-
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,
[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-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: [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
GEOTECHNICAL INSTRUMENTATION NEWS
8 Geotechnical News, September 2007
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