Applications of Optical Fiber Sensing Technology in Monitoring of Geotechnical Structures

  • X. W. Ye
  • T. Liu
  • Dan-Feng Zhang
  • Jun Li
  • Zhi-Feng Liu
  • Liang Zhang
Chapter
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Abstract

The geotechnical structures will be inevitably suffered from material degradation and structural performance deterioration during the in-service operation stage. In order to acquire the on-line structural responses of each phase at the structural whole life-cycle (construction, operation, reinforcement, and rehabilitation), the structural health monitoring (SHM) systems based on the optical fiber sensing technology have been broadly implemented on a variety of geotechnical structures. The optical fiber sensors have received great concerns and been widely used in long-term geotechnical engineering monitoring due to their inherent advantages such as small size, light weight, immunity to electromagnetic interference (EMI) and corrosion, and embedding capability. Various monitoring purposes make optical fiber sensors get great advances in the manufacture and installation techniques as well as the analytical methods and theories. In this paper, the sensing principles of different types of optical fiber sensors are introduced. The applications of optical fiber sensing technology in different fields of geotechnical engineering are presented. The feasibility, reliability and effectiveness of optical fiber sensors in structural monitoring applications through laboratory tests and field experiments are described.

Keywords

Structural health monitoring Geotechnical structures Optical fiber sensors Applications 
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Introduction

The optical fiber sensing technology has gained significant growth and development due to the advance of laser technology, optical fiber communication technology, modern manufacturing technology, and computer technology. In comparison with the traditional sensors, the optical fiber sensors have some unique advantages, such as small dimension, good resolution and accuracy with low loss, free from electromagnetic interference and corrosion resistance. Therefore, the optical fiber sensing technology has received much attention and is widely employed in the fields of civil engineering, mechanical engineering, and aerospace engineering. In recent years, the optical fiber sensing technology is increasing used to monitor the structural condition and behavior of geotechnical structures.

Usually, the geotechnical structures are too complicated to be accurately understood because of the complexity of geological conditions and the nonlinear characteristics of soil and rock materials. In this connection, the field monitoring will play an essential role in the evaluation of safety state and performance of the geotechnical structures [1]. With the development of laser and instrument technology, the optical fiber sensing technology becomes increasingly mature by overcoming the difficulties in the practical applications. Different types of optical fiber sensors with unique features and advantages are designed and produced for the specific requirements. According to the demodulation principle, the optical fiber sensor can be categorized into different types [2], such as light intensity modulation type sensor, phase modulation type sensor, and wavelength modulation type sensor. In the paper, the sensing principles of three kinds of optical fiber sensors are described, and the applications of optical fiber sensors to geotechnical structures are summarized.

Principles of Optical Fiber Sensing Technology

Generally, a simple optical fiber sensing system consists of a light source, an optical fiber, an optical fiber sensor, a light acceptor, and a modulator element. The optical fiber plays a role in transferring the original light to the optical fiber sensor and then transferring the signal to the light acceptor. The fiber core, cladding layer, and coating layer make up the optical fiber whose constituent is silicon dioxide. The refractive index of the fiber core is higher than the cladding layer’s by reason that the fiber core contains a small amount of impurity, germanium for instance. When written into the optical fiber, a beam of white light generates the total reflection at the interface between the fiber core and the cladding layer. The optical fiber sensor bears the function of measurement. The optical fiber sensor can be divided into two categories. One is the functional sensor, and the other is the non-function sensor. The functional sensor is that the sensor is inscribed in the optical fiber leading to the optical fiber has the functions of measurement and transmission. The non-functional sensor only has the function of transmission because that the sensor is an independent element. The transmission principle of the fiber core is shown as Fig. 1. In which, n 1 = refractive index of fiber core, and n 2 = refractive index of cladding layer. The acceptor transfers the received signal to the modulator element in which the signal is modulated and analyzed.
Fig. 1

Transmission principle of fiber core

Light Intensity Modulation Type Sensor

The sensing principle is that the measurand (the refractive index of cladding layer, the geometry of optical fiber) will cause the variation of light intensity written into the optical fiber. The distributed optical fiber sensor which is based on the optical time domain reflection technique belongs to the light intensity modulation type sensor. The most obvious advantage is that the distributed optical fiber sensor can realize the distributed measurement along the optical fiber of temperature, strain and other information. The sensing principle [3, 4] of the distributed optical fiber sensor can be illustrated in Fig. 2.
Fig. 2

