The 6th International Forum on Opto-electronic Sensor-based Monitoring in Geo-engineering, Nanjing, China, 3-5 Nov., 2017
A Study on the Application of BOTDR in the Deformation Monitoring for Tunnel Engineering
A.B. Author1*, C.D. Other2
1Department of Earth Science, Nanjing University, Nanjing 210046, China
2Department of Earth Science, Nanjing University, Nanjing 210046, China
*E-mail:
ABSTRACT: In this paper, the basic principle and features of the Brillouin optical time-domain reflectometer (BOTDR) are introduced, and the feasibility of its application in the deformation monitoring for the tunnel engineering is analyzed throughout a tunnel monitoring engineering in Nanjing City, China. Some of factors influencing the measured results are discussed based on in-situ tests, including temperature, vibration and sensitivity. The monitoring results indicate that the BOTDR is fully feasible to be applied in the tunnel monitoring; the measured deformation of the monitored tunnel is only local and tiny, which attributes to the opening and closure of the cracks and joints that is caused by temperature, and the tunnel is under safe condition. It can be anticipated that the BOTDR will be a very effective measure tool to make the distributed deformation monitoring and the health diagnosis for the tunnel engineering.
KEYWORDS: BOTDR, tunnel engineering, deformation, distributed monitoring, health diagnosis
1
The 6th International Forum on Opto-electronic Sensor-based Monitoring in Geo-engineering, Nanjing, China, 3-5 Nov., 2017
1 INTRODUCTION
Recently, the Brilliouin Optical Time Domain Reflectometer (BOTDR) has been recognized as a powerful distributed fiber optic sensor with its real-time monitoring, long measurable distance, high measurement accuracy and high durability, and has begun to be applied in various infrastructure engineering’s deformation monitoring and health diagnosis such as tunnel, dike, bridges and subway [1-3].
Supported by the “985” Project of Nanjing University and the key project of Education Ministry of China, the BOTDR fiber-optic monitoring laboratory for large infrastructure engineering monitoring was set up in 2000 at Nanjing University. In 2001 and 2002, BOTDR has successfully been applied in the deformation monitoring of two tunnels, i.e. Nanjing Gulou Tunnel and Xuanwuhu Lake Tunnel. Herein the partial monitoring results obtained from Nanjing Gulou tunnel are presented in this paper.
2 CASE STUDY
2.1 The Project Scheme
2.1.1 Selection of the Optical Fiber
Based on many experiments, the jacketed SM optical fiber from Corning Co Ltd is selected in the field monitoring project.
2.1.2 Installation of Optical Fiber
In light of the deformation of the tunnel and 1m distance resolution of BOTDR, the optical fibers were installed on the surface of the concrete arch with three configurations:
(1) Overall Adhesion Method (OAM) that means the optical fiber is entirely affixed to the surface of the concrete arch as shown in Figure 1. The method is designed to examine the whole deformation of the tunnel such as the whole uneven subsidence.
(2) Fixed-point Adhesion Method (FAM) that means the optical fiber is bonded on the fixed points on the surface of the concrete arch at a certain interval as shown in Figure 1. The install method is used so as to detect the partial deformation caused by the crack zones which width is less than the distance resolution of BOTDR.
(3) W-shaped Fixed-point Adhesion Method (W-FAM) that is designed as in Figure 2 in order to enhance the measurement accuracy and sensitivity of BOTDR to the deformation caused by the single or several crack(s) which width is much less than the distance resolution of BOTDR, inasmuch as the method can magnify the strain of optical fiber located within the distance resolution. This method was mainly employed in the middle part of the tunnel, where the cracks and joints more developed.
Figure 1 Configurations of the OAM and FAM fibers
Five lines of optical fibers were installed, four of them are affixed to the concrete wall in the west side of the western arch using overall adhesion and fixed-point adhesion method mixed with Fixed-point Adhesion Method respectively, one of them was affixed to the arch along which the optical fiber was cinctured three times using the fixed-point adhesion method. All of optical fibers were centralized to connect to an optical cable after they were set, and then link to the management room located in the middle of the tunnel for BOTDR monitoring.
