quantitative evaluation of optical fiber/soil interfacial behavior...

IEEE SENSORS JOURNAL, VOL. 15, NO. 5, MAY 2015 3059

Quantitative Evaluation of Optical Fiber/SoilInterfacial Behavior and Its Implications

for Sensing Fiber SelectionCheng-Cheng Zhang, Hong-Hu Zhu, Jun-Kuan She, Dan Zhang, and Bin Shi

Abstract— An adequate understanding of the interface betweenoptical fibers and geomaterials is a prerequisite for applyingdistributed optical fiber sensor systems to strain monitoringin geoengineering. This contribution reports a quantitativeinvestigation of the fiber/soil interfacial behavior regarding theinfluence of fiber types and normal pressures. A simplifiedmodel describing the progressive failure of a fiber/soil interfacewas briefly illustrated. Results of a series of pullout testson three different soil-embedded optical fibers under variousnormal pressures were interpreted by this model, through whichthe fiber/soil interfacial behaviors were quantified. The resultsshowed that the mechanical properties of the three fiber/soilinterfaces were similar. Optical microscopic images indicated thatthe soil particles and the fibers were merely loosely contacted.This led to the formation of a fiber/soil interface susceptible tothe normal pressure: 1) the interfacial bond was tightened and2) the deformation measurement range was widened under highnormal pressures. Moreover, the criterion for selecting a strainsensing fiber for geoengineering applications was discussed interms of interfacial bond, deformation measurement range, ratioof peak to residual interfacial shear strength, Young’s modulusof fiber, and so forth. An assessment of the three fibers revealsthat, for ground deformation measurement, each fiber has itsown advantages as well as limitations. In field or laboratoryapplications, a combination of different types of fibers may obtainthe best measurement results.

Index Terms— Deformation measurement, interfacial bond,normal pressure, optical fiber sensor, selection criterion, soil.

I. INTRODUCTION

S INCE the invention of the first optical fiber sensor (OFS)in the late 1970s [1], optical fiber sensing technologieshave been developed rapidly. Various quasi- andfully-distributed monitoring technologies have emerged todate, including Optical Time-Domain Reflectometry (OTDR),

Manuscript received November 4, 2014; revised December 10, 2014;accepted December 10, 2014. Date of publication December 31, 2014; dateof current version April 1, 2015. This work was supported in part bythe Open Fund of the State Key Laboratory of Geohazard Prevention andGeoenvironment Protection under Grant SKLGP2011K011, in part by theNational Key Technology Research and Development Program of China underGrant 2012BAK10B05, in part by the National Basic Research Program(973 Program) of China under Grant 2011CB710605, in part by the NationalNatural Science Foundation of China under Grant 41302217. The associateeditor coordinating the review of this paper and approving it for publicationwas Dr. M. N. Abedin.

The authors are with the School of Earth Sciences and Engineering, NanjingUniversity, Nanjing 210023, China (e-mail: [email protected];[email protected]; [email protected]; [email protected];[email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2014.2386881

Brillouin Optical Time-Domain Reflectometry/Analysis (BOTDR/A), Brillouin Optical Frequency-DomainReflectometry/Analysis (BOFDR/A), Brillouin OpticalCorrelation-Domain Reflectometry/Analysis (BOCDR/A),and Fiber Bragg Grating (FBG). Compared with conventionalsensors, OFSs have some inherent advantages such asimmunity to electromagnetic interference, insensitivity tocorrosion, high precision and tiny size. In the past fewdecades, OFSs have already shown outstanding performancesin health monitoring of civil infrastructures [2]–[4]. Recently,these sensors have been adopted to monitor strains ordisplacements of a variety of geotechnical structures, such asslopes [5]–[9], earth-retaining walls [10], foundations [11],tunnels [12]–[14], and dams [15]. These pilot case studieshave greatly extended the real-world applications of thesesensors, and have preliminarily verified their capability incondition evaluation of geo-structures.

However, full potential of OFS systems has not beenexploited in geo-engineering due to the variable mechanicalbehaviors of geo-materials, and the complexity of geologicalconditions. Firstly, soils and rocks, in contrast to artificialmaterials, are heterogeneous, anisotropic and porous media.Their properties are easily influenced by the surroundingenvironments, possibly resulting in undesirable deteriorationof the fiber/soil interface. Secondly, the measurement oflarge strains and displacements is frequently required ingeo-engineering [16]. Fig. 1 shows the typical ranges of strainexperienced in geo-engineering applications. Undoubtedly,OFSs are capable of sensing minute deformations. Sometimeslarge deformations, usually encountered during catastrophicevents (e.g. landslides and lateral spreading; see Fig. 1),should be captured. It is also of great importance to recordlarge deformations in geotechnical and geomechanical modeltests so that the failure mechanism of the prototypes can bedetermined [17]. Unfortunately, the ultimate strain of popularsilica optical fibers is merely around 5% [4]. Polymer opticalfibers (POFs) are potential alternatives to silica optical fibersowing to their high elastic strain limits [18], but to dateonly a limited efforts have been made to apply these newfibers to geotechnical monitoring. In summary, it is hardto ensure an intimate contact between embedded OFSs andthe surrounding geo-materials, especially when the soil orrock mass is undergoing large deformations, and/or when theembedded OFSs are under low normal pressures. Consideringthat the OFS systems installed will be in service for months

1530-437X © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

3060 IEEE SENSORS JOURNAL, VOL. 15, NO. 5, MAY 2015

Fig. 1. Typical ranges of strain experienced in geoengineering applications. Strain ranges are from [16].

or even years, it is hard for geotechnical practitioners to geta clear understanding of the monitoring results obtained bysoil-embedded sensing fibers.

