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applied sciences Article

Experiment and Numerical Study on Deformation Measurement of Cast-in-Place Concrete Large-Diameter Pipe Pile Using Optical Frequency Domain Reflectometer Technology Lei Gao 1,2,3, *, Yunhao Gong 1 , Hanlong Liu 4 , Baoquan Ji 1 , Yining Xuan 5 and Yuan Ma 6 1 2 3 4 5 6

*

Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China; [email protected] (Y.G.); [email protected] (B.J.) Key Laboratory of Hydraulic and Waterway Engineering of the Ministry of Education, Chongqing Jiaotong University, Chongqing 400074, China National Engineering Research Center for Inland Waterway Regulation, Chongqing Jiaotong University, Chongqing 400074, China School of Civil Engineering, Chongqing University, Chongqing 400045, China; [email protected] School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, China; [email protected] GTL Technology & Service Co., Limited, Hong Kong, China; [email protected] Correspondence: [email protected]; Tel.: +86-025-8378-7687

Received: 3 August 2018; Accepted: 22 August 2018; Published: 24 August 2018







Abstract: The Cast-in-place concrete large-diameter pipe (PCC) pile has been used as the foundation reinforcement and embankment in China due to its low cost and high bearing capacity. The deformation of PCC pile under different vertical loads is very important for the application of engineering. In order to study the deformation characteristics of PCC pile, a small-scale model test was carried out. The new distributed measuring technology, named Optical Frequency Domain Reflectometer (OFDR), was applied to measure the strain on the PCC pile. A single mode fiber (SMF) was used, and the methods of layout, packaging and protection of optical fiber are introduced in detail. The obtained data was dealt with by wavelet transform, and the strain curves were analyzed based on the experiments. The finite element (FE) analysis model was established by COMSOL Multiphysics, and the numerical results compared with the experiment results. It showed that the optical fiber sensor can measure the strain of PCC pile, and that the deformation of PCC pile can be successfully obtained by OFDR technology. The strain of the pile decreases with depth and increases with loading. The measured result agrees well with numerical simulation result. The potential application of OFDR technology to PCC pile in situ and PCC energy pile is discussed. Keywords: PCC pile; strain; optical fiber; Optical Frequency Domain Reflectometer (OFDR) technology; numerical simulation; wavelet transform

1. Introduction PCC pile is developed using a new technique combining the advantages of pre-stressed concrete (PC) pile, drilled pile, and steel pile for ground improvement of highway and railway engineering [1]. Compared with other rigid concrete pile such as square pile, the PCC pile is simple in conformation, fast to construct, less concrete and more cost-efficient [2]. It has been widely used for pile foundation in China, it has yielded great social and economic benefits. Liu H.L. and many other researchers have carried out a series of studies on PCC pile, including studying its bearing behavior under vertical and horizontal loads, its characteristics of negative friction, the squeezing soil effect and so on [3–8]. Appl. Sci. 2018, 8, 1450; doi:10.3390/app8091450

