sensors Article

Study on the Deformation Measurement of the Cast-In-Place Large-Diameter Pile Using Fiber Bragg Grating Sensors Lei Gao 1,2, *, Kai Yang 1,2 , Xiaorui Chen 1,2 and Xiangjuan Yu 1,2 1

2

*

Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering, Hohai University, Nanjing 210098, China; [email protected] (K.Y.); [email protected] (X.C.); [email protected] (X.Y.) Research Institute of Geotechnical Engineering, Hohai University, Nanjing 210098, China Correspondence: [email protected]

Academic Editors: Christophe Caucheteur and Tuan Guo Received: 14 December 2016; Accepted: 15 February 2017; Published: 3 March 2017

Abstract: Compared with conventional piles such as the circle pile, the cast-in-place large-diameter pile (PCC pile) has many advantages: the lateral area of PCC pile is larger and the bearing capacity of PCC pile is higher. It is more cost-effective than other piles such as square pile under the same condition. The deformation of the PCC pile is very important for its application. In order to obtain the deformation of the PCC pile, a new type of quasi-distributed optical fiber sensing technology named a fiber Bragg grating (FBG) is used to monitor the deformation of the PCC pile. The PCC model pile is made, the packaging process of the PCC model pile and the layout of fiber sensors are designed, and the strains of the PCC model pile based on FBG sensors are monitored. The strain of the PCC pile is analyzed by the static load test. The results show that FBG technology is successfully applied for monitoring the deformation of the PCC pile, the monitoring data is more useful for the PCC pile. It will provide a reference for the engineering applications. Keywords: PCC pile; strain; deformation; fiber Bragg grating (FBG); sensor

1. Introduction As a new soft foundation reinforcement technology, the cast-in-place large-diameter pile (PCC pile) has many advantages. With the large diameter of the PCC pile, the amount of concrete is little and the bearing capacity of pile is higher. Compared with other solid concrete piles, the PCC pile has a large contact area between the pipe pile and the soil mass, and it increases the friction resistance of the pile. At the same time, the diameter of the PCC pile is more than 1 m, the per unit area using pile is less under the same load, it can effectively reduce the amount of work and save expenditure. The PCC pile can be used in the highway, high-speed railway, municipal roads, and other large area soft foundation treatment engineering projects, and it achieves good social and economic benefits. Liu Hanlong and others carried out a series of PCC pile designs, engineering applications, and testing, and it was put forward that the pile has high bearing capacity, high quality, low cost, and so on. The large scale model test was used to analyze the pile soil interaction characteristics and bearing capacity of the PCC pile under horizontal load. The variable impedance of the computational model and wave equation of the PCC pile are studied [1–5]. Recently, the optical fiber sensing technology has been used in the monitoring of civil engineering structures [5–11]. Fiber Bragg gratings (FBG), as one of the optical fiber technologies, have many advantages such as the distributed, interference, and corrosion resistance. Compared to the conventional monitoring technologies, FBG technology is formed by using the phase mask method to