In light of these negative effects, some practical measureshave been taken to improve the optical fiber/soil interfacialcontact. For instance, Zhu et al. [19] employed a numberof heat shrinkage tubes to roughen the surface of BOTDAsensing fibers that were directly embedded in a model slope.The interfacial bond between the soil mass and the fiberswith periodic enlargements was therefore tightened. A similarapproach was reported by Li et al. [20] in their model tests.On the other hand, much research has been carried out tocharacterize the interfacial behavior between OFSs and man-made materials, or the strain transfer efficiency from hostmaterials to fiber cores [21]–[26]. On theoretical grounds,a few interaction theories have been proposed [27]–[31],but these models can hardly be extended to interpret thefiber/soil interaction as they do not explicitly account fornormal pressure. Additionally, in contrast to sensors attachedto concrete or polymer composites, the installation of fibers insoil rarely involves the utilization of adhesive layers such asepoxy. It is therefore imperative to carry out a comprehensiveinvestigation into the fiber/soil interfacial behavior.

This paper focuses on the evaluation of the fiber/soilinterfacial behavior and its implications for the selection ofsensing fibers for geo-engineering applications. The pullouttest results of soil-embedded optical fibers were interpretedusing a fiber/soil interaction model, in which the progressivefailure of a fiber/soil interface was highlighted. The influ-ences of fiber type and normal pressure on the interfacialbehavior were analyzed and discussed. Some suggestions weregiven on sensing fiber selection for geotechnical monitoringpurposes.

II. BRIEF OVERVIEW OF THE THEORETICAL FIBER/SOILINTERACTION MODEL

A simplified fiber/soil interaction model has been proposedby Zhang et al. [32], which provides a method to charac-terize the fiber/soil interfacial behavior through pullout tests.An illustration of this model is shown in Fig. 2. This modelassumes a tri-linear interfacial shear stress-displacement rela-tion, and thus the progressive failure of a fiber/soil interfacecan be divided into five consecutive phases during pulloutprocesses: pure elastic phase (Phase I), elastic-softening phase(Phase II), pure softening phase (Phase III), softening-residualphase (Phase IV), and pure residual phase (Phase V). Foreach phase, the relation between pullout force F0 and pulloutdisplacement u0 is derived as shown in (1) at the bottomof the next page [32], where G1, G2, τmax and τres arefour independent parameters (Fig. 2(b)), with G1 and G2being the interfacial shear stiffnesses corresponding to theelastic branch and softening branch of the τ − u curve,respectively, τmax being the peak interfacial shear strength,τres being the residual interfacial shear strength; D, L and Eare the diameter, length and Young’s modulus of the fiber,respectively; Ls and Lr are the lengths of the softeningzone and the residual zone (Fig. 2(d)), respectively; andαi = √4Gi/E D (i = 1, 2).

Based on the above model, two characteristic displacementsare defined as [32]

⎧⎪⎨

⎪⎩

ueff = τmaxG1

upeff = τmaxG1

+ τmaxG2

− τresG2

+ 2τresDE

L2(2)

where ueff and upeff are the effective and partially effectivedisplacements, respectively. Three working states of a soil-embedded optical fiber are proposed accordingly: effective

ZHANG et al.: QUANTITATIVE EVALUATION OF OPTICAL FIBER/SOIL INTERFACIAL BEHAVIOR AND ITS IMPLICATIONS 3061

Fig. 2. Overview of the simplified optical fiber/soil interaction model (after [32]). (a) Pullout mechanism of an optical fiber out of soil matrix. (b) Tri-linearinterfacial shear stress-pullout displacement relation. OA, AB and BC denote elastic, softening and residual branches, respectively. (c) A typical pulloutforce-displacement curve. Five pullout phases (i.e., Phases I, II, III, IV and V) are marked, and three working states (i.e., effective, partially effective andineffective states) of optical fiber are shaded for clarity. (d) Evolution of the distribution of interfacial shear stress along the fiber length during the pulloutprocess.

state (0 ≤ u ≤ ueff), partially effective state (ueff < u ≤ upeff),and ineffective state (u ≥ upeff). Detailed formulation andillustration of the model can be found in [32].

To facilitate the identification of the working state of a strainsensing fiber, another parameter is highlighted here: the ratioof peak to residual interfacial shear strength that is defined as

k = τmaxτres

(3)

A higher value of k implies that it will be easier forgeotechnical engineers to determine whether or not a strain

sensing fiber is in the effective state. Therefore, this parametermay be beneficial for determining the most appropriate strainsensing fiber for geotechnical monitoring.