Sensing principle of distributed optical fiber sensor

The white light written into the optical fiber will be transferred to the distributed optical fiber sensor which located at the measurement area from one end via the optical fiber. Under the affect of the measurand, there is a backscattered light occurring due to the interaction mechanism between the propagating light pulse and the optical fiber which carrying the temperature and strain state along the optical fiber. Then the backscattered light is transferred to the light acceptor through the optical fiber coupler. The formula shown in Fig. 2 gives a way to analyze the collected backscattered light. The location of the variation of the measurand can be determined by use of the formula.

Phase Modulation Type Sensor

The sensing principle is that the measurand will cause the phase shift of light written into the optical fiber, and the measurand can be measured by demodulating the phase shift utilizing the interferometer. The optical fiber interferometric sensor is based on this kind of sensing principle. The optical fiber interferometric sensor is suitable for the case that the measurement objection with the requirements of high accuracy and large dynamic range. Michelson interferometer [5], Fabry-Perot interferometer [6], and MachZehnder interferometer are the sensors which are the frequently-used sensors belong to the interferometric sensor. The sensing principle [7, 8] of Extrinsic Fabry-Perot interferometer (EFPI) can be described in Fig. 3. In which, the R 1 = the reference reflection, and the R 2 = the sensing reflection.
Fig. 3

Sensing principle of EFPI

The white light emitted by the light source will be transferred into the Fabry-Perot cavity passing through single-mode and reflector, and 3% of the white light (the reference reflection) will be reflected when via the first reflector. The rest white light whose name is the sensing reflection will be reflected by the next reflector. The reference reflection and the sensing reflection will interfere with each other. The measurand will be determined by demodulating and analyzing the interference.

Wavelength Modulation Type Sensor

The fiber Bragg grating (FBG) sensor belongs to the wavelength modulation type sensor. According to the Bragg’s law, the sensing principle of an FBG sensor is that when a white light transmits to the Bragg grating, the light at a particular wavelength is reflected which is related to both the grating period and the effective index of refraction. The Bragg wavelength can be expressed by [9]
$$ \lambda_{B} = 2n_{eff}\Lambda $$
(1)
where λ B  = the original wavelength, Λ = the grating period, and n eff  = the effective index of refraction.
The measurand (such as temperature and strain) can vary the effective index of refraction and the grating period which leads to the variation of wavelength of the reflected light. The measurand can be distinguished by demodulating the wavelength shift of the reflected white light utilizing the interferometer. The sensing principle [10, 11] of an FBG sensor can be described in Fig. 4.
Fig. 4

Sensing principle of FBG sensor

In a general way, the temperature and strain independently influence the wavelength shift. The shift of wavelength caused by temperature and strain can be calculated by
$$ \Delta \lambda_{B} = \lambda_{B} \left[ {\left( {1 - P_{e} } \right)\Delta \varepsilon + \left( {\alpha + \zeta } \right)\Delta T} \right] $$
(2)
where Δλ B  = the wavelength shift caused by temperature and stress, P e  = the elastic coefficient, α = the coefficient of thermal expansion, ζ = the thermo-optical coefficient, Δε = the variation of strain, and ΔT = the variation of temperature. The measurand will be determined by demodulating and analyzing the wavelength shift based on the above formula.

Applications of Optical Fiber Sensing Technology

In order to better understand the health condition of the geotechnical structures under various loading and environmental conditions, the analysis of real-structure responding data is imperative. Therefore, the measurement instruments applied into field tests are essential for the responds monitoring of real structures. However, the reliability and effectiveness of the instruments should be proved and verified before applied into field monitoring. The following application examples of optical fiber sensing technology in geotechnical structures demonstrate the reliability and effectiveness of optical fiber sensors.