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2 of 12 of PCC pile are important for optimum design and construction. Now, the standard for measuring the strain of pile is based upon electronic gauges; this method has the under vertical and horizontal loads, its characteristics of negative friction, the squeezing soil effect disadvantages of difficult layout, low survival rate, poor stability, and low precision. Furthermore, and so on [3–8]. The deformation characteristics of PCC pile are important for optimum design and this method cannot measure the deformation of the pile accurately and comprehensively. In order to construction. Now, the standard for measuring the strain of pile is based upon electronic gauges; this obtain the of PCC pile, a layout, new measuring method should be used to monitor method has deformation the disadvantages of difficult low survival rate, poor stability, and low precision.the strain. Distributed Sensor (DOFS) beenpile responsible tremendous Furthermore, this Optical methodFiber cannot measure thetechnologies deformation have of the accuratelyforand advances in strain Thedeformation optical fiber both a sensor and a transmission channel along comprehensively. In measurement. order to obtain the of is PCC pile, a new measuring method should be to monitor thethe strain. theused entire length of fiber [9]. DOFS technologies have been developed for monitoring geotechnical Distributed Optical Fiber Sensor (DOFS)Domain technologies have been (OFDR), responsible for Bragg tremendous engineering, such as Optical Frequency Reflectometer Fiber Grating (FBG), advances in strain measurement. The optical fiber is both a sensor and a transmission channel along Analysis Brillouin Optical Time Domain Reflectometer (BOTDR), Brillouin Optical Time Domain the entire length of the fiber [9]. DOFS technologies have been developed for monitoring geotechnical (BOTDA), Brillouin Optical Frequency Domain Analysis (BOFDA), Brillouin Optical Correlation engineering, such as Optical Frequency Domain Reflectometer (OFDR), Fiber Bragg Grating (FBG), Domain Analysis (BOCDA), and so on. Compared with conventional sensors, the fiber optic sensor has Brillouin Optical Time Domain Reflectometer (BOTDR), Brillouin Optical Time Domain Analysis benefits including resistance to corrosion, high precision, and small size.Optical Among these technologies, (BOTDA), Brillouin Optical Frequency Domain Analysis (BOFDA), Brillouin Correlation the BOTDR, BOTDA, BOFDA, and technologies are fully distributed, FBG technology Domain Analysis (BOCDA), and so on.BOCDA Compared with conventional sensors, the fiberwhile optic sensor is quasi-distributed. The spatial resolutions are: OFDR (1 mm in the sensing range of 50–70 m), has benefits including resistance to corrosion, high precision, and small size. Among these BOTDR (1 mthe in the sensing rangeBOFDA, of 50 km), cm in the sensing of 2 kmwhile and 2 m in the technologies, BOTDR, BOTDA, andBOTDA BOCDA(2technologies are fullyrange distributed, FBG technology is 150 quasi-distributed. resolutions OFDR mm in the sensing sensing range of km), BOFDAThe (20 spatial cm in the sensingare: range of 3(1km), BOCDA (7 cmrange in the sensing of 50–70ofm), BOTDR m in the sensing range of 50 km), BOTDA (2 cm theBOTDA sensing range of 2BOFDA km range 5 km). The(1strain precisions are OFDR (1 µε), BOTDR (40inµε), (20 µε), (2 µε), and 2 m in the sensing range of 150 km), BOFDA (20 cm in the sensing range of 3 km), BOCDA (7 cm BOCDA (10 µε), FBG (1 µε). The OFDR is a type of DOFS technologies which has shorter measuring in the sensing range of 5 km). The strain precisions are OFDR (1 με), BOTDR (40 με), BOTDA (20 με), time, higher sensitivity, and higher spatial resolution than other frequently-used DOFS technologies BOFDA (2 με), BOCDA (10 με), FBG (1 με). The OFDR is a type of DOFS technologies which has like BOTDR, BOTDA, BOFDA, BOCDA, and so on [10–23]. shorter measuring time, higher sensitivity, and higher spatial resolution than other frequently-used technology was developed by Eickhoff in 1981 basic principle of OFDR is shown DOFSOFDR technologies like BOTDR, BOTDA, BOFDA, BOCDA, and [24]. so onThe [10–23]. in Figure 1. The continuous light emitted by a tunable light source is divided intoistwo parts: one is OFDR technology was developed by Eickhoff in 1981 [24]. The basic principle of OFDR shown sent out into the test fiber,light producing backscattering light, which through in Figure 1. The continuous emittedRayleigh by a tunable light source is divided intopasses two parts: one isthe coupler as signal light, thefiber, lightproducing is coupled in the backscattering photodetector; thewhich otherpasses is reflected back to the coupler as sent out into the test Rayleigh light, through the coupler as signal light,light, the light coupled in thelight photodetector; thein other reflected back toInthe coupler as a analyzer, a reference andisthe reference is coupled the is photodetector. the spectrum reference light, and the reference light is coupled in the photodetector. In the spectrum analyzer, the mixed signal of reference light and backward Rayleigh scattering light from the photothe detector are mixed signalThe of reference light and backward RayleighRayleigh scatteringscattering light from light the photo detectorposition are measured. optical frequency of the backward at different of the measured. The optical frequency of the backward Rayleigh scattering light at different position of the optical fiber is different because the laser light source of OFDR technology is tunable to that light source. optical fiber is different because the laser light source of OFDR technology is tunable to that light The position information of the optical fiber can be obtained by detecting the light frequency, while the source. The position information of the optical fiber can be obtained by detecting the light frequency, strain/temperature information of theofoptical fiber fiber can be by comparing thethe spectrum of while the strain/temperature information the optical canachieved be achieved by comparing referenceofand signaland lights respectively. spectrum reference signal lights respectively.

Figure 1. Basical principle of OFDR technology [24].

Figure 1. Basical principle of OFDR technology [24].

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The optical backscatter reflectometry (OBR) instrument is based on OFDR technology, manufactured by LUNA company. It has successfully been applied in different areas for strain and temperature measurement. Henault et al. [25] used OBR to monitor the mechanical behavior of a representative-scale reinforced concrete structural element. It was found that the strain measured by OBR was consistent with both the experimental data obtained from conventional vibrating wire gauges and the theoretical predicted strain. Nguyen et al. [26] measured the transient temperature inside a single channel of a wash-coated monolith reactor using OBR. The capability of OFDR technology in temperature monitoring has been tested successfully. Villalba et al. [27] bonded optical fibers to the surface of a concrete slab. The OBR was then used to measure the strain of the concrete under load. The result verified the potential of OFDR in Structural Health Monitoring (SHM). Grave et al. [28] embedded a distributed fiber-optic sensing sensor within a composite joint, and used OBR to monitor the strain. The monitoring results agreed well with the results of Finite Element Analysis. Sierra-Pérez et al. [29] used FBG, OBR, and strain gauges to measure the blades of a 13.5 m wind turbine blade prototype. The results were compared, and showed that OBR could define sensors every 10 µm. The resolution and accuracy of OBR presented a strong inverse dependency of the defined sensors. Andrea et al. [30] monitored crack initiation and propagation in the glue line by FBG and OBR respectively. The experimental results showed that the great advantage of the OBR was that the entire length of fiber could be used for sensing strain, thus providing better spatial resolution compared to conventional FBG arrays. The feasibility of OFDR in aeronautical construction and structural engineering has been verified. However, the OFDR in strain and temperature measurement of geotechnical engineering has not been widely reported on to date. The deformation of concrete pile is very important in the refinement of geotechnical engineering. Compared with aeronautical materials such as steel, aluminum, and so on, concrete material has rough surface; as such, the bonding of optical fiber is difficult. Its roughness makes it hard to maintain fastness. When compared with concrete structures such as slab, girder, and so on, the pile is buried in soil where the optical fiber needs protection due to the friction and pressure from the surrounding soil. The application of OFDR to concrete pile is worthy of further study. The deformation of pile is important for foundation. In order to measure the deformation of PCC pile, a small-scale model test of a single PCC pile was carried out. OFDR technology was used to measure the strain of PCC pile. The method and process of laying the optical fiber, applying vertical loads, dealing with data, etc. are introduced in detail. The strain curves were analyzed according to the experimental situation, a numerical model was established by COMSOL Multiphysics, and the experimental results and numerical results are compared. Finally, the potential for the application of OFDR to PCC pile and PCC energy pile in situ was discussed. 2. Small-Scale Model Test 2.1. Model Test Equipment The small-scale model test equipment was composed of a fairly rigid model container, a loading device, and a measuring device. The small-scale model test equipment is shown in Figure 2. The internal size of the model container was 265 × 200 × 230 cm (104.331 × 78.740 × 90.551 inch) (length × width × height). As shown in Figure 3a, there are two main beams and two secondary beams, which are both made of special steel. They are fixed over the test tank by four I-shaped pillars. The beams were embedded partly into the test tank when the concrete was poured. The length of main beams is 235 cm, and they are 150 cm (59.055 inch) apart. The length of secondary beams is 150 cm (59.055 inch), and they are 100 cm (39.370 inch) apart. The loading device (see Figure 3b) mainly consists of the pressure equipment, the mechanics column, and the base plate. The pressure equipment was a mechanical jack with a height of 300 mm, the piston had a diameter of 80 mm, a maximum stroke of 200 mm, and a maximum loading of 250 kN. The pressure applied to the PCC pile is shown on the screen of a digital