It is worth noting that this model assumes that the soildeforms under one-dimensional condition and the deformationis parallel to the fiber axis (x-axis in Fig. 2(a)). Therefore,the fiber remains straight during the course of soil deformation.However, for the circumstance that the deformation of soilis lateral, the fiber will bend. In such a case, the interactionbetween fiber and soil will be much more complex [6], [7],which lies outside the scope of this study.

F0 =

⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨

⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

π DG1 tanh α1 L

α1u0 (Phase I)

π DG2 cot α2 Lsτmaxα2τres

u0 + π Dτmaxα2 sin α2 Ls

− (1 + G2G1

)π D cot α2 Lsτ 2max

α2τres(Phase II)

−π DG2 tan α2 Lα2

u0 + (1 + G2G1

)π D tan α2 Lτmax

α2(Phase III)

π D2 E

4Lru0 + π DLrτres

2− π D

2 E

4Lr(τmax

G1+ τmax

G2− τres

G2) (Phase IV)

π Dτres L (Phase V)

(1)


Fig. 3. Details of the three test optical fibers. (a) A photograph showingthe fibers. (b) Structures of the fibers (unit: μm). The outer diameters are900, 1200 and 1800 μm for OF-1, OF-2 and OF-3, respectively. Detailedparameters are listed in Table I.

TABLE I

MAIN PROPERTIES OF THE THREE OPTICAL FIBERS

III. LABORATORY TESTS AND RESULTS ANALYSIS

A. Test Materials and Procedures

Three types of tight-buffered standard silicon optical fibersare selected in this test study (see Fig. 3(a)). Their struc-tures are sketched in Fig. 3(b). Detailed parameters of thesefibers are summarized in Table I. A poorly graded sandysoil collected from Xianlin District, Nanjing, China is usedin the pullout tests. This soil has an average grain sizeof 0.510 mm and a uniformity coefficient of 3.75. The max-imum dry density and the optimum moisture content of thissoil are 2.06 g/cm3 and 10.8%, respectively.

The test samples were prepared using the method presentedin [32]. The diameter of the cutting ring was changed to79.8 mm so that a longer fiber/soil bonding length could beobtained. For all the samples, the soil dry density was kept at1.80 g/cm3, which indicated a relative density of 87.4%.

During the tests, the pullout force and displacement weremeasured by a force gauge and a dial gauge, respectively, asshown in Fig. 4. A constant displacement rate of 0.72 mm/minwas applied during the tests. For OF-1, normal pressuresranging from 0 kPa to 120 kPa were used. The other two fibers,i.e. OF-2 and OF-3, were tested under the normal pressuresranging from 0 kPa to 90 kPa.

Fig. 4. Schematic drawing of the test setup (not to scale).

B. Pullout Test Curves

Fig. 5 depicts the pullout force-displacement curves forOF-1, OF-2 and OF-3 under different normal pressures.As expected, the shape of these curves is similar to thatpredicted by the proposed model shown in Fig. 2(c). Simplyfrom a visual inspection of Fig. 5, the maximum pullout forceincreases with the normal pressure, and so is the correspond-ing displacement that is required to mobilize the maximumpullout force. This trend holds for all the three types ofoptical fibers. However, under high normal pressures, theoptical fibers failed—the silica cores were broken and thecoatings underwent plastic deformations—when the pulloutdisplacement continuously increased, as denoted by the dashedlines in Fig. 5. It is indicated that an optical fiber, if deeplyburied in the soil mass, may fail due to tensile breakage insteadof pullout when measuring large deformations. This is an issueof great importance because distributed OFSs bear heavily onfiber failures. Care should therefore be taken during the designof OFS systems either in the laboratory or field.

C. Quantification of Interface Properties

The mechanical properties of the fiber/soil interfaceswere quantified by fitting the experimental results to theaforementioned interaction model. Some fitting curves areshown in Fig. 6. The agreement between the experimental andthe predicted results is reasonably good, providing an indica-tion of possibly using the interaction model to describe thefiber/soil interfacial behavior. The obtained parameters, i.e. theinterfacial shear strengths and stiffnesses, the characteristicdisplacements, and the ratio of peak to residual interfacialshear strength, were used to characterize the fiber/soil interfacefor different optical fibers and under various normal pressures.The results are as follows.

Fig. 7 presents the influence of fiber type and normalpressure on the peak and residual interfacial shear strengths(τmax and τres), and their ratio k. The dashed lines are linearfits to the data in Figs. 7(a) and 7(b), and constant fitsin Fig. 7(c). The fitting parameters in Fig. 7 and those tofollow are summarized in Table II. Fig. 7(a) shows that bothτmax and τres increase with the increase of normal pressure.Taking OF-3 as an example, as the normal pressure increasesfrom 0 kPa to 60 kPa, τmax increases by 156% from 23.04 kPato 58.93 kPa, and τres increases by up to 200% from 11.14 kPato 33.43 kPa for OF-3. As expected, the fiber/soil interfacialshear strength is remarkably improved under high normal pres-sures. However, as stated previously, a normal pressure that istoo high will cause the tensile failure of a strain sensing fiber.


Fig. 5. Curves of pullout force versus pullout displacement for: (a) OF-1, (b) OF-2, and (c) OF-3 under different normal pressures (NPs) obtained fromthe pullout tests. The optical fibers failed during the pullout process under NP = 120 kPa, 90 kPa, and 90 kPa for OF-1, OF-2, and OF-3, respectively.The failure of the fibers were characterized by the broken of the silica cores and the plastic deformations of the coatings.