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change the refractive index of the optical fiber [12–15]. The FBG sensors can measure the distribution of strain, temperature, displacements, and so on with high accuracy and stability [16–23]. This technology has been successfully applied to monitor the field performance of reinforced slopes, dikes, and bridges. The deformation characteristics of the PCC pile are very important for the optimum design and construction of the PCC pile. However, as a new type of pile, the strain distribution, bearing characteristics, and other aspects of the PCC pile are still not clear for its application. At present, the traditional strain gauges are used to monitor the strain of the pile body, but the traditional strain sensor is easy to destroy and it has electromagnetic interference, the full deformation characteristics of the PCC pile cannot be obtained, and it is difficult to achieve the optimal design and construction of the PCC pile. Therefore, it is important to study the deformation characteristics of the PCC pile by using a new technology and method, which is important for the application of the PCC pile. FBG as the latest optical monitoring technology have high monitoring precision of structures and concrete. The new optical fiber sensing technology (FBG) will be used to monitor the deformation of the PCC pile, the special packaging of the optical fiber sensor, the coordination of the pile body, the protection technology of the optical fiber sensor, and the data analysis and processing of the optical fiber monitoring technology, all of which are studied in this paper. Moreover, the deformation characteristics of the PCC pile are analyzed. The strain distribution of the PCC pile is obtained to provide basic data, which will provide a new understanding of the PCC pile through the static load model test. This study will provide technical support for the optimization design, construction, and application for the PCC pile. 2. Materials and Methods 2.1. The PCC Model Pile and Soil In order to obtain the deformation of the PCC pile, it is important to learn whether the bearing capacity of the PCC pile can meet the design requirement, so it is necessary to carry out the static load test for the PCC pile. There are two main experimental methods: one is the model test, and the other is the field test. Through a certain test method and monitoring method, the characteristics and laws of the PCC pile can be directly observed. For a more in-depth understanding, it can grasp the law of the PCC pile to provide a good foundation. The deformation of the pile foundation under the action of the load is difficult to obtain, and the stress state is very complex. The field static load test needs to take into account the instrument layout, the surrounding environment conditions, the loading conditions, and a series of complex problems. As such, the results cannot reflect the deformation of the pile body. The field test is not repeatable when the test data is not ideal and will waste manpower and material resources. In light of the above problems, the model test has become an indispensable part for stress and strain variations of the pile foundation in the study. Considering the application of FBG sensors to the PCC pile for the first time, in order to obtain a good strain curve of the pile body, the model test is chosen because it can accurately control the experimental condition only in this way and can avoid adverse effects of the environmental conditions of the site. Therefore, a correct analysis is needed for the PCC pile under the vertical static loading. The purpose of this experiment is mainly so that FBG technology can be applied to the monitoring of the strain of a PCC pile. The processes of the model pile for the PCC pile are customized mainly to make a model pile mold, a concrete mixture ratio is determined, the pouring pile is maintenance, and the maintenance is removed after forming. It can be seen in Figure 1a–c. The common specification of the PCC pile is an outer diameter of 1000 mm and a wall thickness of 120 mm. Based on this situation, the model pile for the PCC pile is designed for an outer diameter of 315 mm and an internal diameter of 200 mm. In order to ensure that the cast is in place to meet the design requirements of the PCC pile, the concrete mix ratio is chosen. The weight ratio of cement/water/sand/gravel is 1:0.47:1.59:3.39. The parameters of the PCC model pile are shown in Table 1.

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Table 1. Parameters of the cast-in-place large-diameter (PCC) model pile. Parameter Area Circumference External diameter

Unit (m2 )

S C (m) D (m)

Model Pile

Parameter

Unit

Model Pile

0.046 0.989 0.315

Pile length Internal diameter Volume

L (m) d (m) V (m3 )

1.5 0.2 0.070

According to the design of the section size of the PCC pile, the PCC model pile is designed. Because the PCC pile is a new type of pile, it is difficult to build a large diameter pipe pile model. Taking into account the cost and construction process of the PCC pile, the design of the PCC pile material used the circular PVC plastic pipe, and two types of PVC pipes are obtained for the diameter of 200 mm and 315 mm, respectively. In order to analyze the strain difference between the inner and outer wall of the PCC pile body, it is necessary to keep the inner wall of the PCC pile in advance to lay the optical fiber sensor, and it will have the purpose of monitoring the inner deformation of the PCC pile. The inner wall of the 200 mm PVC pipe is drilled, and the 2 mm of fine steel wire is then fixed to the pipe so that the groove is built in the concrete when the concrete is poured into the PVC pipe. When pouring concrete, the pipe ring is small. The concrete should be poured into the ring with a small shovel. In the process of laying FBG sensors, the coordination of deformation and the coordination of FBG sensors with the PCC pile are needed to consider the survival rate of FBG sensors, and the survival rate should be enough during the test. The mobile, embedded loading will inevitably produce friction, collision, or other damage on the PCC pile. If the FBG sensors are directly exposed to the surface of the PCC model pile, they are very easy to break, and it is not guaranteed that the FBG sensors along the direction of the axial force of the pile are still in good condition. Therefore, it needs to slot along the pile body, and the depth of the slot is about 2~3 mm. The optical fiber sensor can be wrapped completely in the groove of the pile body, and the friction is not affected by the optical fiber. The slot should be kept in the direction of the groove along the pile so that bending is avoided. At the same time, the groove should be smooth to prevent slotting too deep or shallow, and the out-of-flatness of groove will lead the optical fiber bending to affect the monitoring data quality on the inside. Because the FBG sensors are only monitoring the axial strain in the PCC pile, at the beginning, the process of the fixed line slot has a great influence on the monitoring data. The direction of the slot and the monitoring data can correctly reflect the strain characteristics of the PCC pile body. The sensing fiber has a flexible structure, the model pile has a rigid structure, and, in order to accurately obtain the deformation of the PCC model pile, the optical fiber must be closely connected with the model pile. The deformation and coordination can thus be achieved. Therefore, the packaging, the selection of the binder, the layout, and other aspects of the process should meet this requirement. The binder and two kinds of epoxy resin AB glue are used. The sensing optical fiber is thin and fragile, especially on the outside of the PCC pile; the layout process and test process is easy to damage, and the lower survival rate of the sensors will affect the collection of the test data. The protection of the sensing optical fiber is an important step in the test. Therefore, the optical fiber can be exposed on the outside of the hose connection protection in the layout and packaging process. Then, a soft cotton is used to protect the optical fiber, which will prevent possible damage during the preparation of the test. The specific layout steps are as follows and can be seen in Figure 1d–f.





