Spending Yong Institute Qi Jun, Gao Wenbin
(Monitoring Center for Photoelectric Sensing Engineering of Nanjing University, Nanjing, Jiangsu, 2 10093)
Distributed optical fiber sensing technology, such as Brillouin scattering time domain reflectometer (BOTDR), is a cutting-edge technology developed in recent years and widely used in the world. This paper mainly introduces the application of BOTDR distributed optical fiber sensing technology in tunnels, foundation pits and pavements. A large number of monitoring data accumulated in the process of engineering monitoring show that BOTDR distributed optical fiber sensing technology is a brand-new and reliable monitoring method, and its application in engineering practice provides a new idea for engineering monitoring, so it will have broad development prospects.
BOTDR optical fiber sensing; Engineering monitoring; meet an emergency
1 Introduction
With the increasing demand for engineering safety, many new sensing and monitoring technologies have been developed in recent years. They do not simply improve the traditional sensing and monitoring technology, but fundamentally change the sensing principle, thus providing brand-new monitoring methods and ideas. Among them, BOTDR distributed optical fiber sensing technology has attracted worldwide attention. It uses ordinary communication optical fiber and implants it into buildings in a way similar to the nervous system to obtain comprehensive strain and temperature information. This technology has become a research and development topic in developed countries such as Japan, Canada, Switzerland, France and the United States. This technology is still in the development stage in China, and has been successfully applied to some tunnel engineering monitoring, and gradually extended to other engineering fields.
With the support of the 985 Project of Nanjing University and the key project of the Ministry of Education, the photoelectric sensing engineering monitoring center of Nanjing University built the first BOTDR distributed optical fiber strain monitoring laboratory for large-scale basic engineering in China, carried out a series of experimental studies, and successfully applied this technology to the actual monitoring of underground tunnels and other projects, and achieved a number of important achievements, laying a solid foundation for the wide application of this technology in quality monitoring and health diagnosis of various large-scale basic and geological engineering in China.
Principle of 2 BOTDR distributed optical fiber sensing technology
Brillouin scattering is affected by both strain and temperature. When the temperature along the optical fiber changes or there is axial strain, the frequency of backscattered Brillouin light in the optical fiber will drift, and the frequency drift has a good linear relationship with the strain and temperature change of the optical fiber. Therefore, by measuring the frequency drift (vB) of natural Brillouin light backscattered in optical fiber, the distribution information of temperature and strain along the optical fiber can be obtained. The strain measurement principle of BOTDR is shown in figure 1.
In order to get the strain distribution along the fiber, BOTDR needs to get the Brillouin scattering spectrum along the fiber, that is, get the vB distribution along the fiber. The measuring principle of BOTDR is very similar to OTDR (Optical Time Domain Reflector). Pulsed light is incident from one end of the optical fiber at a certain frequency, and the incident pulsed light interacts with acoustic phonons in the optical fiber to produce Brillouin scattering, in which the backscattered Brillouin light returns to the incident end of the pulsed light along the original path of the optical fiber. After a series of complex signal processing, the power distribution of Brillouin backscattering along the optical fiber can be obtained after entering the light receiving part and signal processing unit of BOT-DR, as shown in figure 1 (b). The distance z from the scattering position to the incident end of pulsed light, i.e. BOTDR, can be calculated by the formula (1). Then, according to the above method, the frequency of incident light is changed at a certain interval and the measurement is repeated, so that the spectrum of Brillouin scattering light at each sampling point on the optical fiber can be obtained.
Figure 1 BOTDR strain measurement schematic diagram
As shown in figure 1 (c), theoretically, the Brillouin backscattering spectrum is Lorentz-shaped, and the frequency corresponding to its peak power is Brillouin frequency shift vB. If the fiber is stretched axially, the Brillouin frequency shift of the stretched fiber will change, and the strain can be obtained through the linear relationship between the change of frequency shift and the strain of the fiber. Where: c-the speed of light in vacuum;
Essays on Geological Disaster Investigation and Monitoring Techniques and Methods
Refractive index of n- fiber;
T refers to the time interval between emitting pulsed light and receiving scattered light.
At present, the most advanced BOTDR monitoring equipment in the world is represented by the latest generation AQ8603 BOTDR optical fiber strain analyzer developed by NTT Company of Japan. Table 1 is the main technical performance index of AQ8603.
