Celebrating 50 Issues of Geotechnical Instrumentation Newsbrunet/Articles cimentation...

Celebrating 50 Issues ofGeotechnical Instrumentation News

Gord McKenna

“Put your hand in your pocket and drillseparate boreholes!” and so began thewords of wisdom in John Dunnicliff’sGeotechnical Instrumentation News(GIN) – an edited section of the last fiftyissues of BiTech’s Geotechnical Newsmagazine.

John often writes that it is “our GIN”but truly it’s his GIN, his heart and soul– his readers and contributors help keepit going by writing, arguing, comment-ing, and sharing. John is an inspirationto the entire geotechnical instrumenta-tion community – a community that hehas helped to create, to bring together,and to nurture. A self-styled “stickler”and “curmudgeon”, John’s insights andthoroughness from his famous RedBook, his instrumentation shortcourses, and his writings in GIN echo inour ears as we struggle to measure up tohis clear standard of care and thorough-ness. Getting people to present and ar-gue passionately about the intricacies ofgeotechnical instrumentation in GIN isone more way that John helps us keephigh standards even when we’re kneedeep in the mud, tricking sensors a hun-dred feet into the solid earth.

Leafing through fifty issues of GIN,I’m struck by the breadth of ourgeotechnical instrumentation commu-nity – it’s not just soil and rock – it’sconcrete and steel, it’s tunnels anddams, pavements and shafts, piles andtires, it’s the permafrost, the seafloor,and residual soils in the tropics, it’sshallow, deep, and everything in be-tween. It’s about understanding thepast, signing off on the present, predict-ing the future.

We often allow ourselves to forgetwhat a small community we are, andhow so many of us are geographicallyisolated. Geotechnical instrumentationis a small industry – there are only a fewmanufacturers, a few key workshopsand conferences, a few major projectson the go at any given time. It takesnearly a generation to work the kinksout of new technologies – many don’tsurvive the harsh field conditions, andmany old favourites are no longer man-ufactured (yet must still be lovinglyread and maintained). GIN offers a run-ning dialog of new technologies, im-provements on old ones, ideas andunderstanding for continuous improve-ment. Issues are sprinkled with adagesfrom Ralph Peck and Karl Terzaghi toremind us where we have been, wherewe are going, and the need for both un-derstanding and the utmost care. GINprovides a place for us to meet – itbrings us together.

John uses GIN to bring togethermanufacturers, designers, field practi-tioners, spec writers, and procurementspecialists to get the most out ofgeotechnical instrumentation. We taketo heart the need to understand exactlyhow each sensor works – how each onereacts differently with the ground, andhow every borehole is different. Nocookbook or standard can cover all con-ditions, but there are some important re-curring themes in GIN that include:• The Golden Rule: Every instrument

on a project should be selected andplaced to assist with answering aspecific geotechnical question; ifthere is no question, there should be

no instrumentation• The person held responsible should

be the one with the greatest vestedinterest in the data

• If an instrument is not working per-fectly before installation, there’s notmuch hope of it working well afterinstallation (so make absolutely sureit is working perfectly before you in-stall it)

• Don’t allow dust to grow on data• We almost never know the actual

value of the quantity being mea-sured, so we must resort to othermethods to ensure accuracy

• Never shy from an opportunity to in-terrogate sensors under controlledcondition – to ensure their accuracy,and to better understand their behav-iour in the ground.

• Installation is always a professionalendeavour, not something to be leftto the inexperienced or to simply thelowest bidder.GIN is a wonderful mix of articles

about the success and failure of instru-ments and instrumentation programs,users’ complaints and answers frommanufacturers, short book reviews,checklists for doing things right, andplugs for upcoming instrumentationconferences and workshops. Weaving itall together is John’s running commen-tary on everything from liquid levelgages, to measuring unsaturatedpore-water pressures, to comparing ac-curacy, resolution, and precision, towriting specification packages for thosewho insist on assigning their instrumen-tation programs to the lowest bidder.Mixed in for good measure are pointers

24 Geotechnical News, September 2007

on cricket, pleas for good grammar andpunctuation, and an ongoing explana-tion to North Americans on what itmeans to be British (“two nations sepa-rated by a common language”).