The line slot and point are fixed along the direction of the PCC pile with the ring. The worker set aside in the groove corresponding to 2 mm of the groove and cut out about 2 mm for the groove. In a PCC pile body under the selection of symmetrical grooves, the paste line of FBG sensors is fixed. On the selected groove, with a marker of the FBG points from the pile top with 20 cm, the middle is every 30 cm between two raster points, with a total of 5 points. An optical fiber loose tube is used at the top of the pile, which enables the 0.9 mm optical fiber in the optical fiber loose tube to strengthen the protection, and this will avoid the destruction.

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For the fixed-point paste, the FBG sensor is fixed in the predetermined layout position; the AB glue is used for the FBG sensors around theis paste and in the groove of the PCC the pile,  For the fixed-point paste, the FBG sensor fixed in thefixed predetermined layout position; ABwhich provides the FBG very littlethe pre-stress. glue is used forsensors the FBGwith sensors around paste and fixed in the groove of the PCC pile, which For the protecting, epoxy adhesive is used and fixed in the groove to protect the provides the FBG the sensors withresin very little pre-stress.  For the protecting, the epoxy resin adhesive is used and fixed in the groove to protect the FBG FBG sensors. sensors.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 1. Process of fiber Bragg grating(FBG) (FBG) sensor sensor package. Fixed thethe moulds; (b) Concreting; Figure 1. Process of fiber Bragg grating package.(a)(a) Fixed moulds; (b) Concreting; (c) the PCC model pile; (d) Fixed and Cut; (e) Fixed-point paste; (f) Comprehensive paste. (c) the PCC model pile; (d) Fixed and Cut; (e) Fixed-point paste; (f) Comprehensive paste.

Due to the dual sensitivity of the temperature and strain of the sensing fiber, the influence of

Due to the dual sensitivity of theoftemperature and strain of the sensing fiber, the influence of temperature variation on the strain the PCC model pile is considered in the test process. In the case of the sensing optical is not affected the force, theconsidered strain of optical fiber caused byIn the temperature variation on thefiber strain of the PCC by model pile is in the testisprocess. of the surrounding temperature the strain of is thecaused PCC by case temperature of the sensing optical fiber issoil. notTherefore, affected the by influence the force,ofthe strain ofon optical fiber pile should be monitored in the testTherefore, process. In the order to obtainofthe real strain ofon thethe PCC model temperature of the surrounding soil. influence temperature strain of pile the PCC under the vertical load, the effect of the temperature on the strain of the pile is considered, and a pile pile should be monitored in the test process. In order to obtain the real strain of the PCC model hollow PVC tube is buried near the PCC pile with a spacing of 30 cm. In the monitoring of the pile under the vertical load, the effect of the temperature on the strain of the pile is considered, and a deformation process, the FBG sensors are fixed into the tube, sand is added so that it can monitor the hollow PVC tube is buried near the PCC pile with a spacing of 30 cm. In the monitoring of the pile temperature of soil. In this case, the temperature of the soil can be obtained, it can be used to deformation the FBGeffect sensors arestrain fixedof into tube, is added so that it can monitor eliminateprocess, the temperature on the thethe PCC pile,sand and the deformation characteristic of the temperature of soil. In this case, the temperature of the soil can be obtained, it can be used to eliminate the PCC model pile in the static load is obtained. the temperature on the of the pile, and the is deformation characteristic of the In order effect to simulate thestrain situation, the PCC soft soil foundation simplified, the clay soil around the PCC pile hasinathe height of load 0.6 m,isand the lower layer uses sand with a height of 1.6 m. Soil samples are model pile static obtained. taken from model and soil testthe is carried out,foundation and the physical propertiesthe of the layer are the In order tothe simulate the the situation, soft soil is simplified, claysoil soil around obtained as shown in Table 2. pile has a height of 0.6 m, and the lower layer uses sand with a height of 1.6 m. Soil samples are taken from the model and the soil test is carried out, and the physical properties of the soil layer are obtained Table 2. Physical indexes of soil layers. as shown in Table 2. Material Soil Water Content w (%) Natural Density ρ (g/cm3) Thickness (m) Table 2. Physical indexes of soil clay 19.22 1.93layers. 0.6 sand 1.33 1.70 1.6 Material Soil Water Content w (%) Thickness (m) Natural Density ρ (g/cm3 ) 2.2. Static Load Model Test of the PCC Pile clay 19.22 1.93 0.6