Table 1 AQ8603 main technical performance indexes of optical fiber strain analyzer
3 tunnel safety monitoring
The application of BOTDR distributed optical fiber sensing technology in tunnels in China is becoming more and more mature. During the construction of several tunnel deformation monitoring systems, we have formed a set of successful experiences, which provide a solid technical foundation for the popularization of this technology in geotechnical and geological engineering safety monitoring.
3. 1 optical fiber laying
In order to accurately reflect the strain state of the measured structure, the optical fiber must be closely connected with the structure and laid on the structure. Laying quality is directly related to the actual effect of monitoring, so it is of great significance in engineering application.
According to the design principle of optical fiber monitoring system, combined with the actual engineering situation and the characteristics of AQ8603 stress distributed optical fiber sensor, there are basically two laying methods: comprehensive laying and fixed-point laying, as shown in Figure 2.
Figure 2 Full-scale connection and fixed-point connection
3. 1. 1 Full continuous laying
The sensing optical fibers are respectively connected and arranged along the depth direction and cross section of the tunnel. The sensing optical fiber laid along the depth direction is used to monitor the longitudinal deformation of the tunnel, while the optical fiber laid along the cross section is used to monitor the transverse deformation of the tunnel.
The characteristic of comprehensive continuous laying is that it can monitor the health of the tunnel in the whole process, and the monitoring object is the whole tunnel, and the monitoring result is the deformation of the whole tunnel. In this connection mode, a specific laying process is applied, and the sensor fiber is stuck on the concrete surface according to the designed line by using mixed glue (mainly epoxy resin) with excellent experimental effect, and the optical cable is connected at the end of the sensor fiber to transmit the monitoring signal to the tunnel monitoring center.
3. 1.2 Fixed-point continuous laying
The characteristic of this connection mode is to focus on monitoring the deformation of potential (or assumed) deformation sites such as deformation joints and stress concentration areas. The monitoring objects are potential (or assumed) deformation places such as deformation joints, and the monitoring results are the stress and strain characteristics of potential (or assumed) deformation places such as deformation joints. The laying method of this connection method is almost the same as the whole connection method, but the difference is that some special points are selected on the design and construction surface for pasting, that is, a fixed point is determined for the optical fiber every 1m ~ 1.5m, and pasted on the concrete wall to detect the deformation of the local joints of the tunnel (see Figure 3). In some characteristic positions, according to the actual situation, joint sensors are installed at specific positions of specific lines to monitor the deformation of deformation joints (see Figure 4).
Fig. 3 Schematic diagram of tunnel joint wiring
3.2 Deformation calculation
Because of the complicated reasons of tunnel deformation, there are not only overall deformation caused by temperature, but also local deformation caused by cracks and dislocations in different directions, so it is sometimes difficult to convert tunnel strain measured by BOTDR into deformation. Therefore, the feasible solutions are as follows: firstly, the optical fiber monitoring network is reasonably arranged to monitor the overall strain and local strain of the tunnel and its direction respectively, and the overall deformation and local deformation of the structure are calculated according to the deformation characteristics; Secondly, the corresponding calculation method should be adopted to transform the strain of optical fiber into the deformation of tunnel.
Fig. 4 Schematic diagram of weld sensor
For example, for uniform strain, the deformation can be calculated by the following formula:
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Where ε is the strain, d is the length of the strain segment, and δ is the deformation.
For uneven deformation, optical fiber can be laid by fixed-point connection within a certain distance. The strain between two bonding points is approximately considered as uniform strain, and the non-uniform deformation along the fiber can also be obtained according to the above formula.
If the overall uneven settlement occurs in the tunnel, the settlement deformation can be approximately calculated according to the deflection calculation method (see Formula (3)):
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Where ε 1 and ε2 are the strains of two optical fibers laid at the top and bottom of the structure, respectively, and d is the distance between the two optical fibers.
In addition, combined with numerical simulation technology, deformation calculation can be realized. The strain of optical fiber can be used as the boundary condition or known condition of numerical calculation, and various deformations of different parts of the structure can be obtained by finite element or finite difference calculation method.
In short, the calculation of tunnel deformation from strain to deformation is often complicated, but as long as the optical fiber monitoring network is arranged reasonably and the correct calculation method is adopted, the calculation of tunnel deformation can get satisfactory results.
4 foundation pit deformation monitoring
Deformation monitoring of foundation pit is one of the basic problems in geotechnical engineering, and the importance of foundation pit stability is self-evident. Over the past six months, the research group has successfully applied BOTDR technology to many deep and large foundation pit projects in Nanjing through a large number of indoor and outdoor experimental studies, and achieved some very valuable results.
