ation represents, perhaps half.Next on the thought agenda is thefact that the boomers possess agreat deal of institutional knowl-edge, knowledge that is critical toorganizational continuity andsuccess. We have already estab-lished that folks are living longer,more productive lives. Industry isrealizing that these graying work-ers still have plenty to offer.Heres an interesting statistic - theaverage age of a constructionworker in our part of the industryis 55. The same holds true for thecivil engineering profession. Em-ployers are now discovering thatcontrary to the assumption thatolder workers may cost you morebecause of health expenses,health related absenteeism, lossof focus, etc., in fact older healthyworkers may cost you less. Thoseover 65.6 are not only collectingsocial security payments, but theyare on Medicare as well. If an em-ployer is able to offer flex timeand fewer hours, older workersare able to supplement the em-ployers pay check with their owndraws on social security and/orretirement plans. Employers arealso discovering that these folks

by and large have a work ethicthat is not found in younger folks.For people of this ilk, work islife, not something you have todo as little of as possible and getpaid as much as possible. Theseworkers are not running home totake young children to soccerpractice, ballet, piano lessons, orthe orthodontist. They are notcommitted to attending parentteacher association meetings, orlinking their vacations to schoolholiday breaks. The ones thatwant to work, or have to work, ap-preciate having the opportunity.They dont think they are owedanything. They relish the chanceto continue to contribute to acompanys objectives. Whilethere may be initial problemswith older workers having to re-port to youngsters they quicklyget over it. These seasoned citi-zens want the work, they need thework. They will do the work.

None of the above precludesthe need to aggressively recruityoung folks into our industry onthe design and the constructionsides of the coin. I have alreadywritten about the need to beginthe recruitment process early and

often. We are competing forfewer young people with lots andlots of choices. We must make ourprofession attractive. But thatsanother topic.

Heres the point dont dis-count the value of keeping yourolder employees. Dont be afraidto bring ambitious seniors back tohelp mentor the younger folks.The blend of experience andhopefully wisdom, with exuber-ant youthful energy and excite-ment is a terrific combination forany company.

Its shift the paradigm time.Please dont misunderstand my mo-

tive in reprinting this it has nothing todo with hopes for my own future. Just agood sermon for others!

ClosurePlease send contributions to this col-umn, or an article for GIN, to me as ane-mail attachment in MSWord, tojohn@dunnicliff.eclipse.co.uk, or byfax or mail: Little Leat, Whisselwell,Bovey Tracey, Devon TQ13 9LA, Eng-land. Tel. and fax +44-1626-832919.

Happy Landings!

Overview of Fiber Optic SensingTechnologies for GeotechnicalInstrumentation and Monitoring

Daniele InaudiBranko Glisic

IntroductionFrom many points of view, fiber opticsensors are the ideal transducers forstructural health monitoring. Being du-rable, stable and insensitive to externalperturbations, they are especially usefulfor long-term health assessment of civilstructures and geostructures. Many dif-ferent fiber optic sensor technologiesexist and offer a wide range of perfor-mances and suitability for different ap-plications. In the last few years, fiber

optic sensors have made a slow but sig-nificant entrance in the sensor pan-orama. After an initial euphoric phasewhen fiber optic sensors seemed on theverge of becoming prevalent in thewhole world of sensing, it now appearsthat this technology is mainly attractivein the cases where it offers superior per-formance compared with the moreproven conventional sensors. The addi-tional value can include an improvedquality of the measurements, a better re-

liability, the possibility of replacingmanual readings and operator judgmentwith automatic measurements, an easierinstallation and maintenance or a lowerlifetime cost. Finally, distributed fibersensors offer new exciting possibilitiesthat have no parallel in conventionalsensors.

This article reviews the four main fi-ber optic sensor technologies: Fabry-Prot Interferometric Sensors Fiber Bragg Grating Sensors

GEOTECHNICAL INSTRUMENTATION NEWS

4 Geotechnical News, September 2007

SOFO Interferometric Sensors Distributed Brillouin Scattering and

Distributed Raman Scattering Sen-sors

and their practical implementation inthe form of packaged sensors and read-out instruments.