1.33 1.70performance of the actual 1.6 soil, the Insand order to ensure that the soil can accurately simulate the soil is compacted at 20 cm. It is careful that the hammer does not destroy the optical fiber sensor on the pile when the Test soil is 2.2. Static Load Model of compacted. the PCC PileIt is set aside for two weeks to ensure that the clay soil can be consolidated, as shown in Figure 2.

In order to ensure that the soil can accurately simulate the performance of the actual soil, the soil is compacted at 20 cm. It is careful that the hammer does not destroy the optical fiber sensor on

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the pile when the soil is compacted. It is set aside for two weeks to ensure that the clay soil can be Sensors 2017, 17, 505 5 of 9 consolidated, as shown in Figure 2.

Figure Figure 2. 2. Soil Soil layered layered compaction. compaction.

shows the the configuration configuration of of the the static static load load model model test. test. The The loading loading system system is mainly Figure 3 shows composed of loading equipment with digital display equipment, the anti-force frame, and a pressure transmission device. device.InIn order to ensure the loading process, the equipment digital equipment is equipped. specially transmission order to ensure the loading process, the digital is specially equipped. The magnitude andofthe change load caninbe reflected in a timely by the The magnitude of load and of theload change load can beofreflected a timely manner by themanner digital display digital display equipment. The model test equipment uses the reinforced concrete structure, equipment. The model test equipment uses the reinforced concrete structure, the length of modelthe is length the the width is 200 cm,cm. and themodel height is 230is cm. The model trough is 265 cm,of themodel widthisis 265 200 cm, and height is 230 The trough equipped with stairs, and equipped with stairs, and the organic glass window is used to observe the filling and stress of soil the organic glass window is used to observe the filling and stress of soil after loading. The channel on after sides loading. Themain channel on end bothwith sidesa of the main girder model groove, and the main both of the girder model groove, andend the with mainabeam and secondary beam is beam and secondary beam is connected. Because the counter forceand frame is through the channel connected. Because the counter force frame is through the channel is with the model, the loadand by is with the model, the load by the channel modelatgroove transfers to trench the model at the bottom of the channel steel model groove transfers tosteel the model the bottom of the so that it guarantees trench so thatframe it guarantees the counter frame forThe thecounter securityforce of the load transfer. The the counter force for the security of theforce load transfer. frame consists of two counter forceand frame consistsbeams of two main ofcm, special steel. Two main beams secondary made of beams special and steel.secondary Two mainbeams beamsmade are 150 and the overall main of beams are 150 cm,isand the overall span ofbeam the main beam is 235 the secondary is 100 span the main beam 235 cm; the secondary is 100 cm, and thecm; overall span of thebeam secondary cm, and the cm. overall of theequipment secondaryuses beam 150 cm. pressure equipment usesthe thediameter Jack, the beam is 150 Thespan pressure theisJack, the The height of the Jack is 300 mm, height of the Jack is 300 mm, the diameter of the piston is 80 mm, the maximum stroke is 200 mm, of the piston is 80 mm, the maximum stroke is 200 mm, and the maximum load is 250 kN. The static and the load is 250 as kN. Theasstatic load test uses FBG technology as and wellthe as the traditional load testmaximum uses FBG technology well the traditional resistance strain gauge, strain of inner resistance strainwall gauge, and the strain of innerThe wall andsensor outerused wall in of this the experiment pile are monitored. The wall and outer of the pile are monitored. FBG is produced FBG sensor used in this experiment is produced by Suzhou Nanzhi Company of China. by Suzhou Nanzhi Company of China. The wavelength range of FBG Sensors is 1510~1590 The nm. wavelength range of FBG Sensors is 1510~1590 nm. Theby FBG a SM125 demodulator The FBG demodulator is a SM125 demodulator produced thedemodulator Optics MicronisCompany, (852 Century produced the Optics Micron Company, Century Place NE,isAtlanta, GAto30345 America) and Place NE, by Atlanta, GA 30345, USA) and the(852 SM125 demodulator designed measure the stress, the SM125 demodulator is designed to measure the stress, temperature, and pressure of the low temperature, and pressure of the low speed variation. The scanning frequency of each channel with variation. The scanning frequency of each channel 1 Hz can monitored the same 1speed Hz can be monitored at the same time. While SM125 can with be extended to be 16 channels at at any time to time. While SM125 can be extended to 16 channels at any time to monitor more than 40 FBG sensors, monitor more than 40 FBG sensors, SM125 is mainly based on the demodulation technology of the SM125 is mainly basedand on is thesuitable demodulation technology of the The fiberresolution Fabry Perot filter and is suitable fiber Fabry Perot filter for long-term monitoring. of SM125 demodulator for2.5 long-term monitoring. The resolution of SM125 demodulator is 2.5 pm. is pm.