Fig. 6. Model predictions plotted against experimental data under: (a) NP = 0 kPa, (b) NP = 15 kPa, and (c) NP = 60 kPa. The dashed lines denotepredicted results.

Fig. 7. Comparison of: (a) peak and (b) residual interfacial shear strengths between different optical fibers under the impact of normal pressure. The dashedlines are linear fits to the data in (a) and (b), and constant fits in (c).

In addition, although τmax or τres increases at a similarrate with the normal pressure, the fiber/soil interfacial shearstrengths for the three optical fibers is obviously different.The peak and residual shear strengths of the OF-2/soil andthe OF-3/soil interface are both higher than those of theOF-1/soil interface. Fig. 7(c) shows the correlation betweenthe ratio of interfacial shear strength k and the normal pressure.It is observed that k is not significantly influenced by thenormal pressure.

The effect of normal pressure on the interfacial shearstiffnesses G1 and G2 is shown in Fig. 8. Constant fits are used

to fit the data. The results indicate that the stiffness G1 of theOF-2/soil interface is the largest, then the OF-3/soil interface,and the OF-1/soil interface being the smallest. However, theimpact of fiber type on G2 is less significant than that on G1.From Fig. 8(b), it is observed that G2 of the OF-2/soil interfaceunder the normal pressure of 60 kPa is extremely large, butconsidering that the descending branch of the pullout force-displacement curve is sensitive to the test condition, constantfits can still be used.

Fig. 9 shows the correlation between the normal pressureand the two characteristic displacements. Referring to Fig. 9,


TABLE II

EMPIRICAL RELATIONS BETWEEN MODEL PARAMETERS AND NORMAL PRESSURE OF THE THREE OPTICAL FIBERS

Fig. 8. Comparison of interfacial shear stiffnesses: (a) G1 and (b) G2 betweendifferent optical fibers under the impact of normal pressure. The dashed linesare constant fits to the data.

Fig. 9. Comparison of: (a) effective and (b) partially effective displacementsbetween different optical fibers under the impact of normal pressure. Thedashed lines indicate linear regressions.

one can say that both the effective and partially effectivedisplacements increase with an increasing normal pressure.Taking OF-1 as an example, when the normal pressureincreases from 0 kPa to 60 kPa, ueff increases by 146%from 0.731 mm to 1.799 mm, and upeff increases by 150%from 1.173 mm to 2.932 mm. It can be deduced that themeasurement range of a strain sensing fiber is significantlywidened under high normal pressures.

IV. DISCUSSION

A. Influence of Normal Pressure on theFiber/Soil Interfacial Behavior

In the literature, the influence of normal pressure on thereinforcement/soil interfacial behavior has been extensivelyinvestigated [33]–[37]. The typical dimension of a soil rein-forcement, for instance, a model soil nail often has a diameter

Fig. 10. Optical microscopic images of an untested sample. The soil particlesand the fiber were loosely contacted, except for a few particles penetrated intothe fiber surface. This is supposed to be the reason why the fiber/soil interfaceis easily affected by the normal pressure.

of a few centimeters, which is one order-of-magnitude morethan that of the strain sensing fiber used in this study. Compar-atively, Tang et al. [38] and Zhu et al. [39] performed pullouttests and quantified the interfacial shear strength between a48 μm-diameter polypropylene fiber and soil. If the formerstudies are classified as macroscopic and the latter beingmicroscopic, the current study can be regarded as being some-where between the two cases. Because the average grain sizeof soil is 0.510 mm, which is comparable with the diameterof the sensing fibers (D = 0.9∼1.8 mm), the behavior of anoptical fiber/soil interface may be different from those in themacro- and microscopic studies.

Fig. 10 shows the optical microscopic images of an untestedsample. It is seen that although the soil particles wereinter-locked or inter-bonded after compaction, the sample wasnot a tight complex where the soil particles and the fiberwere merely loosely contacted, except for a few soil particlespenetrated into the fiber surface (see Fig. 10(b)). This issupposed to be the reason why the fiber/soil interface is easilyaffected by the normal pressure. Generally, the normal pressurecan affect the fiber/soil interfacial behavior in the followingtwo aspects.

On one hand, when a normal pressure is applied on thetest sample, it is re-compacted. Specifically, the higher thenormal pressure, the more inter-locked or inter-bonded the soilparticles will be, and the more particles will probably penetrateinto the fiber surface, causing considerable temporary orpermanent indentations along the fiber surface. O’Rourke [33]investigated the interfacial properties of sand/polymer inter-faces and found that the polymer surfaces were roughened


Fig. 11. Comparison of: (a) peak and (b) residual interfacial apparentcoefficients of friction between different optical fibers under the impact ofnormal pressure. The dashed lines show fits to power functions.

under high normal stresses. Han [34] also reported thatthe FRP surface was scratched under high normal stressesduring interface shear tests. As a result, the increased surfaceroughness and the tightened particle-particle and particle-fibercoupling under higher normal pressures contribute to a tighterinterfacial bond as indicated in Fig. 7.