As we all know, the causes of foundation pit deformation are complex and diverse, but in general, the main reasons are the horizontal displacement and basement uplift caused by foundation pit excavation. Traditional monitoring methods, such as earth pressure box and inclinometer tube, often have shortcomings such as low accuracy, poor corrosion resistance, large loss and waste of manpower due to the limitation of their own sensing methods. Through research, the research group successfully developed a patented distributed optical fiber sensing system (distributed optical fiber sensing intelligent inclinometer) for foundation pit displacement monitoring based on BOTDR technology.
Fig. 5 Distributed optical fiber sensing system for foundation pit displacement monitoring
As shown in fig. 5, the sensor is developed by combining traditional inclinometer equipment with advanced BOTDR technology. The purpose of applying traditional inclinometer device is: ① inclinometer can reflect the deformation of soil ideally and is a good material; (2) The inclinometer tube itself has a groove, and there is no need for manual slotting; (3) This material is a common monitoring material for foundation pit, which is convenient, easy to obtain and economical; (4) Use materials consistent with traditional monitoring methods to facilitate the comparison between old and new technologies. In short, the composition of the system is to stick the optical fiber on the inclinometer tube according to a certain construction technology with special glue verified by indoor and outdoor tests and engineering practice to form a sensing system, which we call distributed optical fiber sensing intelligent inclinometer tube. The sensor has all the advantages of distributed optical fiber sensor and can be used for quasi-real-time monitoring.
The monitoring results obtained by distributed optical fiber sensor using BOTDR technology are axial physical information (strain, temperature, etc.). ) along the fiber optic sensor. Therefore, how to use optical fiber sensor to obtain the horizontal deformation along the foundation pit has become the core of the problem. Through research, the horizontal deformation of foundation pit is approximately calculated by calculating deflection.
According to the knowledge of material mechanics, the deflection of each point along the line can be calculated by the following formula.
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Where: εx is the measured strain of the optical fiber at the point to be measured, and its value is the strain difference between the two optical fibers on both sides of the inclinometer; D is the distance between the optical fibers attached to both sides of the inclinometer; The starting point of the integral is a deep strain-free point, and v(x) is the deflection of each point, which can be approximately considered as the horizontal deformation of the foundation pit.
5 continuous reinforced concrete pavement detection
Continuous reinforced concrete pavement (CRCP) is a kind of continuous concrete slab with joints completely omitted, which is used to reduce vibration and noise caused by joints or improve smoothness and driving comfort. For this kind of high-performance pavement structure, the stress state of reinforcement, concrete stress state and crack distribution are the main factors reflecting the pavement performance [8.9]. It is of great significance to use BOTDR, an excellent nondestructive testing technology, to monitor the stress of steel bars and concrete on CRCP pavement and pavement cracks.
Fig. 6 shows the layout of BOTDR distributed optical fiber sensing system in continuously reinforced concrete pavement. There are 1 1 longitudinal steel bars on the pavement. Sensing optical fibers, four temperature compensation optical fibers and five strain sensing optical fibers are symmetrically laid on nine steel bars along the center.
Fig. 7 shows the change of concrete strain on the board surface detected by BOTDR within 5 days after pouring concrete. From the figure, we can clearly see the distribution of concrete strain along the longitudinal surface of the pavement, and can predict the location of possible cracks on the pavement according to the location of the maximum tensile strain. As shown in the figure, cracks are most likely to appear at 79m.
Fig. 6 arrangement of optical fiber sensing system
Fig. 7 strain distribution of concrete on slab surface
Fig. 8 shows the change of steel bar strain detected by BOTDR within 5 days after concrete pouring. It can be clearly seen from the figure that the strain of steel bars is distributed along the longitudinal direction of the pavement. During the hardening of concrete, the strain of steel bars is uneven. Continuous monitoring of reinforcement strain is helpful to predict pavement performance.
The test results show that BOTDR distributed optical fiber sensing system can effectively detect the strain of steel bar and concrete in continuously reinforced concrete pavement slab on line. This shows that BOTDR has good applicability and broad application prospects in similar projects such as pavement slab and bridge slab.