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Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 12 display through the pressure sensor. The measuring device comprises the single mode optical fiber, Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 12 LUNA OBR4600 (see Figure 3c), a computer, and the software. This model test is used for single mode optical fiber, LUNA OBR4600 (see Figure 3c), a computer, and the software. This model test is PCC pile experiment. modefor optical OBR4600 (see Figure 3c), a computer, and the software. This model test is used singlefiber, PCCLUNA pile experiment.

used for single PCC pile experiment.

Figure 2. Schematic diagram of the experiment.

Figure 2. Schematic diagram of the experiment. Figure 2. Schematic diagram of the experiment.

(a) (a)

(b) (b) Figure 3. Cont.

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(c)

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Figure 3. Photos experiment. (a) tank; (b) device; (c) (c) OBR 4600. 3. Photos of of thethe experiment. Test(b) tank; (b)Loading Loading device; OBR 4600. FigureFigure 3. Photos of the experiment. (a) Test(a) tank; Loading device; (c) OBR 4600.

2.2. Model Pile and Soil

2.2. Model Pile and Soil 2.2. Model Pile and Soil

The PCC model pile is shown in Figure 4. The pile was cast with C20 concrete without a

The model PCC model is shown in Figure Thewas pilecast waswith cast C20 withconcrete C20 concrete without a The PCC pile ispile shown in Figure 4. The4. pile without a reinforcement cage in a mould made of large-diameter PVC pipes. Its size is: length (L) 1.5 m (59.055 reinforcement cage in a mould made of large-diameter PVC pipes. Its size is: length (L) 1.5 m reinforcement cage in a mould made of large-diameter PVC pipes. Its size is: length (L) 1.5 m (59.055 inch), inner diameter (d) 0.2 m (7.874 inch), and the outer diameter (d) 0.315 m (12.402 inch). inch), inner diameter (d)inch), 0.2 mand (7.874 the outer diameter (d) 0.315 m (12.402 inch). inch), (59.055 inner diameter (d) 0.2 m (7.874 theinch), outerand diameter (d) 0.315 m (12.402 inch).

Figure 4. Burying the PCC pile.

When the burying of the model pile was finished, the head of the pile is 0.1 m above ground. In order to ensure that the pileFigure is able 4. toBurying bear thethe load uniformly and avoid stress concentration on the PCC Figure 4. Burying thepile. PCC pile. head under vertical load, a cubical base plate cast with C30 concrete was used under the jack. To the foundation, two kinds of soil were filled into the model container. The top soil layer was Whensimulate the burying of the model pile was finished, the head of the pile is 0.1 m above ground. In clay of 0.6 (23.622 inch) depth, andpile the lower layer was sand of 1.6of m the (62.992 depth. Both clay When themburying of the model was finished, the head pileinch) is 0.1 m above ground. order to ensure that the pile is able to bear the load uniformly and avoid stress concentration on the and to sand werethat taken District of the Nanjing, The and soil was placed the model on In order ensure thefrom pileHexi is able to bear load China. uniformly avoid stressinto concentration head undercontainer verticalartificially load, a cubical base plate cast with C30 concrete was used underand the jack. To compacted once each 0.2 m (7.874 inch). basic physical the head under verticaland load, a cubical base plate cast with C30The concrete was used mechanical under the jack. simulate the foundation, two kinds of filled into the model container. The top soil layer parameters the soil andtwo pilesoil arewere shown in were Table 1. The uniaxial compressive strength thewas pilelayer To simulate the of foundation, kinds of soil filled into the model container. Theoftop soil clay of 0.6 m (23.622 inch) depth, and the lower layer was sand of 1.6 m (62.992 inch) depth. Both was 2.8 MPa, and the biaxial compressive strength of pile was 2.36 MPa. These parameters clay were was clay of 0.6 m (23.622 inch) depth, and the lower layer was sand of 1.6 m (62.992 inch) depth. and sand were taken from Hexitests District of Nanjing, China. The soil wasPeople’s placed Republic into theofmodel obtained by laboratory and from the National standard of the China Both clay and sand were taken from Hexi District of Nanjing, China. The soil was placed into the (GB50021-2001) Code for the once Investigation Engineering. container artificially and compacted each 0.2ofmGeotechnical (7.874 inch). The basic physical and mechanical