Figure 3. 3. Static Static load load model model test. test. Figure

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The FBG sensors are made of a variety of sensors such as temperature, strain, stress, The FBGand sensors are made of a variety of FBG sensors such can as temperature, strain, stress,wavelengths, acceleration, acceleration, pressure sensors. Different sensors have different working and pressure sensors. Different FBG sensors can have different working wavelengths, so they can so they can use wave division multiplexing technology in a single optical fiber cascaded multiple use division multiplexing technologyItinhas a single optical cascaded FBG sensor FBGwave sensor for distributed measurement. a small size,fiber a light weight,multiple compatibility with for distributed measurement. It has a small size, a light weight, compatibility with optical fibers, optical fibers, low insertion loss, good long-term stability, and so on. It is particularly suitable for use low insertion loss, goodthat long-term stability, and so on. or It substantially is particularly suitable for useWhen in harsh in harsh environments are inflammable, explosive, electromagnetic. the environments that are inflammable, or substantially electromagnetic. When temperature temperature or stress changes, the explosive, fiber produces an axial strain, which makes thethe grating period or stressatchanges, fiber produces anindex axialof strain, whichfiber makes the grating at the change; the samethe time, the refractive the optical changes, whichperiod causeschange; a wavelength same time, refractive index the optical fiber changes, causes a wavelength shift in the the grating. By using theoflinear relationship betweenwhich the strain and wavelength shift shift in of the the grating. By using the linear relationship between the strain and wavelength shift of the grating, grating, the deformation of the measured object can be calculated. When the strain of the pile is the deformation of the measured can bechange calculated. When the strain the pile isand changed, changed, the optical fiber grating object wavelength of the pile surface is of monitored, it can the optical fiber grating wavelength change of the pile monitored, and it can obtainwith the obtain the strain of the PCC pile under the vertical load.surface When is the FBG wavelength changed strain of the PCC pile under the vertical load. When the FBG wavelength changed with 0.001 nm, 0.001 nm, the corresponding strain value is 1 με. the corresponding 1 µε. The maximumstrain loadvalue is 40 iskN in this test, which is performed 4 times, and the increment of Theload maximum is 40 kNkN. in this which performed 4 times,and andreach the increment vertical vertical at eachload level is 10 Thetest, load will isgradually increase stability. of After the load at each level is 10 kN. The load will gradually increase and reach stability. After the completion of completion of the loading, a dial indicator data is read every two hours; when the per hour the loading,isa less dial than indicator data the is read hours; when the per less than settlement 0.1 mm, pileevery is in two a steady state under the hour load settlement operation. isUnder the 0.1 mm, the is in a steady state under load displacement operation. Under the condition a constant vertical condition ofpile a constant vertical load, the the vertical increases rapidly,ofthe speed increases load, the vertical displacement rapidly, the speed increases gradually, the maximum gradually, when the maximumincreases load or displacement is reached, it must stop thewhen experiment. When load or displacement reached, it must the experiment. Whenthe thetest. pile is crushed, the loading the pile is crushed, theisloading should bestop stopped, thus concluding should be stopped, thus concluding the test. 3. Results 3. Results The strain of the PCC pile is monitored by FBG technology, the FBG sensors are used to monitor The strain ofofthe is and monitored FBG technology, theon FBG are outer used to monitor the deformation thePCC PCCpile pile, the FBGbysensors are arranged thesensors inner and wall of the the deformation of the PCC pile, and the FBG sensors are arranged on the inner and outer wall of the PCC pile body. The deformation characteristics of the PCC pile can thus be more accurately PCC pile body. The sensing deformation of the PCC pilelines can thus beFBG morefiber accurately analyzed. The FBG fiberscharacteristics are used to connect the four of the sensorsanalyzed. together. The FBG sensing fibers are used to connect the four lines of the FBG fiber sensors together. the model the model test is small, and the pile body is shorter with only 1.5 m. In order to ensure the survival test is small, and the pile body is shorter with only 1.5 m. In order to ensure the survival rate the rate of the optical fiber sensors, the single end of the layout is easy to slot and layout, and of it can optical sensors, thebody single endthe of coated the layout is easy to slot andrequirements it can reduce the slotted reduce fiber the slotted pile and epoxy resin. Thisand canlayout, meet the of the pile pile body and the coated epoxy resin. This can meet the requirements of the pile body and fiber sensor. body and fiber sensor. In the process of monitoring based on FBG, the effect of temperature In the process of based the effect of temperature compensation of strain the compensation of monitoring strain on the pileon is FBG, eliminated. After the completion of each loading, theonFBG pile is eliminated. After and the completion loading, the FBG chart wavelengths are read, andsets the of groups wavelengths are read, the groupsofofeach wavelength change are obtained. Four data of wavelength change chart are obtained. Four sets of data with 10 kN, 20 kN, 30 kN, and 40 kNFBG are with 10 kN, 20 kN, 30 kN, and 40 kN are selected for comparison and analysis. The results of the selected forthe comparison andshown analysis. The results of 5, theand FBG of the PCC pile shown in method of PCC pile are in Figures 4 and themethod variation of strain andare deformation Figures 4 and 5, and the variation of strain and deformation characteristics for the PCC model pile can characteristics for the PCC model pile can be seen. be seen.