At the start, some were concernedthat GIN would be too controversial formanufacturers (a.k.a. advertisers) –highlighting disagreements with cli-ents, inviting comments on reliability,or dredging up problems solved longago. John’s solution is to invite com-ment from all interested parties, oftenpublished together, but also often run-ning as major themes over three or fourissues. The reader not only understandsthe solutions, but how people go aboutarriving at them, and an understandingthat there are many things we all strug-gle with together. His success in this re-

gard provides an example of how toapproach such disagreements with in-tegrity. But it requires skill and effort onJohn’s part – GIN works because ofJohn’s commitment to excellence inwriting, in instrumentation, and to get-ting the word out.

How long can we expect John tocarry the torch? He’ll tell you that GINdepends on all of us – keeping the arti-cles coming, hopefully a little fasterthan in the past, and adhering to his 26point manifesto on style. But then he’llconfide that editing GIN is “Part of whoI am.” So I’m assuming the first fifty is-sues are the sign of more good issues,more good controversies, and moregood insights to come.

Thank you, John, for your dedica-tion in bringing our community to-

gether, and offering a thoughtful, welledited, and diverse mix of articles, re-views, notices, philosophy and discus-sion that teaches and inspires all of us.

Gord McKenna, PhD, PEng, PGeo, Se-nior Geotechnical Engineer, BGC En-gineering Inc, Suite 500 - 1045 HoweStreet, Vancouver, BC, Canada, V6Z2A9, Telephone: (604) 684-5900,e-mail: [email protected], www.bgcengineering.ca

Geotechnical News, September 2007 25

GIN & JOHN

do not print keylines

4 colour drawing

Geotechnical Instrumentation News

John Dunnicliff

IntroductionThis is the fifty-second episode of GIN.Two articles this time.

Fiber Optic SensorsIn the previous episode of GIN I men-tioned fascinating updates on fiber opticsensing and time domain reflectometryat the March instrumentation course inFlorida. I said that I hoped to have GINarticles on these two topics. Here aretwo on the first topic. The authors of thetwo art ic les are employed bySMARTEC in Switzerland, a memberof the Roctest Group. Their companymanufactures the fiber optic sensingsystems that are described in the arti-cles.

Time Domain Reflectometry(TDR)There will be a paper on TDR by KevinO’Connor (GeoTDR, Westerville,Ohio) in the proceedings of theFMGM-2007 symposium, to be held inBoston in September. We’ll re-publishthis in GIN, probably in the March 2008episode.

Fully-grouted PiezometersThere will also be a paper in the pro-ceedings of the FMGM symposium byIván Contreras, Aaron Grosser andRich VerStrate (Barr Engineering Co.,Minneapolis) on the use of thefully-grouted method for piezometerinstallation. We’ll also re-publish this inGIN next year.

What Else Can I Tell YouAbout?While trying to answer this I re-read theMay 2007 editorial by Scot Litke, Edi-

tor of Foundation Drilling, the maga-zine of ADSC, The International Asso-ciation of Foundation Drilling. Eventhough the editorial has nothing to dowith instrumentation, no way can I dobetter. So, with Scot’s helps and permis-sion, ‘ere ‘tis, as they say in Devon:

Older Workers Can Fill anImportant Role

Much has been written about thechanging population demographicsin the United States and how it mayaffect the world of work. It has beena traditional assumption in our soci-ety that once one reached the age of65 one was expected to retire. In factseveral vocations mandate retire-ment at this age, if not sooner. Thisassumption is ‘now under review’.