model container artificially and compacted once each 0.2 m (7.874 inch). The basic physical and parameters of the soil and pile are shown in Table 1. The uniaxial compressive strength of the pile mechanical parameters of the soil and pile are shown in Table 1. The uniaxial compressive strength of was 2.8 MPa, and the biaxial compressive strength of pile was 2.36 MPa. These parameters were the pile was 2.8 MPa, and the biaxial compressive strength of pile was 2.36 MPa. These parameters obtained by laboratory tests and from the National standard of the People’s Republic of China were obtained by laboratory tests and from the National standard of the People’s Republic of China (GB50021-2001) Code for the Investigation of Geotechnical Engineering. (GB50021-2001) Code for the Investigation of Geotechnical Engineering.

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Table 1. Physical and mechanical parameters of soil and pile.

Material Material

Pile Pile Clay Clay Sand Sand

and mechanical parameters of soil and pile. Density,Table ρ 1. Physical Modulus, Poisson Cohesion, (kg/m3) E/(MPa) Ratio, ν C/(kPa) Poisson Ratio, ν Cohesion, C/(kPa) Density, ρ (kg/m3 ) Modulus, E/(MPa) 2400 18,000 0.33 18,000 2400 0.33 1930 15 0.3 28.6 15 1930 0.3 28.6 1700 68 0.3 68 1700 0.3 00

Friction Angle, ψ/(°) Friction Angle, ψ/(◦ ) 23.1 23.1 27.9 27.9

2.3. 2.3. Layout Layout of of Optical Optical Fiber Fiber Sensor Sensor The layoutand andprotection protection of the optical is important data acquisition and The layout of the optical fiber fiber sensorsensor is important for datafor acquisition and reliability. reliability. The surface of the concrete is rough; therefore, the optical fiber cannot be adhered to The surface of the concrete is rough; therefore, the optical fiber cannot be adhered to directly and directly and tightly. Duringof the processand of burying and loading of pile, the suffers optical the fiber suffersfrom the tightly. During the process burying loading of pile, the optical fiber friction friction from surrounding soil, which may cause it to break off. A feasible method of material surrounding soil, which may cause it to break off. A feasible method of material packaging and packaging and developed protectionafter was several developed after(see several attempts 5). introduced The main steps are protection was attempts Figure 5). The (see mainFigure steps are in detail, introduced as follows. in detail, as follows.

(a)

(b)

Figure fiberbonded bondedtotothe thepile. pile. Fiber cross section; (b) Fiber thebody. pile Figure 5. 5. Optical Optical fiber (a)(a) Fiber onon thethe pilepile cross section; (b) Fiber alongalong the pile body.

A line was drawn along the axis of the pile, and the notch was cut along the line at a depth of A line was drawn along the axis of the pile, and the notch was cut along the line at a depth of 2~3 mm. The notch protected the optical fiber from breaking off due to friction and collisions during 2~3 mm. The notch protected the optical fiber from breaking off due to friction and collisions during the process of backfilling soil and loading. The notch was cleaned with a brush and blower, because the process of backfilling soil and loading. The notch was cleaned with a brush and blower, because dust and particles in the notch might cause damage. After cleaning, the optical fiber was laid and dust and particles in the notch might cause damage. After cleaning, the optical fiber was laid and gently stretched; it was fixed in the notch at fixing several points with a quick drying adhesive, similar gently stretched; it was fixed in the notch at fixing several points with a quick drying adhesive, similar to glue 502. A thin layer of epoxy resin was smeared into the notch to make the surface smooth; to glue 502. A thin layer of epoxy resin was smeared into the notch to make the surface smooth; complete solidification of the epoxy resin takes 24 h. The surface of the epoxy resin was ground to complete solidification of the epoxy resin takes 24 h. The surface of the epoxy resin was ground to reduce the friction between the resin and soil during the process of loading. The optical fiber left reduce the friction between the resin and soil during the process of loading. The optical fiber left exposed was protected by a special protective sheath of 0.9 mm in diameter. The optical fiber was laid exposed was protected by a special protective sheath of 0.9 mm in diameter. The optical fiber was along the pile in a U shape. The advantage of this was that data can be obtained from both ends of the laid along the pile in a U shape. The advantage of this was that data can be obtained from both ends optical fiber. In case one end is damaged during the experiment, the data can be obtained from the of the optical fiber. In case one end is damaged during the experiment, the data can be obtained from other; the symmetrical layout of optical fiber allowed us to check whether the load is applied with the other; the symmetrical layout of optical fiber allowed us to check whether the load is applied with eccentricity or not. eccentricity or not. 2.4. Data Acquisition 2.4. Data Acquisition Before data acquisition, the fiber had to be welded to the fiber connector cable. It was necessary Before data acquisition, the fiber had to be welded to the fiber connector cable. It was necessary to remove the sheath of the optical fiber and clean the core of the fiber. To keep the stability of the light to remove the sheath of the optical fiber and clean the core of the fiber. To keep the stability of the signal in the measurement, as far as possible, the light loss during connection must be no more than light signal in the measurement, as far as possible, the light loss during connection must be no more 2 dB. A thermal expansion pipe was used to reinforce the connection and the light path was checked. than 2 dB. A thermal expansion pipe was used to reinforce the connection and the light path was The calibration constants of optical fiber were set in the data acquisition software. The initial data checked. The calibration constants of optical fiber were set in the data acquisition software. The initial was obtained before loading. When the loading was applied, the strain data were acquired. The data data was obtained before loading. When the loading was applied, the strain data were acquired. The acquisition of OBR is very convenient.; the required time is in the order of a few seconds. data acquisition of OBR is very convenient.; the required time is in the order of a few seconds.