Selected application examples il-lustrate the practical use of thesesensing systems.

Fiber Optic SensorsThere exists a great variety of fiber opticsensors (FOS) for structural andgeotechnical monitoring. In thisoverview we will concentrate on thosethat have reached a level of maturity, al-lowing a routine use for a large numberof applications. Figure 1 illustrates thefour main types of fiber optic sensors: Point sensors have a single measure-

ment point at the end of the fiber op-tic connection cable, similarly tomost electrical sensors.

Multiplexed sensors allow the mea-surement at multiple points along asingle fiber line.

Long-base sensors integrate themeasurement over a long measure-ment base. They are also known aslong-gage sensors.

Distributed sensors are able to senseat any point along a single fiber line,typically every meter over many ki-lometers of length.The greatest advantages of the FOS

are intrinsically linked to the optical fi-ber itself that is either used as a link be-tween the sensor and the signalconditioner, or becomes the sensor it-self in the case of long-gauge and dis-

tributed sensors.In almost all FOSapplications, theoptical fiber is athin glass fiberthat is protectedmechanically by apolymer coating(or a metal coat-ing in extremecases) and furtherprotected by amulti-layer cablestructure de-signed to protectthe fiber from the

environment where it will be installed.Since glass is an inert material very re-sistant to almost all chemicals, even atextreme temperatures, it is an ideal ma-terial for use in harsh environmentssuch as encountered in geotechnical ap-plications. Chemical resistance is agreat advantage for long term reliablehealth monitoring of civil engineeringstructures, making fiber optic sensorsparticularly durable. Since the lightconfined into the core of the optical fi-bers used for sensing purposes does notinteract with any surrounding electro-magnetic field, FOS are intrinsicallyimmune to any electromagnetic (EM)interferences. With such unique advan-tage over sensors using electrical ca-bles, FOS are obviously the idealsensing solution when the presence ofEM, Radio Frequency or Microwavescannot be avoided. For instance, FOSwill not be affected by any electromag-netic field generated by lightning hit-ting a monitored bridge or dam, nor

from the interference produced by asubway train running near a monitoredzone. FOS are intrinsically safe and nat-urally explosion-proof, making themparticularly suitable for monitoring ap-plications of risky structures such as gaspipelines or chemical plants. But thegreatest and most exclusive advantageof such sensors is their ability to offerlong range dis tr ibuted sensingcapabilities.

Fabry-Prot InterferometricSensorsFabry-Prot Interferometric sensors aretypical example of point sensors and

have a single measurement point at theend of the fiber optic connection cable.

An extrinsic Fabry-Prot Interfer-ometer (EFPI) consist of a capillaryglass tube containing two partially mir-rored optical fibers facing each other,but leaving an air cavity of a few mi-crons between them, as shown in Figure2. When light is coupled into one of thefibers, a back-reflected interference sig-nal is obtained. This is due to the reflec-tion of the incoming light on the twomirrors. This interference can be de-modulated using coherent or low-co-

Geotechnical News, September 2007 5

GEOTECHNICAL INSTRUMENTATION NEWS

Figure 1. Fiber optic sensor types.

Figure 2. Operating principle or a Fabry-Prot cavity sensor.

Figure 3. Examples of geotechnicalsensors based on the Fabry-ProtCavity principle. Depicted are apiezometer and a displacementtransducer.

herence techniques to reconstruct thechanges in the fiber spacing. Since thetwo fibers are attached to the capillarytube near its two extremities (with a typ-ical spacing of 10 mm), the gap changewill correspond to the average strainvariation between the two attachmentpoints shown in Figure 2.

Many sensors based on this principleare currently available for geotechnicalmonitoring, including piezometers,weldable and embedded strain gauges,temperature sensors, pressure sensorsand displacement sensors. Examplesare shown in Figure 3.

