Geo-Frontiers 2011 © ASCE 2011
Development and Application of an Optical Fiber Sensor Based In-place Inclinometer for Geotechnical Monitoring Huafu Pei1, Jianhua Yin1, Honghu Zhu2 and Chengyu Hong1 1
Department of Civil and Structural Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China PH (852) 2766-6065; Fax: (852) 2334-6389; email:
[email protected] 2 School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, China PH (86) 25-83596220; Fax: (86) 25-83596220; email:
[email protected] ABSTRACT Traditional in-place inclinometers are widely used for displacement monitoring of slopes and other geo-structures. But these inclinometers have limitations such as (a) numerous cables requirement, (b) sensitivity to electromagnetic interference, (c) narrow measuring range. The new optical fiber sensor based in-place inclinometer can overcome the above limitations. This paper introduces a new type of in-place inclinometer based on fiber Bragg grating (FBG) sensing technique. A series of FBG sensors are glued onto the surface of a plastic rod for measuring bending strains and axial strains. Based on the beam theory, these strain results are used to calculate the displacement and rotation angle of the rod tip. A series of rods with FBG sensors can be connected together and installed into a geotechnical structure for displacement monitoring. Several calibration tests have been conducted in laboratory and the calibration results are in good agreement with theoretical results. The new FBG-based in-place inclinometers have been applied in a subway tunnel in Shenzhen, China, and a slope site in Sichuan, China. This technology will be used to monitor a retaining wall in Hong Kong. INTRODUCTION Inclinometers are widely used for measuring the internal displacement of geotechnical structures (Dunnicliff 1993), but there are also some drawbacks including (a) there are too many cables, (b) the measurements face electromagnetic interference problems, (c) the conventional inclinometer can only measure the angle between two points, and (d) the gravity based conventional inclinometer can only measure lateral deflections. Recently, a number of researches have been conducted to develop new types of inclinometer (e.g., Moyo et al. 2005; Ho et al. 2006; Yin et al. 2008; Zhu et al. 2010), in which fiber Bragg grating (FBG) sensors
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were adopted as strain sensors. The FBG sensing technology has been successfully utilized for slope stability monitoring (1999; Yoshida et al. 2002, Zhu 2009). In this paper, the authors developed a new type of in-place inclinometer which can overcome the main disadvantages of conventional inclinometer. Based on the beam theory, the inclinometer can measure the displacement profile of geotechnical structures, such as slopes, excavations, and foundations. PRINCIPLE OF FBG SENSING TECHNIQUE According to Bragg’s law, when a broadband source of light has been injected into the fiber, FBG reflects a narrow spectral part of light at certain wavelength, which is dependent on the grating period and the refractive index of fiber (Hill et al. 1993, Morey et al. 1989). In the reflected spectrum of an FBG sensor, the wavelength at which the reflectivity peaks is called the Bragg wavelength λB and can be calculated by
λB = 2neff Λ
(1)
where neff is the effective core index of refraction; Λ is the periodicity of the index modulation. For a standard single mode silica fiber, the relationship between the Bragg wavelength, strain, and temperature of the sensing fiber is simplified as (Othonos and Kalli 1999) Δλ B
λB 0
= (1 − peff ) Δε + (α + ξ ) ΔT
(2)
where λB 0 is the original Bragg wavelength under strain free and 0 oC condition; α is the coefficient of thermal expansion; ξ is the thermo-optic coefficient; peff is the photo-elastic parameter; Δε and ΔT are the variation of strain and temperature, respectively. BASIC THEORY OF IN-PLACEMENT INCLINOMETER
L v
ε1
θ
ε2
Figure 1. Schematic illustration of a cantilever beam
In the new inclinometer, a smart rod is employed, which functions as a beam, as shown in Figure 1. A series of FBG sensors are glued on the rod in the longitude direction. The relationships between the strains of the FBG sensors and the displacement and angle of the rod tip can be expressed by
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v = C1ε1
(3)
tan θ = C2ε 2
(4)
where v and θ are the displacement and the rotation angle at the free end, respectively; C1 , C2 are calibration coefficients; L and R are the length and the radius of the smart rod; L FBG1 , L FBG 2 are the distances of FBG1 and FBG2 to the free end, respectively; ε1 , ε 2 are the strains of FBG1 and FBG2, respectively. The sensing bar can be connected to each other and inserted in an inclinometer casing. The continuous deformation in horizontal or vertical direction can be measured using this technology. For Section i, The relative lateral displacement for this section is calculated by
Δ d i = v i + Δ v i = v i + Δ L i sin θ i
(5)
where vi is the lateral deflection of the ith flexible sensor bar; Δvi = ΔLi sinθ i is the lateral deflection due to rotation (θ i ) of the i-th rigid tube; ΔLi is the length of the i-th rigid tube. Due to the lateral movement and rotation of Sections 1 to i-1, the total lateral movement at the end of Section i shall be calculated as i −1
di = di −1 + Δdi + ( Li + ΔLi ) sin ∑ θ j 1
i −1
= di −1 + ( vi + ΔLi sin θi ) + ( Li + ΔLi ) sin ∑ θ j
(6)
1
CALIBRATION IN LABORATORY
In laboratory, several smart rods are fabricated. The special plastic rod is 1m in length and its radius is 10cm. FBG sensors are glued on the surface of this rod. In calibration tests, one end of the rod was free and the other end was fixed (Figure 2(a)). Displacements (Figure 2(b)) were applied onto a rigid tube in stages, which was connected to the smart rod. High-accuracy linear variable displacement transformers (LVDTs) were used to measure the displacements of two loading points. Calibration results (Figure3) show that the test results are in good agreement with theoretical values and the calibration coefficients C1 , C2 of this in-place inclinometer equal 38.596 and 100.884, respectively.