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3. Numerical Simulation Simulation 3. Numerical In order to study study the the deformation deformationof ofthe themodel modelpile pileunder undervertical verticalloads, loads,a numerical a numerical model model is is established by COMSOL Multiphysics software. The software realizes the simulation of real established by COMSOL Multiphysics software. The software realizes the simulation of real physical physical phenomena by partial solvingdifferential partial differential equations or differential partial differential equations based phenomena by solving equations or partial equations based on the on theelement finite element Multiphysics is widely used in scientific and finite method.method. COMSOLCOMSOL Multiphysics is widely used in scientific research andresearch engineering engineering to itscomputational efficient computational performance and outstanding multi-field calculations calculations owing to itsowing efficient performance and outstanding multi-field directdirect-coupling capabilities [31,32]. Compared other FE COMSOL coupling analysisanalysis capabilities [31,32]. Compared with other with FE software, thesoftware, COMSOLthe Multiphysics Multiphysics software superior coupling in multi-field coupling calculation. model test prepared software is superior in is multi-field calculation. This model testThis is prepared for is single PCC for single PCC pile and PCC energy pile experiments. Considering the advantages of the pile and energy pile experiments. Considering the advantages of the software, software, and the and the characteristics of the experiment, the COMSOL Multiphysics software wastoused to conduct an characteristics of the experiment, the COMSOL Multiphysics software was used conduct an equalequal-sized numerical model of soil and pile (see Figure 6). sized numerical model of soil and pile (see Figure 6).

Figure 6. 6. Numerical of soil soil and and pile. pile. Figure Numerical model model of

In this simulation, soil and pile are both regarded as incompressible materials. The soil is an In this simulation, soil and pile are both regarded as incompressible materials. The soil is an elastoplastic material, and the constitutive model uses the Drucker-Prager (D-P) model. The pile is a elastoplastic material, and the constitutive model uses the Drucker-Prager (D-P) model. The pile is a homogeneous isotropic linear elastomer, and subject to the William-Warnke (W-W) criterion. The homogeneous isotropic linear elastomer, and subject to the William-Warnke (W-W) criterion. The pile pile body and the soil body were continuously coordinated and deformed. Creep was not considered. body and the soil body were continuously coordinated and deformed. Creep was not considered. The physical and mechanical parameters used in the numerical simulation are shown in Table 1. The physical and mechanical parameters used in the numerical simulation are shown in Table 1. The meshes of the model include two parts with soil meshes and pile meshes, which are all The meshes of the model include two parts with soil meshes and pile meshes, which are all tetrahedral meshes, the meshes arranged by free subdivision. The maximum size of a mesh unit is tetrahedral meshes, the meshes arranged by free subdivision. The maximum size of a mesh unit is 0.367 m, and the minimum is 0.0661 m; the maximum growth rate is 1.5. The curvature factor is 0.6 0.367 m, and the minimum is 0.0661 m; the maximum growth rate is 1.5. The curvature factor is 0.6 and the resolution of the narrow region is 0.5. When calculating, the number of iteration meshes is 4, and the resolution of the narrow region is 0.5. When calculating, the number of iteration meshes is 4, and the maximum depth of the unit is 4 m. There were 6 areas, 33 boundaries, 65 sides, and 42 vertices and the maximum depth of the unit is 4 m. There were 6 areas, 33 boundaries, 65 sides, and 42 vertices in the numerical calculations, and the freedom degree of the solution was 199,401. in the numerical calculations, and the freedom degree of the solution was 199,401. In the stress field, the lower boundary of the sand was under fixed constraint, i.e., without In the stress field, the lower boundary of the sand was under fixed constraint, i.e., without displacement in three directions. The boundaries around the two layers of soil were all under displacement in three directions. The boundaries around the two layers of soil were all under horizontal horizontal constraints (roller bearing). The displacement only occurs in the direction of the Z axis, constraints (roller bearing). The displacement only occurs in the direction of the Z axis, which is along which is along the length of the pile. The top of the clay layer had a free boundary in which was the length of the pile. The top of the clay layer had a free boundary in which was contact with the contact with the surface of pile. The load was applied to the head of pile. surface of pile. The load was applied to the head of pile. 4. Results 4. Results The axial axial strain strain of of PCC PCC model modelpile pileat atdifferent differentdepths depthsunder underaavertical verticalload loadwas wasmeasured measuredusing using The a a OBR4600 instrument. The strain curves were obtained using OFDR technology. With the obtained OBR4600 instrument. The strain curves were obtained using OFDR technology. With the obtained data, the data, thedeformation deformation characteristics characteristics of of the the PCC PCC pile pile could could be be analyzed. analyzed. The The strain strain data data measured measured by by the U-shape part of the optical fiber was affected by optical loss. The effect of optical loss should get rid of from the measure data. Additionally, traditional electronic strain gauges were bonded to the