On the other hand, the normal pressure will restrain the soildilation around the fiber. In Fig. 11, the peak and residualinterfacial apparent coefficients of friction, calculated fromf ∗p = τmax/σv and f ∗r = τres/σv, were plotted against thenormal pressure. Both f ∗p and f ∗r are found to decrease as thenormal pressure increases. It is because under lower normalpressures, the soil undergoing dilating is less restrained. On thecontrary, the dilatancy of soil is less significant under highernormal pressures. Similar phenomena on other soil inclusionshave been reported [35], [40]. Fig. 11 shows that OF-2/soilinterface has the highest f ∗p while the OF-1/soil interface hasthe lowest f ∗p , but the difference is reduced under highernormal pressures. For f ∗r , the difference is less obvious forthe three interfaces compared with f ∗p . Furthermore, powerfunctions were used to fit the data, which demonstrated favor-able fitting results (see Table II). Frost and Han [40] relatedthe parameters of the power functions to the roughness of theFRP/sand and steel/sand interface. They found that a smoothsteel/sand interface corresponds to low constants and powerindices, whereas a rough FRP/sand interface corresponds tohigh parameters. However, the constants and power indicesreported here do not corroborate their observations. It mightbe due to the diameter differences of the sensing optical fibers.In fact, apart from the surface roughness, the dimension effectis another factor affecting the dilatancy of soil during shear.These effects are interactional and lead to complex interactionbetween an optical fiber and the surrounding soil.

Additionally, the influence of normal pressure onthe contribution of each pullout stage to the partiallyeffective displacement is discussed. The impact of normalpressure on the progressive pullout behavior of a soil inclusionhas seldom been studied before. Fig. 12 demonstrates thatthe progressive pullout response of OF-2 and OF-3 is morepronounced and more sensitive to the normal pressurethan that of OF-1. It might be due to the fact that theYoung’s modulus of OF-1 is higher than that of OF-2and OF-3. For a rigid optical fiber, the fiber/soil relativedisplacement is much uniformly distributed along thefiber, and consequently the two transitional stages wheredifferent stress states exist make only a small contribution

to the total displacement. However, under high normalpressures, it becomes difficult to mobilize the fiber/soilrelative displacement, especially for the part away from thepullout force. As a consequence, the distribution of relativedisplacement along the fiber becomes non-uniform, andtherefore the pullout process is more progressive.

B. Selection of an Appropriate Optical Fiber forGround Deformation Monitoring

There are important issues to consider when choosing astrain sensing optical fiber for a monitoring project relatedto geotechnical engineering. Iten et al. [7] argued thatfiber protection and strain transfer efficiency are two vitalsubjects in field projects. A strong protection is prerequisite formonitoring in hash environments whereas the strain transferefficiency ensures we know the exact strain values. The othercrucial issue is the bonding property between a fiber andsoil [6], [8], which is the focus of this section.

Interfacial shear strength and interfacial shear stiffnessare two basic parameters to characterize the behavior of asoil inclusion/soil interface [37], [39], [41]. The interfacialbonding becomes tighter as the parameters increase. FromFigs. 7(a), 8(a) and 11(a), one can infer that the OF-2/soilinterfacial bonding is the tightest, then the OF-3/soil interfacialbonding, and the OF-1/soil interfacial bonding being theloosest. Therefore, OF-2 is preferable regarding the interfacialbonding. Effective and partially effective displacements(ueff and upeff) are the other two important parameters toevaluate the performance of a soil-embedded optical fiber.As plotted in Fig. 9, OF-3 is preferred in terms of bothueff and upeff . OF-2 is similar in ueff to OF-3, but is largerthan OF-1 in upeff . These results indicate that a tight inter-facial bonding does not guarantee a satisfactory deformationmeasurement range. In fact, a stiff interface will even reducethe effective and partially effective displacements according toEquation (2). Because monitoring large strains and displace-ments is sometimes required in geo-engineering, for instance,the monitoring of creeping landslides in mountainous regions,both the interfacial bonding and the deformation measurementrange should be taken into account. Finally, the raito k of thethree fibers shown in Fig. 7(c) indicate that it will be easier toassess whether OF-1 is in an effective working state comparedwith other fibers.

In addition to the fiber/soil interfacial behavior, the relativeYoung’s modulus between an optical fiber and soil should alsobe considered. From a geotechnical engineer’s perspective,an optical fiber embedded in soil can alter the stress and strainfield distributions of soil around the fiber. In addition, theinterface are more likely to debond due to the large differencein modulus between fiber and soil. Because most of naturallydeposited soils fall in the modulus range of 0.30-200 MPa [42],which is much less than that of a typical strain sensing fiber, anoptical fiber with low modulus is desirable when it is directlyintegrated into soil. According to the uniaxial tensile tests onthe optical fibers performed in this study, OF-3 is preferred.

From the foregoing analyses, it may be concluded thatOF-2 is suitable for measuring minute deformation because


Fig. 12. Simulation results of the contribution of each pullout stage to the partially effective displacement. (a)–(c) Influence of normal pressure on thecontribution. (d) Influence of fiber type on the contribution (NP = 60 kPa).

the interfacial bond between OF-2 and soil is tight whereasthe measurement range is narrow. OF-3 is preferred whilemeasuring large deformations given that the Young’s modulusis low and the measurement range is wide. However,OF-1 is favorable in terms of ratio k. In summary, each fiberhas its advantage as well as limitations for ground deformationmonitoring. Depending on laboratory or field applications,different optical fibers will be needed to obtain the bestmonitoring data.