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Rigid boundary
LVDTS
Rigid tube
FBG1
FBG2 Li
Δ Li
Two moving devices
(a) Initial stage LVDTS
FBG2
FBG1
θi
Δvi
vi
Li
ΔLi
(b) Testing stage Figure 2. Setup of the calibration tests
(a) Calibration results of displacement (b) Calibration results of angle Figure 3. Typical calibration results of the new in-place inclinometer APPLICATIONS
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A landslide monitoring project has been carried out in Beichuan County, Sichuan Province of China. The geo-materials in the top layers of the most mountainous areas become extremely loose due to the earthquake happened on 12th May 2008. Thus landslides and debris flows occurred frequently. In order to monitor the internal movement of a loose slope situated at Weijiagou Village of Beichuan County. Two sets of FBG in-place inclinometers have been installed in this slope. The preliminary monitoring results are shown in Figure 4. A super high building named Ping An International Financial Centre is being constructed in Shenzhen, China. The tower building is 588m (115 floors) in height. The construction area of this tower is 328530m2. The excavation is in a trapezoidal shape (Figure 5). The area and depth of excavation is about 17495m2 and 30.8m, respectively. Municipal roads exist around this excavation. This site is located next to Shopping Park Station of Shenzhen Metro Line 1. The boundary of the subway tunnel and the nearest station exit are about 19 and 6 meters far from the excavation edge, respectively. Recently, the new type of FBG in-place inclinometers was installed in the subway to monitor the settlement during the process of excavation (Figure 4). Several sets of FBG tilt meters and crack meters were also used to monitor the inclination and possible cracks in the tunnel. These sensors were multiplexed to form a quasi-distribution sensing array.
(a) Displacements of Borehole 1
(b) Displacements of Borehole 2
Figure 4. Monitoring results from FBG in-place inclinometers at a slope site
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In-place inclinometer (200m)
Figure 5. Instrumentation of the subway tunnel using the FBG in-place inclinometers
Another monitoring project is in plan in Anderson Road, Hong Kong Island. Several piles (Figure 6) in the retaining wall will be instrumented using FBG in-place inclinometers for monitoring horizontal displacements. Brillouin Optical Time Domain Analysis (BOTDA) cables will be adhered on the steel bars to measure their strains. The results of in-place inclinometers and BOTDA cables will be compared to each other and safety assessment of the retaining wall will be conducted. BOTDA cables In-place inclinometer BOTDA cables Retaining wall
(a) Cross section of the inclinometer casing (b) Diagram sketch of the retaining wall
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Figure 6. The retaining wall to be monitored by FBG in-place inclinometers and BOTDA cables CONCLUSION
In this study, the author developed a new type of FBG inclinometer based on the beam theory, which overcomes main drawbacks of traditional inclinometers. This in-place inclinometer can measure the displacements and angles simultaneously. The laboratory calibration results confirmed the effectiveness of this in-place inclinometer. In a landslide monitoring project, two FBG inclinometers has been successfully installed at a slope site and preliminary monitoring results are described. The in-place inclinometers were also applied for monitoring subway settlement during the foundation excavation of Ping An International Financial Centre in Shenzhen and will be used to measure horizontal displacements in a retain wall in Hong Kong. REFERENCES
Dunnicliff, J. (1993). Geotechnical Instrumentation for Monitoring Field Performance, John Wiley & Sons Inc, New York. Hill, K. O., Malo, B., Bilodeau, F., Johnson, D. C. and Albert, J. (1993). Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask. Applied Physics Letters, 62(10), 1035-1037. Ho, Y. T., Huang, A. B., and Lee, J. T. (2006). Development of a fibre Bragg grating sensored ground movement monitoring system. Measurement Science and Technology, 17(7), 1733-1740. Morey, W. W., Meltz, G. and Glenn W. H. G. (1989). Fiber Optic Bragg Grating sensors. Proceedings of SPIE, Boston, 1169, 98-107. Moyo, P., Brownjohn, J. M. W., Suresh, R., and Tjin, S. C. (2005). Development of fiber Bragg gating sensors for monitoring civil infrastructure. Engineering Structures, 27(12), 1828-1834. Othonos, A., and Kalli, K. (1999). Fiber Bragg Gratings: Fundamentals and Applications in Telecommunications and Sensing. Artech House, London. Yin, J. -H., Zhu, H. -H., Fung, K. W., Jin, W., Mak, L. M., and Kuo, K. (2008). Innovative optical fiber sensors for monitoring displacement of geotechnical structures. The HKIE Geotechnical Division 28th Annual Seminar, Hong Kong, 287-294. Yoshida, Y., Kashiwai, Y., Murakami, E., Ishida, S., and Hashiguchi, N. (2002). Development of the monitoring system for slope deformations with fiber Bragg grating arrays. Proceedings of SPIE, 4694, 296-303. Zhu, H. -H. (2009). Fiber optic monitoring and performance evaluation of geotechnical structures. PhD thesis, The Hong Kong Polytechnic University, Hong Kong, China. Zhu, H. -H., Yin, J. -H., Jin, W., and Kuo, K. T. M. (2010). Health monitoring of foundations using fiber Bragg gratin sensing technology. China Civil Engineering Journal, 43(6), 109-115. (in Chinese)
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