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was affected by optical loss. The effect of optical loss should 8 ofget 12 rid of from the measure data. Additionally, traditional electronic strain gauges were bonded to the surface of model pile near the optical fiber cable, as shown in Figure 4; however, the strain gauges broke. This proves the high survival rate of the optical fiber sensor. sensor. Figure 7 shows the strain curves of the left and right right sides sides of of the the model model pile pile at at 16 16 kN kN and and 24 24 kN. kN. The measurement results from the optical fiber by OBR can be analyzed. The strain of the model pile at depths from 0.1 m to 1.3 m was measured without any breaks. This illuminates the effectiveness of the layout, packaging, and protection of the optical fiber. The strain data of the model pile at depths from 0.0 m to 0.1 0.1 m m was was not not recorded, recorded, because because of the the influence influence of the the outer outer environment. environment. Every Every datum is 1 με, µε, it shows the advantage of OFDR in measuring strain along the optical fiber with high spatial resolution and accuracy. The strain curves of the left side of the pile at 16 kN and 24 kN have similar variation trends. The The strain strain gets gets smaller smaller from from the the top to the bottom of the pile, which corresponds with the deformation characteristic of the pile at a vertical load in general. These results confirm the stability and accuracy of OFDR technology indirectly. indirectly. The strain load, which should be the same in an situation, had strain of ofboth bothsides sideson onthe thesame same load, which should be the same inideal an ideal situation, obvious differences. This may have been caused by the small eccentricity of the load. There are many had obvious differences. This may have been caused by the small eccentricity of the load. There are fluctuations on curves, which are bigger those on those the strain curves of metal composite; many fluctuations on curves, which arethan bigger than on the strain curvesand of the metal and the this was because concrete a rougher surface surface than that theofmetal or the In composite; this wasthe because the had concrete had a rougher thanofthat the metal or composite. the composite. addition, the pile was subjected to the friction from surrounding soil during loading. The fluctuations In addition, the pile was subjected to the friction from surrounding soil during loading. The fluctuations of different in position andand range, which is dueistodue the local heterogeneity of the concrete of both bothsides sidesare are different in position range, which to the local heterogeneity of the material. concrete material. Strain (με)

Strain (με) 5

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0.6 0.8

16kN (L) 16kN (R)

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Position of pile (m)

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0.6 0.8

24kN (L) 24kN (R)

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(a)

(b)

Figure7. of two two sides sidesofofmodel modelpile. pile.(a)(a)Strain Strain curves Strain curves 24 Figure 7. Strain Strain curves curves of curves at at 16 16 kN;kN; (b) (b) Strain curves at 24atkN. kN.

Position of pile (m)

Figure 88 shows shows the the average average strain 16 kN kN and and 24 24 kN. kN. The The strain strain of of the the pile pile Figure strain curves curves of of model model pile pile at at 16 gets larger larger with with load load increasing. increasing. The The two two curves curves have have the the same same variation variation trend. trend. There There is is an an abnormal abnormal gets fluctuation below the position of 1.05 m, which gets larger with load increasing; this is because the fluctuation below the position of 1.05 m, which gets larger with load increasing; this is because the model pile pile was was given given the the small model small load load twice twice for for testing testing the the feasibility feasibility of of loading loading and and the the monitoring monitoring devices. The loading caused movement of the model pile and disturbance of the surrounding devices. The loading caused movement of the model pile and disturbance of the surrounding soil. soil. Due to the properties of the equipment, the features of the signal, and effects of the environment, Strain (με) is a barrier for analyzing the deformation the obtained data contained much signal noise; this 0 5 10 15 20 25 30 0.0 characteristics. There are many methods to deal with the optical fiber data with a noisy signal, e.g., wavelet transform. The wavelet transform is a signal analysis method of time–frequencies. It has 0.2 the characteristics of a multi-resolution analysis (MRA), and the ability to describe the local features 0.4 of a signal both in time and the frequency domain. Therefore, this method is suitable for resolving 0.6 the data obtained by OBR to eliminate the signal noise. Figure 9 shows the process of using wavelet transform to deal with the amplitude data obtained by OBR on 3 levels. From Figure 9, the signal noise 0.8 reduces notably, i.e., from level1.01 to 3. 1.2

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Figure 8. Average strain curves of model pile.