V. CONCLUDING REMARKS

In this study, the fiber/soil interfacial behavior was evaluatedquantitatively. Laboratory pullout tests were conducted onthree types of optical fibers under various normal pressures.The experimental data were interpreted using a simplifiedinteraction model. Predictions from the model comparedsatisfactorily with the pullout test results. Potential applicationof this model may also be promising for other optical fibersand other environmental conditions.

The normal pressure has a significant impact on thefiber/soil interfacial behavior. Under high normal pressures,the interfacial bond is tightened, the deformation measurementrange is widened, and the soil dilation effect around the fiberis restrained. However, an intensifying progressive failure ofsoil-embedded optical fiber is not observed with the increaseof normal pressure.

The criterion for selecting a strain sensing fiber for grounddeformation measurement is discussed based on interfacialshear strengths, interfacial shear stiffnesses, ratio of peak toresidual interfacial shear strength, effective and partially effec-tive displacements, and Young’s modulus of fiber. An assess-ment of the three fibers indicate that a combination of differentoptical fibers may obtain the best measurement results.

It should be noted that this study does not consider theinfluence of lateral soil deformation on the fiber/soil inter-facial behavior and the above conclusions only applies tostraight optical fibers undergoing axial deformation. Furtherinvestigations will be conducted to capture the fiber/soilinteraction mechanism when the soil deformation is underthree-dimensional conditions.

ACKNOWLEDGMENT

The assistance provided by C.-S Tang, J.-F. Yan, F.-D. Wu,Y. You, Y.-C. Wei and T. Xu during the tests is gratefully

acknowledged. C.-C. Zhang would like to thank Y. Zhou forher continued help during this study.

REFERENCES

[1] E. Udd, “Early efforts to initiate the field of fiber optic smart structuresat McDonnell Douglas,” Proc. SPIE, vol. 3330, pp. 12–18, Jul. 1998.

[2] H.-N. Li, D.-S. Li, and G.-B. Song, “Recent applications of fiber opticsensors to health monitoring in civil engineering,” Eng. Struct., vol. 26,no. 11, pp. 1647–1657, Sep. 2004.

[3] B. Glisic and D. Inaudi, Fibre Optic Methods for Structural HealthMonitoring. New York, NY, USA: Wiley, 2007.

[4] C. K. Y. Leung et al., “Review: Optical fiber sensors for civil engineeringapplications,” Mater. Struct., pp. 1–36, Nov. 2013, doi: 10.1617/s11527-013-0201-7.

[5] Y.-T. Ho, A.-B. Huang, and J.-T. Lee, “Development of a fibre Bragggrating sensored ground movement monitoring system,” Meas. Sci.Technol., vol. 17, no. 7, pp. 1733–1740, Jul. 2006.

[6] M. Iten and A. M. Puzrin, “BOTDA road-embedded strain sensingsystem for landslide boundary localization,” Proc. SPIE, vol. 7293,pp. 729316-1–729316-12, Apr. 2009.

[7] M. Iten, A. M. Puzrin, and A. Schmid, “Landslide monitoringusing a road-embedded optical fiber sensor,” Proc. SPIE, vol. 6933,pp. 693315-1–693315-9, Apr. 2008.

[8] B.-J. Wang, K. Li, B. Shi, and G.-Q. Wei, “Test on application ofdistributed fiber optic sensing technique into soil slope monitoring,”Landslides, vol. 6, no. 1, pp. 61–68, Mar. 2009.

[9] H.-H. Zhu, A. N. L. Ho, J.-H. Yin, H. W. Sun, H.-F. Pei, and C.-Y. Hong,“An optical fibre monitoring system for evaluating the performance ofa soil nailed slope,” Smart Struct. Syst., vol. 9, no. 5, pp. 393–410,May 2012.

[10] A. Yashima, S. Tsuji, K. Yoshida, and Y. Yokota, “A new opticalfibre sensor to assess the stability of geogrid-reinforced soil walls,”Geosynthetics Int., vol. 16, no. 4, pp. 238–245, Aug. 2009.

[11] Y. Lu, B. Shi, G. Q. Wei, S. E. Chen, and D. Zhang, “Applica-tion of a distributed optical fiber sensing technique in monitoringthe stress of precast piles,” Smart Mater. Struct., vol. 21, no. 11,pp. 115011-1–115011-9, Nov. 2012.

[12] B. Shi et al., “A feasibility study on the application of fiber-optic distributed sensors for strain measurement in the Taiwan straittunnel project,” Marine Georesour. Geotechnol., vol. 21, nos. 3–4,pp. 333–343, 2003.

[13] H. Mohamad, K. Soga, P. J. Bennett, R. J. Mair, and C. S. Lim,“Monitoring twin tunnel interaction using distributed optical fiberstrain measurements,” J. Geotech. Geoenviron. Eng., vol. 138, no. 8,pp. 957–967, Aug. 2012.