Figure 2 Configuration of the W-FAM fiber
According to the scheme above, a series of installation technics were taken to affix the optical fiber on the tunnel, including fluting, smoothing, grinding, cleaning, bottoming, sticking, checking etc. A special epoxy resin was used as adhesive, meanwhile mini OTDR was used to inspect the optical loss attenuation and breakpoint caused during installing. The FSM-16R splicer was used to fuse the optical fibers.
3 DEFORMATION ANALYSIS
The deformation of tunnel is mainly local, which is caused by the cracks and joints distributed on the tunnel sidewall and arch. Based on the strain distribution measured by Fixed-point Adhesion Method, the abnormal segments or points distributed on the measured strain spectrum were specially analyzed and the measured strain value at the abnormal segments or points can be converted into the deformation value using the following formula:
(1)
where is the length of the strained fiber, is the measured strain and is the deformation value, in which the sign convention is that the positive sign indicates the tensile due to the crack opening and negative sign denotes the compressive due to the crack closure.
The measured deformation of the tunnel mainly attributed to the temperature after analyzing the temperature changes during monitoring. The temperature difference at the tunnel entrance is more than the central part of the tunnel, so that the deformation of cracks distributed in the entrance is also larger than that of other part of the tunnel. Figure 8 shows the deformation and temperature of the measured point SP 27 changes with time. It is obvious that the deformation has a good relationship with temperature. The opening or closure of the cracks is dependent on the decline or rise of temperature.
Table 1 Some crack openings in the tunnel
Position NO. / Position ( from South Entrance) /m / Max. /mm / Min. /mmSP 13, / 450m / 0.139 / -0.022
SP 15 / 405m / 0.128 / -0.025
SP 23 / 190m / 0.135 / -0.015
SP 25 / 138m / 0.127 / -0.008
4 CONCLUSIONS
The analytical solutions for orthotropic density functionally graded cantilever beams derived in this paper by the superposition principle and the trial-and-error method are very explicit and convenient, and are also useful for study of other problems with more complicated loads and boundary conditions. Moreover, these analytical solutions can serve as benchmarks for numerical methods such as the finite element method, the boundary element method, etc.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural Science Foundation of China (Grant No. 41230636) and the National Basic Research Program of China (973 Program) (Grant No. 2011CB710605).
REFERENCES:
[1] Ohno, H., Naruse, H. Kihara, M., Shimada, A. Industrial applications of the BOTDR optical fiber strain sensor. Optical Fiber Technology, 2001, 7(1): 45-64.
[2] Zhu, H.H., Wang, Z.Y., Shi, B., Wong, J.K.W. Feasibility study of strain based stability evaluation of locally loaded slopes: insights from physical and numerical modeling. Engineering Geology, 2016, 208: 39-50.
[3] Zhang, C.C., Zhu, H.H., Shi, B. Role of the interface between distributed fibre optic strain sensor and soil in ground deformation measurement[J]. Scientific Reports, 2016: 36469.
[4] Habel, W.R., Hofmann, D., Senze, A., Kowalle, G. Detection of a slipping soil area in an open coal pit by embedded fibre-optic sensing rods[C]. Proceedings of 5th International Forum on Opto-electronic Sensor-based Monitoring in Geo-engineering, 2014, Nanjing, China: 1-7.
[5] Oka, K., Ohno, H., Kurashima, T., Matsumoto, M., Kumagai, H., Mita, A., Sekijima, K. Fiber optic distributed sensor for structural monitoring. Proceedings of 2nd International Workshop on Structural Health Monitoring, 1999, Stanford, USA: 672-679.
[6] Dunnicliff, J. Geotechnical instrumentation for monitoring field performance. New York: John Wiley and Sons, 1993.
1