Slopes

Pei et al. [12] developed a new type of FBG-based in-place inclinometer which considered the deformation of the segment in the calculation. The inclinometer was fabricated and calibrated in the laboratory and used to monitor the deformation of a slope in China. The results showed that the new type of inclinometer was robust and reliable for monitoring the field slope in a harsh environment. Li et al. [13] built a slope model in the laboratory for simulating the slope deformation in the rainy season. The optical fiber BOTDR sensors were embedded into different positions of the slope, and the FBG sensors were mounted on the surface of the slope at different position. The research showed that the optical fiber BOTDR sensor and the FBG sensor are promising in the slope monitoring field. Ding et al. [14] designed a fiber optic sensing net for slope surface deformation monitoring by embedding the net at a certain depth in slope. Pi et al. [15] installed in situ inclinometer based FBG sensing technology in highway slope for the deformation monitoring, and proposed a new approach for slope stability evaluation.

Foundation Pits

Huang et al. [16] utilized the FBG sensors to monitor the strain curve of reinforced concrete support beam in deep excavation during the excavation process, which proved the FBG sensor is feasible for internal force monitoring of reinforced concrete support beam. Liu et al. [17] proposed a method for on-line monitoring of pit horizontal displacement using BOTDR-based on distributed optical fiber sensor. The successful application of the method in the site test demonstrated the feasibility and effectiveness of the method.

Piles

Kister et al. [18] deployed FBG sensors in reinforced concrete foundation piles for strain and temperature monitoring and structural health condition assessment. Lee et al. [19] used an optical fiber sensor system based on the single path multiplexing technique for the axial load strain monitoring in piles. Weng et al. [20] mounted quasi-distributed fiber Bragg grating (FBG) strain sensors on the surface of pipe piles to monitor the performance of pipe piles.

Dams/Dikes

Notother et al. [21] embedded a distributed sensor system based on Brillouin optical frequency domain analysis into the soil body of river dikes for detection of critical soil displacement, and proposed a method to deal with the issue of local fluctuations of the Brillouin gain attribute to birefringence. Zhu et al. [22] developed rod-type embedded sensors based on FBG sensing technology for the internal deformation monitoring of dam model. The quasi-distributed FBG sensors were axially mounted on the surface of sensor. The researchers utilized the sensors to monitor the deformation of a dam model under overload condition. The results showed that the novel sensor is feasible and effective for the dam deformation monitoring.

Soil Nails

Pei et al. [23] studied the performance of a glass fiber-reinforced polymer (GFRP) bar soil nail in a pullout test in laboratory using FBG sensing technology. Zhu et al. [24] developed an FBG-based monitoring system for the monitoring of average strain of host material as well as localized strain. The system then was used to monitor the strain distributions along soil nails in Hong Kong. The monitoring results showed that the monitoring system was applicable for field soil nail monitoring.

Tunnels

Zhao and Qiu [25] installed FBG sensors on the surface of the tunnel secondary lining to monitor the deformation status of tunnel cross-section. The monitoring data showed that the FBG sensors could relative realistically monitor the strain distributions of tunnel. Mao et al. [26] embedded distributed optical fiber sensors into tunnel secondary lining concrete by air-blowing and vacuum grouting technology, and proposed a method of time series analysis for the tunnel health state assessment.

Conclusions

The sensing principles of the light intensity modulation type sensor, phase modulation type sensor, wavelength modulation type sensor were described. A review of applications of optical fiber sensing technique to geotechnical structures such as the slope, foundation pit, pile, soil nail, and dams/dike was made. The application examples demonstrate that the reliability and effectiveness of the optical fiber sensing technology and indicate that the optical fiber sensors have been used in almost all fields in geotechnical engineering. The optical fiber sensors will have a better application prospect due to the unique advantages and the constant development of manufacture technology and analytical theory.

Notes

Acknowledgements

The work described in this paper was jointly supported by the National Science Foundation of China (Grant No. 51308493) and the Guangdong Provincial Academy of Building Research Group Co., Ltd. (Grant No. 2013-43).

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Copyright information

© Springer Nature Singapore Pte. Ltd. and Zhejiang University Press 2018

Authors and Affiliations

  • X. W. Ye
    • 1
  • T. Liu
    • 1
  • Dan-Feng Zhang
    • 2
  • Jun Li
    • 2
  • Zhi-Feng Liu
    • 2
  • Liang Zhang
    • 2
  1. 1.Department of Civil EngineeringZhejiang UniversityHangzhouChina
  2. 2.Guangdong Provincial Academy of Building Research Group Co. LtdGuangzhouChina

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