There are several important fac-tors that contribute to the re-evalua-tion of this societal expectation.Firstly, there is the fact of numbers.The folks on the front end of thebaby boom generation, those bornbetween 1946 and 1950, are gettingreal close to retirement age. At closeto 76 million, the boomers are thelargest population block in our na-tion. Secondly, people are living lon-ger, more productive lives. Theaverage life expectancy in the U.S.for a man is now up to 83. Womencan expect, on average, to live till 88.Of course not all will, an unfortunateaspect of averaging, but many, manymore will. Thirdly, retirement isn’twhat it used to be, in a number ofways. The economics of retirementhave changed drastically. The dayswhen one could lay back and getalong on social security are long

gone. As people live longer, moreactive lives, their expectations forhow they will live in their retirementyears has changed. Few are now sat-isfied with the notion of kickingback and heading to the porchrocker. It used to be assumed thatonce one reached ‘a certain age’ theywouldn’t need the kind of revenuestream that they had earlier in theirlives, as after all, they weren’t goingto be as active. Guess again. If any-thing retirement has become to meanfor many, a more active life doingthings one ‘wanted’ to do but forwhich one could never find the time.Translation, if you expect to con-tinue to do more, and social securityisn’t going to cut it, you had better besaving and investing aggressively.

Here’s another gotchya - Ameri-cans aren’t very good at saving. Byand large wages have not kept upwith inflation over the long haul. So,what have we got here? A formulafor working past 65. At this timeonly about 15 percent of those over65 are in the workforce. In 2006, 28percent of those in the workforcewere age 50 plus, up from less than20 percent 20 years earlier. This willcontinue to change dramaticallyover the next 10 years as the boomersshow up as quasi-retirees in largenumbers.

Now the interesting part - it is notonly that folks need to work longer.It is that our economy needs them todo so. Remember that those 76 mil-lion baby boomers are not being re-placed by Gen-Xers or Gen-Yers innear the numbers the previous gener-


GEOTECHNICAL INSTRUMENTATION NEWS

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.Here’s 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-ployer’s 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 don’t think they are owedanything. They relish the chanceto continue to contribute to acompany’s 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 that’sanother topic.

Here’s the point – don’t dis-count the value of keeping yourolder employees. Don’t 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.

It’s shift the paradigm time.Please don’t 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, [email protected], 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-Pérot Interferometric Sensors• Fiber Bragg Grating Sensors



• 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-Pérot InterferometricSensorsFabry-Pérot Interferometric sensors aretypical example of point sensors and

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

An extrinsic Fabry-Pérot 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-



Figure 1. Fiber optic sensor types.

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

Figure 3. Examples of geotechnicalsensors based on the Fabry-PérotCavity 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-Pérot 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-Pérot cavity) or a spectrometer.Precision of the order of 1 με 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



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 μmindependently 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 ofYoung’s 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



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: [email protected],email: [email protected]

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



between the two will be explained later)are used to detect and localize seepagein dams and dikes, allowing the moni-toring of hundreds of kilometers along astructure with a single instrument andthe localization of the water path withan accuracy of 1 or 2 meters. Distrib-uted strain sensors are also used to de-tect landslide movements and to detectthe onset of cracks in concrete dams.

Early applications of this technologyhave demonstrated that the design andproduction of sensing cables, incorpo-rating and protecting the optical fibersused for the measurement, as well astheir optimal locating and installation inthe structure under scrutiny, are criticalelements for the success of any distrib-uted sensing instrumentation project.

This article presents advances in dis-tributed sensing systems and in sensingcables design for distributed tempera-

ture and strainmeasurements .The article alsoreports on a num-ber of significantfield applicationexamples of thistechnology.