Figure 8 shows the average strain curves of model pile at 16 kN and 24 kN. The strain of the pile gets larger with load increasing. The two curves have the same variation trend. There is an abnormal fluctuation below the position of 1.05 m, which gets larger with load increasing; this is because the model given the small load twice for testing the feasibility of loading and the monitoring Appl. Sci.pile 2018,was 8, 1450 9 of 13 devices. The loading caused movement of the model pile and disturbance of the surrounding soil. Strain (με) 0.0

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Due to the properties of the0.4equipment, the features of the signal, and effects of the environment, the obtained data contained much signal noise; this is a barrier for analyzing the deformation 0.6 characteristics. There are many methods to deal with the optical fiber data with a noisy signal, e.g., 0.8 wavelet transform. The wavelet transform is a signal analysis method of time–frequencies. It has the 1.0 characteristics of a multi-resolution analysis (MRA), and the ability to describe the local features of a 16kN 24kN signal both in time and the frequency domain. Therefore, this method is suitable for resolving the 1.2 data obtained by OBR to eliminate the signal noise. Figure 9 shows the process of using wavelet 1.4 transform to deal with the amplitude data obtained by OBR on 3 levels. From Figure 9, the signal Figure 8. 1Average curves of model pile. noise reduces notably, i.e., from level to 3. strain Figure 8. Average strain curves of model pile. Strain(με)

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Figure the strain data. (a) (a) Initial data; (b) (b) Level 1; (c) Figure 9. 9. Process Processusing usingwavelet wavelettransform transformtotodeal dealwith with the strain data. Initial data; Level 1; Level 2; (d) Level 3. (c) Level 2; (d) Level 3.

The spatial resolution of OFDR technology is high. The data passing through the wavelet The spatial resolution of OFDR technology is high. The data passing through the wavelet transform is very fluctuant, especially for material with tough surfaces; as such, it was difficult to transform is very fluctuant, especially for material with tough surfaces; as such, it was difficult to find a variation trend. The polynomial fitted method is a good way to match the curve with many find a variation trend. The polynomial fitted method is a good way to match the curve with many data. The strain curves are smoothed out with a wavelet transform, and matched with a polynomial. data. The strain curves are smoothed out with a wavelet transform, and matched with a polynomial. The test results are shown in Figure 10, with a comparison of the Finite Element (FE) analysis results. The test results are shown in Figure 10, with a comparison of the Finite Element (FE) analysis results. From Figure 10, the FE curves of 16 kN and 24 kN have the same trend where the strain gets From Figure 10, the FE curves of 16 kN and 24 kN have the same trend where the strain gets smaller from the top to the bottom of the pile. These trends are consistent with the test curves of 16 smaller from the top to the bottom of the pile. These trends are consistent with the test curves of 16 kN kN and 24 kN. The matched curves of the test results agree well with those of FE analysis. The average and 24 kN. The matched curves of the test results agree well with those of FE analysis. The average difference between them is less than 20%. This verifies the parameter accuracy of the numerical model. difference between them is less than 20%. This verifies the parameter accuracy of the numerical model. The measurement of OBR is very sensitive; a slight external disturbance can affect the measurement result significantly. Movement of the device during the measurement process may disturb the data. The effect of temperature was not considered in this short time measurement, i.e., only two hours, but the temperature was still the cause of poor correlations, especially in the upper part. The results of the test are smaller than those of the FE analysis in most parts of the pile, particularly in the upper part. Because the load was applied using a jack, and little settlement of pile

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The measurement of OBR is very sensitive; a slight external disturbance can affect the measurement result significantly. Movement of the device during the measurement process may disturb the data. The effect of temperature was not considered in this short time measurement, i.e., only two hours, but the temperature was still the cause of poor correlations, especially in the upper part. The test areREVIEW smaller than those of the FE analysis in most parts of the pile, particularly in Appl. results Sci. 2018,of 8, xthe FOR PEER 10 of 12 the upper part. Because the load was applied using a jack, and little settlement of pile could occur duringoccur the process loading, theloading top of the wasof smaller than thesmaller designed value. could duringofthe processthe ofloading loading,onthe on pile the top the pile was than the The difference resultsbetween of the test and FE gradually smaller from top tofrom the bottom designed value.between The difference results of the test andgets FE gradually getsthe smaller the top of the the bottom pile at the same the loading smaller whensmaller transferring along the pile, duethe to to of the pileload, at the same load,becomes the loading becomes when transferring along the friction surrounding soil. The test soil. results of the analysis in the pile, due to from the friction from surrounding Thewere test bigger resultsthan werethose bigger thanFEthose of the FE lower parts of the pileparts at 24 of kN. This bekN. dueThis to the abnormal of the lower parts of of the the analysis in the lower the pilemay at 24 may be due deformation to the abnormal deformation pile, which caused by loading. strain the pile The increases load increasing. addition, lower partswere of the pile, which wereThe caused byofloading. strainwith of the pile increasesInwith load the variations strain curves under different loads are inunder consistent. Thisloads shows that PCC pileThis is a increasing. In of addition, the variations of strain curves different are in the consistent. kind ofthat friction pile, pile it illuminates the effectiveness of wavelet transform and polynomial fitting in the shows the PCC is a kind of friction pile, it illuminates the effectiveness of wavelet transform process of obtaining data using OFDRoftechnology. and polynomial fitting in the process obtaining data using OFDR technology. Strain(με) 0.0

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Figure Figure 10. 10. Comparison Comparison between between the the results results of of the the test test and and FE. FE.