As an example, this technology hasbeen installed for the monitoring of amining dam. Located in the mountainsnorth of Santiago, Chile, El Mauro tail-ings dam is being built as part of the LosPelambres mine project. Approxi-mately 1.4km wide, El Mauro will havea final height of 240m at an altitude of938m asl. Work began on the infrastruc-ture of the dam following environmen-tal approval received in 2004. Expectedto cost around US$450M, the dam isscheduled to be completed in 2007.

In September 2005, Los Pelambresselected Fabry-Prot fiber optic sensorsfor the instrumentation at El Mauro afirst example of fiber optic instrumentsfor this type of application. The instru-ments include piezometers, tempera-ture sensors and seismographs.

Because they are immune to electro-magnetic interferences, static electric-ity and frequent thunderstorms that arefound at high altitudes, fiber optic in-struments offer in this case an importantadvantage over the traditional vibratingwire technology. They are more rugged

in such a harsh environment and allowvery long cable lengths without theneed of any lightning protection. This isimportant because Los Pelambres mineis located at an altitude of 3200m wheredry air produces static electricity. Thearea is also affected by earthquakes,which are monitored by the installationof seismographs connected to the fiberoptic instruments so that high-speed dy-namic measurements can be taken dur-ing a seismic event. This system allowsthe dam to be monitored throughout itsconstruction and all other phases of itslife.

Fiber Bragg Grating SensorsFiber Bragg Grating Sensors are themost prominent example of multi-plexed sensors, allowing measurementsat multiple points along a single fiberline.

Bragg gratings are periodic alter-ations of the density of glass in the coreof the optical fiber produced by expos-ing the fiber to intense ultraviolet light.The produced gratings typically have alength of about 10 mm. If light is cou-pled in the fiber containing the grating,the wavelength corresponding to thegrating period will be reflected while allother wavelengths will pass through thegrating undisturbed, as shown in Figure4. Since the grating period is strain andtemperature dependent, it becomes pos-sible to measure these two parametersby analyzing the spectrum of the re-flected light. This is typically done us-ing a tunable fi l ter (such as aFabry-Prot cavity) or a spectrometer.Precision of the order of 1 and 0.1 Ccan be achieved with the bestdemodulators. If strain and temperature

variations are ex-pected simulta-neously, i t isnecessary to use afree referencegrating that mea-sures the tempera-ture only andemploy its read-ing to correct thestrain values.Set-ups allowingthe simultaneousmeasurement of

strain and temperature have been pro-posed, but their reliability in field con-ditions has yet to be proved. The maininterest in using Bragg gratings residesin their multiplexing potential. Manygratings can be produced in the same fi-ber at different locations and tuned toreflect at different wavelengths asshown in Figure 4. This allows the mea-surement of strain at different placesalong a fiber using a single cable. Typi-cally, 4 to 16 gratings can be measuredon a single fiber line. It should be

pointed out that since the gratings haveto share the spectrum of the source usedto illuminate them, there is a trade-offbetween the number of grating and thedynamic range of the measurements oneach of them.

Because of their short length, FiberBragg Gratings can be used as replace-ments for conventional strain gages,and installed by gluing them on metalsand other smooth surfaces. With ade-quate packaging they can also be usedto measure strains in concrete over gagelength of typically 100 mm.

SOFO Interferometric SensorsThe SOFO Interferometric sensors are

GEOTECHNICAL INSTRUMENTATION NEWS

6 Geotechnical News, September 2007

Figure 4. Chain for Fiber Bragg Grating sensors containingstrain and temperature sensors. Each sensor reflects aspecific wavelength.

Figure 5. SOFO sensor installed on arebar. The plastic pipe contains thecoupled measurement fiber and a freeun-coupled reference fiber. Themetallic anchors at both ends of thewhite plastic pipe define the gagelength.

long-base sensors, integrating the mea-surement over a long measurement basethat can reach 10m or more.

The SOFO system is a fiber opticdisplacement sensor with a resolution inthe micrometer range and excellentlong-term stability. It was developed atthe Swiss Federal Institute of Technol-ogy in Lausanne (EPFL) and is nowcommercialized by the authors com-pany, SMARTEC in Switzerland.