DistributedFiber OpticSensorsUnlike electricalsensors and local-ized fiber opticsensors, distrib-uted sensors offerthe unique charac-teristic of beingable to measure

physical parameters, in particular strainand temperature, along their wholelength, allowing the measurements ofthousands of points from a single read-out unit. The most developed technolo-gies of distributed fiber optic sensorsare based on Raman and Brillouin scat-tering. Both systems make use of a non-linear interaction between the light andthe glass material of which the fiber ismade. If an intense light at a knownwavelength is shone into a fiber, a verysmall amount of it is scattered backfrom every location along the fiber it-self. Besides the original wavelength(called the Rayleigh component), thescattered light contains components atwavelengths that are higher and lowerthan the original signal (called theRaman and Brillouin components).These shifted components contain in-formation on the local properties of the

fiber, in particularits strain and tem-perature. Figure 1shows the mainscattered wave-lengths compo-nents for a stan-dard optical fiber.If λ0 is the wave-length of the orig-inal signal gener-ated by thereadout unit, thescattered compo-nents appear both

at higher and lower wavelengths.The two Raman peaks are located

symmetrically to the original wave-length. Their position is fixed, but theintensity of the peak at lower wave-length is temperature dependant, whilethe intensity of the one at higher wave-length is unaffected by temperaturechanges. Measuring the intensity ratiobetween the two Raman peaks thereforeyields the local temperature in the fibersection where the scattering occurred.

The two Brillouin peaks are also lo-cated symmetrically at the same dis-tance form the original wavelength.Their position relative to λ0 is howeverproportional to the local temperatureand strain changes in the fiber section.Brillouin scattering is the result of theinteraction between optical and ultra-sound waves in optical fibers. TheBrillouin wavelength shift is propor-tional to the acoustic velocity in the fi-ber that is related to its density. Sincethe density depends on the strain and thetemperature of the optical fiber, we canuse the Brillouin shift to measure thoseparameters. For temperature measure-ments, Brillouin scattering is a strongcompetitor against systems based onRaman scattering, while for strain mea-surements it has practically no rivals.

When light pulses are used to inter-rogate the fiber it becomes possible, us-ing a technique similar to RADAR, todiscriminate different points along thesensing fiber through the differenttime-of-flight of the scattered light.Combining the radar technique and thespectral analysis of the returned lightone can obtain the complete profile ofstrain or temperature along the fiber.Typically it is possible to use a fiberwith a length of up to 30 km and obtainstrain and temperature readings everymeter. In this case we would talk of adistributed sensing system with a rangeof 30 km and a spatial resolution (notethat “spatial resolution” is a standardconcept of distributed sensing, eventhough this is not 100% correct inmetrological terms) of 1 m. Figure 2schematically shows an example of dis-tributed strain and temperature sensing.Systems based on Raman scatteringtypically exhibit temperature accuracy



Figure 1. Light scattering in optical fibers and its use for strainand temperature sensing. At every section of fiber, the incomingwavelength 0 is scattered backwards. The backscattered lightcontains new wavelengths that carry information about thestrain and temperature conditions at the location where thescattering occurred.

Figure 2. Schematic example of a distributed strain and tem-perature measurement.

of the order of ± 0.1°C and a spatial res-olution of 1m over a measurementrange up to 8 km. The best Brillouinscattering systems offer a temperatureaccuracy of ± 0.1°C, a strain accuracyof ±20 microstrain and a measurementrange of 30 km, with a spatial resolutionof 1 m. The readout units are portableand can be used for field applications.

The optical fibers themselves areonly 1/8 of a millimeter in diameter andare therefore difficult to handle and rel-atively fragile. For practical uses, it istherefore necessary to package them ina larger cable, much like copper con-ductors are incorporated in an electricalcable.