[14] F. Wang, D. M. Zhang, H. H. Zhu, H. W. Huang, and J. H. Yin, “Impactof overhead excavation on an existing shield tunnel: Field monitoringand a full 3D finite element analysis,” CMC, Comput. Mater. Continua,vol. 34, no. 1, pp. 63–81, 2013.

[15] H.-H. Zhu, J.-H. Yin, L. Zhang, W. Jin, and J.-H. Dong, “Monitoringinternal displacements of a model dam using FBG sensing bars,” Adv.Struct. Eng., vol. 13, no. 2, pp. 249–261, Apr. 2010.

[16] R. J. Mair, “Developments in geotechnical engineering research: Appli-cation to tunnels and deep excavations,” Proc. Inst. Civil Eng., CivilEng., vol. 97, no. 1, pp. 27–41, Feb. 1993.


[17] D. J. White, W. A. Take, and M. D. Bolton, “Measuring soil deformationin geotechnical models using digital images and PIV analysis,” in Proc.10th Int. Conf. Comput. Methods Adv. Geomech., 2001, pp. 997–1002.

[18] W. Yuan et al., “Humidity insensitive TOPAS polymer fiber Bragggrating sensor,” Opt. Exp., vol. 19, no. 20, pp. 19731–19739, 2011.

[19] H.-H. Zhu, B. Shi, J. Zhang, J.-F. Yan, and C.-C. Zhang, “Distributedfiber optic monitoring and stability analysis of a model slope undersurcharge loading,” J. Mountain Sci., vol. 11, no. 4, pp. 979–989,Jul. 2014.

[20] H. Li, H. Sun, Y. Liu, X. Sun, and Y. Shang, “Application of opticalfiber sensing technology to slope model test,” Chin. J. Rock Mech. Eng.,vol. 27, no. 8, pp. 1703–1708, Aug. 2008.

[21] Q. Li, G. Li, and G. Wang, “Elasto-plastic bond mechanics ofembedded fiber optic sensors in concrete under uniaxial tension withstrain localization,” Smart Mater. Struct., vol. 12, no. 6, pp. 851–858,Dec. 2003.

[22] M. J. LeBlanc, “Study of interfacial interaction of an optical fibreembedded in a host material by in situ measurement of fibre enddisplacement—Part 2. Experiments,” Smart Mater. Struct., vol. 14, no. 4,pp. 647–657, Aug. 2005.

[23] Y. Ding, B. Shi, X. Bao, and J. Gao, “Jacket effect on strain measurementaccuracy for distributed strain sensors based on Brillouin scattering,”Opt. Appl., vol. 36, no. 1, pp. 57–67, 2006.

[24] K. T. Wan, C. K. Y. Leung, and N. G. Olson, “Investigation of the straintransfer for surface-attached optical fiber strain sensors,” Smart Mater.Struct., vol. 17, no. 3, pp. 035037-1–035037-12, Jun. 2008.

[25] M. Moradi and S. Sivoththaman, “Strain transfer analysis of surface-bonded MEMS strain sensors,” IEEE Sensors J., vol. 13, no. 2,pp. 637–643, Feb. 2013.

[26] K. T. Wan, “Quantitative sensitivity analysis of surface attached opticalfiber strain sensor,” IEEE Sensors J., vol. 14, no. 6, pp. 1805–1812,Jun. 2014.

[27] F. Ansari and Y. Libo, “Mechanics of bond and interface shear transferin optical fiber sensors,” J. Eng. Mech., vol. 124, no. 4, pp. 385–394,Apr. 1998.

[28] C. K. Y. Leung, X. Wang, and N. Olson, “Debonding and calibrationshift of optical fiber sensors in concrete,” J. Eng. Mech., vol. 126, no. 3,pp. 300–307, Mar. 2000.

[29] M. J. LeBlanc, “Study of interfacial interaction of an optical fibreembedded in a host material by in situ measurement of fibre enddisplacement—Part 1. Theory,” Smart Mater. Struct., vol. 14, no. 4,pp. 637–646, Aug. 2005.

[30] H.-N. Li, G.-D. Zhou, L. Ren, and D.-S. Li, “Strain transfer coeffi-cient analyses for embedded fiber Bragg grating sensors in differenthost materials,” J. Eng. Mech., vol. 135, no. 12, pp. 1343–1353,Dec. 2009.

[31] L. Yuan, L. Zhou, and J. Wu, “Investigation of a coated optical fiberstrain sensor embedded in a linear strain matrix material,” Opt. LasersEng., vol. 35, no. 4, pp. 251–260, Apr. 2001.

[32] C.-C. Zhang, H.-H. Zhu, B. Shi, and J.-K. She, “Interfacial characteri-zation of soil-embedded optical fiber for ground deformation measure-ment,” Smart Mater. Struct., vol. 23, no. 9, p. 095022, Sep. 2014.

[33] T. D. O’Rourke, S. J. Druschel, and A. N. Netravali, “Shear strengthcharacteristics of sand-polymer interfaces,” J. Geotech. Eng., vol. 116,no. 3, pp. 451–469, Mar. 1990.

[34] J. Han, “An experimental and analytical study of the behavior of fiber-reinforced polymer piles and pile-sand interactions,” Ph.D. dissertation,Dept. Civil Environ. Eng., Georgia Inst. Technol., Atlanta, GA, USA,1997.