The measurement set-up useslow-coherence interferometry to mea-sure the length difference between twooptical fibers installed on the structureto be monitored (Figure 5), by embed-ding in concrete or surface mounting.The measurement fiber is pre-tensionedand mechanically coupled to the struc-ture at two anchorage points in order tofollow its deformations, while the refer-ence fiber is free and acts as temperature

reference. Both fi-bers are installedinside the sameplastic pipe andthe gage lengthcan be chosen be-tween 200mmand 10m. TheSOFO readoutunit , shown inFigure 6, mea-sures the lengthdifference be-tween the mea-surement fiberand the referencefiber, by compen-sating it with a matching length differ-ence in its internal interferometer. Theprecision of the system is of 2 mindependently from the measurementbasis and its accuracy of 0.2% of themeasured deformation even over yearsof operation.

The SOFO system has been used tomonitor more than 300 structures,including bridges, tunnels, piles, an-chored walls, dams, historical monu-ments, nuclear power plants as well aslaboratory models. An example of suchan application was the monitoring ofcast-in-place piles during a load test. Anew semi-conductor production facilityin the Tainan Scientific Park, Taiwan, isgoing to be founded on a soil consistingmainly of clay and sand with poor me-chanical properties. To assess the foun-dation performance, it was decided toperform an axial compression, pulloutand flexure test in full-scale on-site con-

dition. Four meterlong SOFO sen-sors were selectedin order to coverthe whole lengthof the pile withsensors, and ob-tain averagedstrains over longpile sections. Thepile was dividedinto eight sec-tions. In the caseof axial compres-sion and pullouttests, a simple

sensor arrangement was used: the eightsensors were installed in a single chain,placed along one of the main rebars, onesensor in each section (A1 to A8), asshown in Figure 7. To detect andcompensate for a possible load eccen-tricity, the top cell was equipped withone more sensor (B1) installed on theopposite rebar with respect to the pileaxis.

As a result of monitoring, valuableinformation concerning the structuralbehavior of the piles was collected. Im-portant parameters were determinedsuch as distributions of strain, normalforces, displacement in the pile, distri-bution of frictional forces between thepile and the soil, determination ofYoungs Modulus, ultimate load capac-ity and failure mode of the piles as wellas qualitative determination of mechan-ical properties of the soil (three zonesare indicated in Figure 7).

For the flexure test, a parallel ar-rangement was used: each section con-tained two parallel sensors (as in section1 of Figure 7) installed on two oppositemain rebars, constituting two chains ofsensors. This sensor arrangement al-lowed determination of the average cur-vature in each cell, calculation ofdeformed shape and identification ofthe plastic hinge depth (failure loca-t ion) . A diagram of horizontaldisplacement for different steps of loadas well as the failure location on the pileis shown in Figure 8. More details canbe found in Glisic et al (2002).

This example shows an interestingapplication of long-gauge fiber optic

Geotechnical News, September 2007 7

GEOTECHNICAL INSTRUMENTATION NEWS

Figure 6. Portable SOFO systemreadout unit.

Figure 7. Sensor location and results obtained by monitoringduring the axial compression test of a cast-in-place pile.

Figure 8. Deformed shapes of the pile and identification offailure location.

sensors. The use of long-base SOFOsensors allows the gapless monitoringof the whole length of the pile, and pro-vides average data that is not affected bylocal features or defects of the pile.

Distributed Brillouin Scattering andDistributed Raman ScatteringSensorsDistributed sensors are able to sense atany point along a single fiber line (asshown in Figure 1), typically every me-ter over many kilometers of length.