6 conclusion
Distributed optical fiber sensing technology is still in its infancy in China. Although some success has been achieved in some fields such as tunnels and foundation pits, there are still many research work to be further carried out, including two aspects: first, the distributed optical fiber sensing monitoring technology itself is further improved; The second is to constantly solve the technical problems in engineering monitoring. It is believed that with the continuous development and maturity of this technology, more and more large-scale infrastructure projects will adopt this technology for distributed monitoring and health diagnosis, and its application prospect is very broad and immeasurable.
Fig. 8 Strain distribution of reinforcement
refer to
[1]Horiguchi T, Kurashima T, Tate Da M. Tensile strain dependence of Brillouin frequency shift in Shi Ying fiber. IEEE Photonics Technology Express, 1989,1(5):107 ~108
[2]Ohno H, Naruse H, Kihara M, Shimada A, industrial application of BOTDR optical fiber strain sensors. Optical fiber technology, 200 1, 7( 1):45~64.
[3] Wu Zhisheng, Yutaka Takahashi, Jin Hao, Ping Songguang, optical fiber sensing detection of cracks in concrete structures. Proceedings of Japan Concrete Society, 2000,22 (1): 409 ~ 414.
[4]Wu Z S, Takahashi T and Sudo K, experimental study on monitoring continuous strain and crack with optical fiber sensor. Concrete research and technology, 2002,13 (2):139 ~148.
[5] Li Chun et al., Distributed optical fiber bidirectional strain sensor for gas pipeline. Optics and laser in engineering, 200 1, (36):4 1~47
[6]Uchiyama H, Sakairi Y, Nozaki T, an optical fiber strain distribution measuring instrument adopting a new detection method. Ando technical bulletin, 2002, (10):52~60.
Huang Minshuang, Chen Weimin, Huang Shanglian. Theoretical analysis of distributed optical fiber tensile strain sensor based on Brillouin scattering. Photoelectric engineering,1995,22 (4):11~ 36.
Cha Xudong, Zhang Qisen, Li Yuzhi, Su Qinggui, Huang Qing. Study on construction technology of continuous reinforced concrete pavement of expressway. Chinese and foreign highways, 23, 2003 (1):1~ 4
[9] Xie Jun, Cha Xudong, compiled. Guide for design of continuous reinforced concrete pavement. Foreign Highway, 2000,20 (5): 4 ~ 6.
[10] Shi Bin et al. Feasibility study of BOTDR strain monitoring technology applied to health diagnosis of large-scale foundation projects. Journal of rock mechanics and engineering. Vol. 22,No. 12, 2003.
[1 1] Shi Bin et al., Application of BOTDR in deformation monitoring of tunnel engineering, structural health monitoring and intelligent infrastructure, Balkema Publishing House, 2003: 1025~ 1030.
Xu, Shi Bin, Zhang Dan, Cui,. Signal processing method of BOTDR optical fiber sensor based on wavelet analysis. Photoelectric Laser, 2003(7).
[13] Xu Haizhong, Shi Baolin, Ding Yong, Cui, botdr distributed strain measurement data processing based on wavelet analysis, structural health monitoring and intelligent infrastructure, Beijing: Science Press, 2003:345~349.
Andy, Lv Zhitao. Optical fiber sensor for bridge monitoring. Highway traffic science and technology, 2003,20 (3): 91~ 95.
Zhang Dan, Shi Bin, Xu, Cui. BOTDR distributed optical fiber sensor and its application in structural health monitoring. Journal of Civil Engineering, 2003,36 (11): 83 ~ 87.
[16],, Xu,, Ding Yong,, Cui, Gao, Application of BOTDR in Structural Bending Monitoring, Structural Health Monitoring and Intelligent Infrastructure, Balkema Press, 2003:27 1~276.
[17],,, Gao,, Xu, Identification and location of cracks in reinforced concrete T-beam structure based on BOTDR,, 2004
Zhang Dan, Shi Bin, Xu, Gao, Zhu Hong. Experimental study on monitoring deformation of reinforced concrete T-beam by BOTDR. Journal of Southeast University (to be published)
[19] Ding Yong, Shi Bin, Wu Zhishen. Optical fiber sensor in geotechnical engineering monitoring. Proceedings of the 4th National Geotechnical Engineering Conference 2003: 283 ~ 29 1.
[20] Ding, Yang, Shi, Wang, Cui, Gao, Chen Jianqing, Chen, 2003. Stability of optical fiber strain sensor under constant stress. Structural Health Monitoring and Intelligent Infrastructure, Balkema Publishing House, 2003:267~270.