Since the Brillouin frequency shiftdepends on both the local strain andtemperature of the fiber, the sensorset-up will determine the actual re-sponse of the sensor. For measuringtemperatures it is necessary to use a ca-ble designed to shield the optical fibersfrom an elongation of the cable. The fi-

ber will therefore remain in itsunstrained state and the frequency shiftscan be unambiguously assigned to tem-perature variations. Measuring distrib-uted strains also requires a speciallydesigned sensor.A mechanicalcoupling betweenthe sensor and thehost s tructurealong the wholelength of the fiberhas to be guaran-teed. To resolvethe cross-sensitiv-ity to temperaturevariations, it isalso necessary toinstall a referencefiber along thestrain sensor. Spe-cial cables, con-taining both freeand coupled fibersallow a simulta-

neous reading of strain and tempera-ture. Figure 3 shows examples oftemperature, strain and combined ca-bles.

Application ExamplesThis section presents brief applicationexamples of distributed sensing for themonitoring of civil and industrial struc-tures.

Temperature Monitoring in aConcrete DamIn this application, a Brillouin scatter-ing sensor system was used to monitorthe temperature development in theconcrete used to build a dam. The



Figure 3. Distributed sensor cables examples. Left-top: temperature sensor;Left-bottom: strain sensor; right: combined strain and temperature sensor.

Figure 4. Contour plot (isothermal lines) of the temperaturemeasurements in °C at the Luzzone Dam 30 days afterconcrete pouring (courtesy of L. Thévenaz).

Figure 5. Plavinu Dam in Latvia.

Figure 6. Strain sensor installation in the Plavinu Daminspection gallery.

Luzzone concrete arch dam was raisedby 17 meters to increase the capacity ofthe reservoir. The raising was achievedby successively concreting 3m thickblocks. The measurements concen-trated on the largest block to be poured,the one that rests against the rock foun-dation at one end of the dam. An ar-mored cable installed in a zigzag patternduring concrete pouring constituted theBrillouin sensor and was placed in themiddle of the concrete block thickness.The cable therefore became embeddedin the concrete.

The temperature measurementsstarted immediately after pouring andcontinued over six months. The mea-surement system was proved reliableeven in the demanding environmentpresent at the dam (dust, snow, and tem-

perature excur-sions). The temp-erature distribu-tions after 30 daysfrom concretepouring areshown in Figure4. Comparativem e a s u r e m e n t sobtained locallywith conventionalthermocouplesshowed agree-ment within theerror of both sys-tems. This exam-ple shows how itis possible to ob-tain a large num-ber of measure-

ment points with relatively simple sen-sors. The distributed nature of distrib-uted sensing make it particularlyadapted to the monitoring of large struc-tures where the use of more conven-tional sensors would require extensivecabling.

Monitoring Bitumen Joints in aDamPlavinu dam belongs to the complex ofthe three most important hydropowerstations on the Daugava River in Latvia(see Figure 5). One of the dam inspec-tion galleries coincides with a system ofthree bitumen joints that connects twoseparate blocks of the dam. Due to abra-sion of water, the joints loose bitumen,and a redistribution of loads in the con-crete arms appears. Since the structure

is nearly 40 years old, the structuralcondition of the concrete can be com-promised because of its ageing. Thus,the redistribution of loads can crack theconcrete and as a consequence the inun-dation of the gallery. In order to increasethe safety and enhance the managementactivities it was decided to monitor theaverage strain in the concrete next to thejoints. A Brillouin scattering system,combined with a strain and temperaturesensing cable is used for this purpose(see Figure 6). The strain sensors arecoupled to the concrete with bolted me-tallic plates every two meters. The read-out unit automatically performs mea-surements every 15 minutes and athreshold detection software sets offwarnings and alarms to the Control Of-fice. Fortunately, so far this has neverbeen the case.

Since it is not possible to predictwhere a crack might appear along thelength of the dam, instrumenting it withconventional discrete sensors, evenlong-gage ones, would have required theinstallation of hundreds of sensors,along with their cables and data acquisi-tion systems. Thanks to distributed sens-ing the same goal can be achieved withjust two cables and a single readout unit.