Figure 4. Strain curve of outer wall for the PCC model pile.

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Figure 5. 5. Strain Strain curve curve of of the the inner inner wall wall for for the the PCC PCC model model pile. pile. Figure

On whole, the the inner innerand andouter outerwall walldeformation deformationofof the PCC pile body consistent with On the whole, the PCC pile body areare consistent with the the overall change in the law, the strain in the upper part of the decline rate is fast, and the lower part overall change in the law, the strain in the upper part of the decline rate is fast, and the lower part of of rate decreases in speed. It can be seen that, when loading increases, the strain of PCC the PCC thethe rate decreases in speed. It can be seen that, when the the loading increases, the strain of the pile pile increases. The strain thepile PCC pile decreases withunder depththe under same This loading. This also also increases. The strain of the of PCC decreases with depth samethe loading. indirectly indirectly the reliability of FBG monitoring for the strain of pile the PCC strain of proves theproves reliability of FBG monitoring for the strain of the PCC body.pile Thebody. strainThe of the inner the wallsmaller is much smaller than the outer wall.be This be small due tomodel the small model wallinner is much than that of the that outerofwall. This may duemay to the pile used in pile this used in this experiment. Withinof a diameter of only is less pile core and itto is form difficult experiment. Within a diameter only 20 cm, there20 is cm, less there pile core soil, and it issoil, difficult the to form the same surrounding environment with thebody, outersopile thefor inner wall same surrounding environment with the outer pile thebody, strainso ofthe thestrain inner of wall the PCC for PCC pile is small. In order study thestrain stressvariation and strain variation lawbody, of thethe pile body, the pilethe is small. In order to study the to stress and law of the pile mean value mean value method will be applied to calculate the stress on the inner and the outer wall. According method will be applied to calculate the stress on inner and the outer wall. According to the section to theofsection sizepile, of the the elastic modulus and the strain be used calculate size the model themodel elasticpile, modulus and the pile strain can pile be used tocan calculate thetoaxial force the axial force of The the PCC pile. The rateand of the inner outer strain of is thehigher PCC pile is of the PCC pile. declining rate declining of the inner outer walland strain of wall the PCC pile in the higher in the upper partinand in the lower. that of thethe upper part of the clay upper part and smaller the smaller lower. This shows thatThis the shows upper part clay is provided withisa provided withresistance a side frictional than provided theupper lowerlayer sand.ofThe side frictional greaterresistance than that greater provided by that the lower sand.byThe theupper soil is layer of the soil is clay, the thickness of the soil is 0.6 m, and the lower soil is sand, which indirectly clay, the thickness of the soil is 0.6 m, and the lower soil is sand, which indirectly shows that the soil shows around the pileeffects body on hasthe different effectsproperties on the mechanical of the aroundthat the the pilesoil body has different mechanical of the PCCproperties pile. As shown PCC pile. As shown in Figures 4 andof 5, strains the distribution of strains theunder PCC model pile under static in Figures 4 and 5, the distribution of the PCC model of pile static load at all levels load at all levels is slowly decreasing. With the increase of the load, the bearing capacity of the PCC is slowly decreasing. With the increase of the load, the bearing capacity of the PCC pile gradually pile gradually increases. The strain data based on the FBG is consistent, that increases. The strain monitoring datamonitoring based on the FBG is consistent, indicating that theindicating application of the technology for the PCC pile is feasible for monitoring model FBGapplication