[35] S. A. Tan, S. H. Chew, and W. K. Wong, “Sand–geotextile interfaceshear strength by torsional ring shear tests,” Geotextiles Geomembranes,vol. 16, no. 3, pp. 161–174, Jun. 1998.

[36] L.-J. Su, T. C. F. Chan, J.-H. Yin, Y. K. Shiu, and S. L. Chiu, “Influenceof overburden pressure on soil–nail pullout resistance in a compactedfill,” J. Geotech. Geoenviron. Eng., vol. 134, no. 9, pp. 1339–1347,Sep. 2008.

[37] C.-C. Zhang, Q. Xu, H.-H. Zhu, B. Shi, and J.-H. Yin, “Evaluations ofload-deformation behavior of soil nail using hyperbolic pullout model,”Geomech. Eng., vol. 6, no. 3, pp. 277–292, Mar. 2014.

[38] C.-S. Tang, B. Shi, and L.-Z. Zhao, “Interfacial shear strength of fiberreinforced soil,” Geotextiles Geomembranes, vol. 28, no. 1, pp. 54–62,Feb. 2010.

[39] H.-H. Zhu, C.-C. Zhang, C.-S. Tang, B. Shi, and B.-J. Wang, “Modelingthe pullout behavior of short fiber in reinforced soil,” GeotextilesGeomembranes, vol. 42, no. 4, pp. 329–338, Aug. 2014.

[40] J. D. Frost and J. Han, “Behavior of interfaces between fiber-reinforcedpolymers and sands,” J. Geotech. Geoenviron. Eng., vol. 125, no. 8,pp. 633–640, Aug. 1999.

[41] H.-H. Zhu, J.-H. Yin, A. T. Yeung, and W. Jin, “Field pullout testingand performance evaluation of GFRP soil nails,” J. Geotech. Geoenviron.Eng., vol. 137, no. 7, pp. 633–642, Jul. 2011.

[42] D. W. Taylor, Fundamentals of Soil Mechanics. New York, NY, USA:Wiley, 1948.

Cheng-Cheng Zhang is currently pursuing the mas-ter’s degree in geological engineering at NanjingUniversity, Nanjing, China. His research interestspans from basic soil behavior to reinforced soilstructures. He is also interested in the developmentand application of fiber optic sensors for geostruc-tural condition monitoring.

Hong-Hu Zhu received the Ph.D. degree in geotech-nical engineering from Hong Kong PolytechnicUniversity, Hong Kong, in 2009. He is currentlyan Associate Professor of Engineering Geologyand Geotechnics with Nanjing University, Nanjing,China. He joined the University of Cambridge,Cambridge, U.K., as a Visiting Scholar, in 2014.His areas of expertise include in the developmentand application of smart monitoring systems forgeostructures, field instrumentation and evaluation ofslope stability and related geohazards, and modeling

of time-dependent behavior of geomaterials.

Jun-Kuan She is currently pursuing the master’sdegree in geological engineering at NanjingUniversity, Nanjing, China. He is mainly involved inengineering geological research and the applicationof fiber optic sensors to geological monitoring.

Dan Zhang received the Ph.D. degree in geologicalengineering from Nanjing University, Nanjing,China, in 2004, where he is currently anAssociate Professor of Engineering Geology. Hisresearch interests include the development andapplication of novel sensing technologies forgeoengineering, in-situ testing, and monitoring andevaluation of geohazards.

Bin Shi received the Ph.D. degree in geologicalengineering from Nanjing University, Nanjing,China, in 1995, where he is currently a Professor ofEngineering Geology and the Head of the Depart-ment of Geological Engineering and InformationTechnology. He has conducted extensive studies ongeoengineering monitoring using Brillouin OpticalTime-Domain Reflectometry and Brillouin OpticalTime-Domain Analysis for over ten years and hasauthored over 80 publications in this field.

/ColorImageDict > /JPEG2000ColorACSImageDict > /JPEG2000ColorImageDict > /AntiAliasGrayImages false /CropGrayImages true /GrayImageMinResolution 150 /GrayImageMinResolutionPolicy /OK /DownsampleGrayImages true /GrayImageDownsampleType /Bicubic /GrayImageResolution 600 /GrayImageDepth -1 /GrayImageMinDownsampleDepth 2 /GrayImageDownsampleThreshold 1.50000 /EncodeGrayImages true /GrayImageFilter /DCTEncode /AutoFilterGrayImages false /GrayImageAutoFilterStrategy /JPEG /GrayACSImageDict > /GrayImageDict > /JPEG2000GrayACSImageDict > /JPEG2000GrayImageDict > /AntiAliasMonoImages false /CropMonoImages true /MonoImageMinResolution 400 /MonoImageMinResolutionPolicy /OK /DownsampleMonoImages true /MonoImageDownsampleType /Bicubic /MonoImageResolution 1200 /MonoImageDepth -1 /MonoImageDownsampleThreshold 1.50000 /EncodeMonoImages true /MonoImageFilter /CCITTFaxEncode /MonoImageDict > /AllowPSXObjects false /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

quantitative evaluation of optical fiber/soil interfacial behavior...

Documents