Monitoring a Gas PipelineAbout 500 meters of a buried 35-yearold gas pipeline, located in Italy, lie in alandslide-prone area. Distributed strainmonitoring was selected in order to im-prove an existing vibrating wire straingage monitoring system. The landslideprogresses with time and could damage



Figure 7. Strain and temperature sensing cables installed on agas pipeline. The picture shows a sensor line attached to the topof the pipeline and one on the side. The sensors are protectedwith an black neoprene pad. Another sensor line is attachedsymmetrically on the opposite side. The temperature sensingcable is also installed on top of the pipe. The vertical tube at thecenter of the picture, brings the optical cables form the pipelineto the junction box.

Figure 8. Strain distribution over the monitored part of thepipeline measured by the three distributed strain sensors.Each curve is composed of 500 individual strain points.

the pipeline until it would be put out ofservice. In the past, three symmetricallylocated vibrating wires strain gageswere installed in several sections at adistance typically of 50 to 100m, cho-sen as the most stressed locations ac-cording a preliminary engineering eval-uation. These sensors were veryhelpful, but could not fully cover thelength of the pipeline as they provideonly local measurements.

Distributed strain and temperaturesensing cables were installed at the pro-ject. Three parallel lines consisting offive segments of strain sensing cablewere installed over whole length of thepipeline for which there were concerns,at approximately 0°, 120° and –120°around the pipeline circumference (seeFigure 7). The sensing cables were ep-oxy-glued to the surface of the pipelinealong their whole length and protectedwith a neoprene mat. This instrumenta-tion allows the monitoring of strain,curvatures and deformed shape of thepipeline every meter (corresponding to

the spatial resolu-t ion of theBrillouin systemin use). The tem-perature sensingcable was in-stalled onto theupper line (0°) ofthe pipeline in or-der to compensatethe strain mea-surements fortemperature andfor leakage detec-tion purposes. Allthe sensors are

connected to a central measurementpoint by means of optical extension ca-bles and connection boxes. They are in-terrogated from this point using onesingle readout unit. Since the landslideprocess is slow, the measurement ses-sions were performed manually once amonth.

In case of need, a dedicated readoutunit can be installed onsite and the datatransmitted wirelessly to the pipelineowners. All the measurements obtainedwith the system are correlated with themeasurements obtained with vibratingwire strain gages placed at a few se-lected locations.

Figure 8 shows the strain changes re-corded after burying the pipeline. Thisfigure contains more than 1,500 strainmeasurement points, a coverage thatcould never be achieved with any con-ventional strain sensing technology.

During the installation of the sensorsand the burying of the pipeline, a gasleakage simulation test was performedby installing an empty plastic tube over

a distance of 50m at the beginning of thefirst monitored segment, connecting thesurface of the pipe at that point with theopen air. Carbon dioxide was injectedinto the tube, cooling down the pipe endto mimic conditions expected in thecase of a gas leakage. A reference tem-perature measurement was performedbefore injecting the carbon dioxide. Af-terwards temperature measurementswere performed every two to ten min-utes and compared with the referencemeasurement. The results of the test arepresented in Figure 9. The test was suc-cessful, and the point of the simulatedleakage was clearly observed andlocalized (encircled area in Figure 9).

ConclusionsThe use of distributed fiber optic sen-sors for the monitoring of civil andgeotechnical structures opens new pos-sibilities that have no equivalent in con-ventional sensors systems. Thanks tothe use of a single optical fiber with alength of tens of kilometers it becomespossible to obtain dense information onthe strain and temperature distributionin the structure. This technology istherefore particularly suitable for appli-cations at large or elongated structures,such as dams, dikes, levees, largebridges and pipelines.

Daniele Inaudi and Branko Glisic,SMARTEC SA, Roctest Group, ViaPobiette 11, 6928 Manno, Switzerland,Tel. +41 91 610 18 00,email: [email protected],email: [email protected]



Figure 9. Results of leakage test; leakage is detected astemperature drop at the leakage location.

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