In fully distributed FOS, the opticalfiber itself acts as sensing medium, al-lowing the discrimination of differentpositions of the measured parameteralong the fiber. These sensors use an in-trinsic property of standard telecommu-nication fibers that scatter a tiny amountof the light propagating through it at ev-ery point along their length. Part of thescattered light returns backwards to themeasurement instrument and containsinformation about the strain and tem-perature that were present at the loca-tion where the scattering occurred.When light pulses are used to interro-gate the fiber, it becomes possible, us-ing a technique similar to RADAR, todiscriminate different points along thesensing fiber by the differenttime-of-flight of the scattered light.Combining the radar technique and thespectral analysis of the returned light itbecomes possible to obtain the com-plete profile of strain or temperaturealong the fiber. Typically it is possibleto use a fiber with a length of up to 30

km and obtain strain and temperaturereadings every meter. In this case wewould talk of a distributed sensing sys-tem with a range of 30 km and a spatialresolution of 1 m.

Although the fiber used for the mea-surement is of standard telecommuni-cation type, it must be protected inside acable designed for transferring strainand temperature from the structure tothe fiber while protecting the fiber itselfform damage due to handling and to theenvironment where it operates. To takefull advantage of these techniques it istherefore important to select the appro-priate sensing cable, adapted to thespecific installation conditions.

The article immediately followingthis one is dedicated to distributed fiberoptic sensors. It presents the differentscattering sensing techniques, known asBrillouin and Raman Scattering, andtheir applications in geotechnical moni-toring.

ConclusionsThe monitoring of new and existingstructures is one of the essential toolsfor modern and efficient managementof the infrastructure network. Sensorsare the first building block in the moni-toring chain and are responsible for theaccuracy and reliability of the data.Progress in sensing technologies comesfrom more accurate and reliablemeasurements, but also from systemsthat are easier to install, use and main-tain. In recent years, fiber optic sensors

have taken the first steps in structuralmonitoring and in particular in civil andgeotechnical engineering. Differentsensing technologies have emerged andevolved into commercial products thathave been successfully used to monitorhundreds of structures. No longer a sci-entific curiosity, fiber optic sensors arenow employed in many applicationswhere conventional sensors cannot beused reliably or where they present ap-plication difficulties.

If three characteristics of fiber opticsensors should be highlighted as thereasons of their present and future suc-cess, we would cite the precision of themeasurements, the long-term stabilityand durability of the fibers and the pos-sibility of performing distributed andremote measurements over distances oftens of kilometers.

ReferenceGlisic, B., Inaudi, D., Nan, C. (2002)

Piles monitoring during the axialcompression, pullout and flexuretest using fiber optic sensors, 81stAnnual Meeting of the Transporta-tion Research Board (TRB), Wash-ington, DC, January 13-17, 2002

Daniele Inaudi and Branko Glisic,SMARTEC SA, Roctest Group, ViaPobiette 11, 6928 Manno, Switzerland,Tel. +41 91 610 18 00,email: inaudi@smartec.ch,email: glisic@smartec.ch

Distributed Fiber Optic Sensors:Novel Tools for the Monitoring ofLarge Structures

Daniele InaudiBranko Glisic

IntroductionDistributed fiber optic sensing offersthe ability to measure temperatures andstrains at thousands of points along asingle fiber. This is particularly interest-

ing for the monitoring of large struc-tures such as dams, dikes, levees, tun-nels, pipelines and landslides, where itallows the detection and localization ofmovements and seepage zones with

sensitivity and localization accuracyunattainable using conventional mea-surement techniques.

Sensing systems based on Brillouinand Raman scattering (the difference

GEOTECHNICAL INSTRUMENTATION NEWS

8 Geotechnical News, September 2007

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 300 /GrayImageDepth 8 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /FlateEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 1200 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages false /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects true /CheckCompliance [ /None ] /PDFX1aCheck false /PDFX3Check false /PDFXCompliantPDFOnly false /PDFXNoTrimBoxError true /PDFXTrimBoxToMediaBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXSetBleedBoxToMediaBox true /PDFXBleedBoxToTrimBoxOffset [ 0.00000 0.00000 0.00000 0.00000 ] /PDFXOutputIntentProfile (None) /PDFXOutputConditionIdentifier () /PDFXOutputCondition () /PDFXRegistryName () /PDFXTrapped /False

/Description >>> setdistillerparams> setpagedevice