural health monitoring". UG ,QW &RQIRQ %ULGJH 0DLQWHQDQFH 6DIHW\ DQG 0DQDJHPHQW%ULGJH0DLQWHQDQFH6DIHW\0DQDJHPHQW/LIH&\FOH3HUIRUPDQFHDQG &RVW, Porto, Portugal, Taylor and Francis/Balkema: 963-964.


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Klar, A. and R. Linker (2010). "Feasibility study of automated detection of tunnel excavation by Brillouin optical time domain reflectometry." 7XQQHOOLQJ DQG 8QGHUJURXQG6SDFH7HFKQRORJ\ 25(5): 575-586. Komatsu, K., K. Fujihashi and M. Okutsu (2002). "Application of optical sensing technology to the civil engineering field with optical fiber strain measurement device (BOTDR)". $GYDQFHG 6HQVRU 6\VWHPV DQG $SSOLFDWLRQV , USA, SPIE-Int. Soc. Opt. Eng.4920: 352-361. Leng, J. S. (2005). "Fiber optic sensors for structural health monitoring of concrete structures." 3URFRISPIE 5634: 18-26. Leng, J. S., R. A. Barnes, A. Hameed, D. Winter, J. Tetlow, G. C. Mays and G. F. Fernando (2005). "Fibre optic sensor protection system and its practical for structural integrity monitoring of concrete structures". 6PDUW6WUXFWXUHVDQG0DWHULDOV6DQ 'LHJR&$8QLWHGVWDWHV, 3URFRI63,(.5765: 528-539. Li, H. (2004). "Recent applications of fiber optic sensors to health monitoring in civil engineering." (QJLQHHULQJ6WUXFWXUHV 26(11): 1647-1657. Masri, S. F., M. S. Agbabian, A. M. Abdel-Ghaffar, M. Higazy, R. O. Claus and M. J. de Vries (1994). "Experimental study of embedded fiber-optic strain gauges in concrete structures." -RXUQDORI(QJLQHHULQJ0HFKDQLFV 120: 1696-1717. Muley, P. S., Y. Yang and W. L. Sham (2011). "Stability and reliability of fiber optic measurement systems: Basic conditions for successful long term structural health monitoring". 1RQGHVWUXFWLYH &KDUDFWHUL]DWLRQ IRU &RPSRVLWH 0DWHULDOV $HURVSDFH (QJLQHHULQJ&LYLO,QIUDVWUXFWXUHDQG+RPHODQG6HFXULW\San Diego, CA, United states, SPIE.7983: 798819-1 to12. Nikles, M., Luc Thevenaz and P. A. R. (1997). "Brillouin Gain Spectrum Characterisation in Single-Mode Optical Fiber." -RXUQDO RI /LJKWZDYH 7HFKQRORJ\ 15(10): 20-28. Ohno, H., H. Naruse and M. Kihara (2001). "Industrial Applications of the BOTDR Optical Fiber Strain Sensor." Optical)LEHU7HFKQRORJ\ 7(1): 45-64. Rivera, E., D. Polyzois, D. J. Thomson and N. Xu (2002). "The development and use of fiber optic sensors for the structural health monitoring of composite (GFRP) structures". $60( ,QWHUQDWLRQDO 0HFKDQLFDO (QJLQHHULQJ &RQJUHVV DQG ([SRVLWLRQ, New Orleans, LA, United states, : 85-89. Silva-Muñoz, R. A. and R. A. Lopez-Anido (2009). "Structural health monitoring of marine composite structural joints using embedded fiber Bragg grating strain sensors." &RPSRVLWH6WUXFWXUHV 89(2): 224-234. Skontorp, A. and J. Cammas (2001). "Static fatigue life of silica optical fibers and the significance of fiber coating and handling". 6PDUW 6WUXFWXUHV DQG 0DWHULDOV 1HZSRUW%HDFK&$8QLWHGVWDWHV3URFRI63,(.4328, 96-105. Zhang, Q. M., P. Y. Zhu, S. L. Wang and Y. B. Leng (2010). "Research of BOTDR on dike strain monitoring". ,QWConf RQ 3UHFLVLRQ ,QVWUXPHQWDWLRQ DQG 0HDVXUHPHQW Kiryu, Japan, Trans Tech Publications.36: 187-191.