5. Discussion 5. Discussion When the OFDR technology is used to measure the strain of PCC pile, the optical fiber length When the OFDR technology is used to measure the strain of PCC pile, the optical fiber length and and spatial resolution need to be carefully considered. The strain of the optical fiber is calculated as spatial resolution need to be carefully considered. The strain of the optical fiber is calculated as an an average change of the reflective spectrum along the fiber. Regarding heterogeneous and tough average change of the reflective spectrum along the fiber. Regarding heterogeneous and tough concrete concrete materials, the length and spacing of the optical fiber were chosen during post processing; as materials, the length and spacing of the optical fiber were chosen during post processing; as such, such, many peaks appear intensively on the strain curves, which make it difficult to show the many peaks appear intensively on the strain curves, which make it difficult to show the variation variation trend. The spatial resolution and strain resolution of the OBR 4600 are 1 cm and trend. The spatial resolution and strain resolution of the OBR 4600 are 1 cm and 1µεrespectively; 1μεrespectively; this satisfies the requirement of strain measurement for engineering construction. It this satisfies the requirement of strain measurement for engineering construction. It can detect the can detect the strain with distributed measurement along the entire optical fiber. It is an ideal method strain with distributed measurement along the entire optical fiber. It is an ideal method to study the to study the mechanical behavior and engineering properties of PCC pile using the optical fiber mechanical behavior and engineering properties of PCC pile using the optical fiber sensor. Real-time sensor. Real-time measurement can be carried out. Once the load is applied, the curve of strain measurement can be carried out. Once the load is applied, the curve of strain changes immediately. changes immediately. Thedeformation characteristics of PCC pile can be obtained. Thedeformation characteristics of PCC pile can be obtained. In engineering construction, the PCC pile is casted in place by vibrating and pulling out of the In engineering construction, the PCC pile is casted in place by vibrating and pulling out of the mould, with a maximum length of 25 m, diameter of 1.0–1.5 m, and tube thickness of 0.12–0.2 m. mould, with a maximum length of 25 m, diameter of 1.0–1.5 m, and tube thickness of 0.12–0.2 m. When OFDR technology is used, the optical fiber can be bonded to the reinforced rebar on strain When OFDR technology is used, the optical fiber can be bonded to the reinforced rebar on strain measurement, through the test demonstrated by Henault [25]. This provides a new method for strain measurement, through the test demonstrated by Henault [25]. This provides a new method for measurement and integrity evaluation of PCC pile in situ. The optical fiber used is a commerciallystrain measurement and integrity evaluation of PCC pile in situ. The optical fiber used is a available product manufactured for the telecommunications industry, and without any specific commercially-available product manufactured for the telecommunications industry, and without protective layer. With the development of optical fiber technology, the price of equipment and any specific protective layer. With the development of optical fiber technology, the price of equipment software for OFDR will get lower and lower, which is beneficial for the wide application of OFDR technology in monitoring the strain of pile. OFDR technology is very sensitive to the outer environment; the special protection should be used for application in the engineering field. Since Rayleigh effects are both temperature and strain sensitive, the PCC energy pile was developed based on the PCC pile. It can make full use of inner space by installing equipment for heat exchange inside the PCC pile. The PCC energy pile provides support for the upper structure, and

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and software for OFDR will get lower and lower, which is beneficial for the wide application of OFDR technology in monitoring the strain of pile. OFDR technology is very sensitive to the outer environment; the special protection should be used for application in the engineering field. Since Rayleigh effects are both temperature and strain sensitive, the PCC energy pile was developed based on the PCC pile. It can make full use of inner space by installing equipment for heat exchange inside the PCC pile. The PCC energy pile provides support for the upper structure, and keeps a stable temperature for the structure. The measurement for temperature and strain of the PCC energy pile is difficult, because it involves the multi-field coupling problem, including the temperature field, strain field, seepage field, and so on. Considering the advantages of OFDR technology, the bearing mechanism and engineering characteristics of PCC pile and PCC energy pile can be studied using OFDR technology. 6. Conclusions OFDR technology was used to measure the strain of PCC pile. A model test for a PCC pile was carried out. The numerical model was established using COMSOL, and the following conclusions were obtained. (1) Based on the model test, the method of layout, packaging, and protection of the optical fiber bonded to the surface of the PCC pile were developed and introduced in detail. The strain of the PCC pile was successfully monitored by OFDR technology. The strain curves of the PCC pile at different vertical loads have the same variation trends, and agree well with the results of finite element calculation. (2) The OFDR has higher spatial resolution and measuring accuracy, both of which can reflect the deformation characteristics of pile. The weak section and abnormal position of the pile can easily be detected. The strain gets smaller from the top to bottom of pile, and increases with loading. These show the superiority of OFDR in the strain monitoring of pile. (3) Wavelet transform is an effective method to deal with the data obtained by using OFDR, in terms of deleting noise signal. Combined with polynomial matching method, the variations of strain can be clearly shown. Author Contributions: L.G. designed the project. Y.G. and Y.M. performed the experiment. B.J. did the numerical simulation. Y.G., L.G. and Y.X. analyzed the data and prepared the manuscript. H.L. reviewed and commented on the manuscript. Funding: This research was funded by the National Natural Science Foundation of China (Grant No. 51508159), the Fundamental Research Funds for the Central Universities of Hohai University (No. 2016B06114, 2017B20614), China Postdoctoral Science Foundation (No. 2014T70468), the Natural Science Foundation of Jiangsu Province (No. BK20130832), the Key Laboratory of Hydraulic and Waterway Engineering of the Ministry of Education and National Engineering Research Center for Inland Waterway Regulation open foundation (SLK2015A01). Conflicts of Interest: The authors declare no conflict of interest.

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