technologyof forFBG the PCC pile is feasible for monitoring the strain. The modelthe teststrain. is veryThe successful, test is very successful, theofmethod of the based onand thethe FBG are reliable, andFBG the the method and process the PCCand pileprocess based on the PCC FBG pile are reliable, strain data of the strain data the useful FBG sensors very useful the noting PCC pile. is worth whenthere the load sensors areof very for theare PCC pile. It is for worth thatIt when thenoting load isthat applied, is a is applied, there is a certain degree of separation between the pile and the counter force frame, certain degree of separation between the pile and the counter force frame, because the soil under the because the soil the bottom of pile pile is is sinking, compressed pilepile is sinking, so theaccurately top of thereach pile bottom of pile is under compressed and the so theand topthe of the force cannot force cannot the set value accurately of the load.reach the set value of the load. Conclusions 4. Conclusions In this this paper, a new optical fiber monitoring monitoring technology technology named named FBG FBG is used to monitor monitor the In deformation of the PCC pile in combination with the static load model test. Based on the strain data of the PCC pile in combination with the static load model test. Based on the strain data thethe PCC pile, thethe deformation characteristics of of thethe PCC pile were studied. of PCC pile, deformation characteristics PCC pile were studied.

(1) (1) Combined Combinedwith withthe thePCC PCCpile, pile,the theconstruction constructionof ofthe the PCC PCC model model pile pile in in the the static static load load test test was was developed; the FBG sensors were first used in the PCC pile, and the fiber laying and protection developed; the FBG sensors were first used in the PCC pile, and the fiber laying and protection process process were were introduced introduced in in detail. detail.

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According to the monitoring strain data of the PCC pile body based on FBG sensors, the strain curve distribution of the PCC pile body were obtained under a different loading, and the deformation variation characteristics of the PCC pile were studied. The inner wall and outer wall deformation of the pile body is consistent with the overall change in the law; the strain in the upper part of the decline rate is fast, and the lower part of the rate decreases in speed. It can be seen that, when the loading increases, the strain of the PCC pile also increases. The strain of the PCC pile decreases with the depth under the same loading. This indirectly proves the reliability of the FBG technology for the strain of the PCC pile body. The strain on the inner wall is much smaller than that of the outer wall, because the model pile used in this experiment is small and there is less soil in the inner wall of the pile than there is in the outer wall of the pile.

This study provides scientific information support and technical support for the optimization design, construction, and application of PCC piles. Acknowledgments: Financial support comes from the Fundamental Research Funds for the Central Universities of Hohai University (No. 2015B25914, 2016B20214, 2016B06114), the Natural Science Foundation of Jiangsu Province (No. BK20130832), the National Natural Science Foundation of China (Grant No. 51508159). Author Contributions: For research articles with several authors, Lei Gao and Xiaorui Chen conceived and designed the experiments; Xiaorui Chen and Kai Yang performed the experiments; Kai Yang and Xiangjuan Yu analyzed the data; Lei Gao, Xiaorui Chen and Kai Yang wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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Study on the Deformation Measurement of the Cast-In-Place Large-Diameter Pile Using Fiber Bragg Grating Sensors.

Compared with conventional piles such as the circle pile, the cast-in-place large-diameter pile (PCC pile) has many advantages: the lateral area of PC...
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