Laboratory Studies on Slope Stability Monitoring Using Distributed Fiber-Optic Sensing Technologies Hong-Hu Zhu, Bin Shi, Jun-Fan Yan, Cheng-Cheng Zhang, Jie Zhang, and Zhan-Pu Song
Abstract
The advances in distributed fiber optic sensing (DFOS) technologies enable automatic, remote and long-distance slope monitoring and early-warning of potential geological disasters. Compared with conventional geotechnical instruments, the fiber optic sensors have a number of advantages such as higher accuracy and repeatability, better durability, and enhanced integration capability. In this paper, the quasi-distributed Fiber Bragg Grating (FBG) and fully-distributed Brillouin Optical Time-Domain Analysis (BOTDA) sensing technologies are applied for monitoring of slope stability problems in laboratory model tests. The sensing principles and the implementation methods are introduced, followed by two case studies. The fiber optic sensors were embedded in the model slopes for strain monitoring of a soil mass during seepage and surcharge loading, respectively. The reliability of the DFOS based slope monitoring systems has been verified through the analyses of the strain monitoring results. Keywords
Fiber optic sensing Slope monitoring Geotechnical instrumentation
Introduction Geotechnical instrumentation plays an important role in evaluating slope stability and identifying potential landslide hazards. Currently, typical shortcomings associated with
H.-H. Zhu (*) School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing 210046, China Nanjing University High-tech Institute at Suzhou, 150 Ren’ai Road, Suzhou 215123, China e-mail:
[email protected] B. Shi J.-F. Yan C.-C. Zhang Z.-P. Song School of Earth Sciences and Engineering, Nanjing University, 163 Xianlin Avenue, Nanjing 210046, China e-mail:
[email protected];
[email protected];
[email protected];
[email protected] J. Zhang Department of Geotechnical Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China e-mail:
[email protected]
many conventional monitoring technologies of slope stability include low long-term accuracy, poor durability, and limited integration capability. Additionally, the collected measurement data from discrete locations can hardly reflect the overall stability of a slope. The advances in distributed fiber optic sensing (DFOS) technologies enable automatic, remote and long-distance slope monitoring and early-warning of geological disasters (Dunnicliff 1993; Shi et al. 2003). These technologies have been successfully applied for the health monitoring of a variety of civil infrastructures, such as bridges, high-rise buildings, and tunnels (Shi et al. 2003; Zhang et al. 2006; Wang et al. 2013). The fiber optic sensors not only have the advantages of immunity to electromagnetic interference, tiny size, high accuracy and repeatability, and excellent durability, but also can be integrated to form a quasidistributed or fully-distributed fiber optic sensing network (see Fig. 1). In recent years, the potential of the DFOS-based slope monitoring system has been recognized all over the world (Yoshida et al. 2007; Ho et al. 2006; Iten et al. 2008;
K. Sassa et al. (eds.), Landslide Science for a Safer Geoenvironment, Vol. 2, DOI 10.1007/978-3-319-05050-8_97, # Springer International Publishing Switzerland 2014
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Distributed Fiber Optic Sensing Technologies Quasi-Distributed Fiber Bragg Grating Technology The photosensitivity in optical fiber was first discovered by Hill et al. (1978). It was found that the Bragg wavelength of the light that a Fiber Bragg Grating (FBG) sensor reflects is strain and temperature dependent. A change in strain or temperature will alter the Bragg wavelength and can be formulated as follows (Othonos and Kalli 1999) ΔλB ¼ 1 λB
peff Δε þ ðα þ ξÞΔT
ð1Þ
where ΔλB is the change in the Bragg wavelength due to strain and temperature changes; λB is the original Bragg wavelength under strain free and 0 C condition; peff is the photo-elastic parameter; and α and ξ are the thermal expansion and thermo-optic coefficients, respectively. With the above features, the FBG sensors with different Bragg wavelengths can be connected in series to form a quasi-distributed sensing array with high accuracy. The FBG sensor is by far the most commonly used fiber optic sensor in civil engineering, as well as in fiber-optic communication as the optical filter.
Fully-Distributed Brillouin Optical Time-Domain Analysis Technologies
Fig. 1 Comparison of different types of slope monitoring technologies. (a) In point-measurement, (b) quasi-distributed measurement, (c) fullydistributed measurement
Shi et al. 2008; Wang et al. 2009; Pei et al. 2011; Zhu et al. 2011, 2012). However, the performance evaluation of slopes based on the DFOS monitoring results is quite different from conventional methods and requires further investigation. In this paper, the DFOS technologies have been applied to strain monitoring within small-scale model slopes in two commonly encountered conditions. The strain variations measured by the fiber optic sensors indicate the evolution of stability condition of the model slopes under increased surcharge loading and water seepage process, respectively. The effectiveness of the DFOS system in monitoring slope stability is verified.
In the sensing technology of Brillouin optical time-domain analysis (BOTDA), the Brillouin frequency shift of the scattering light of a single-mode optical fiber has a linear relationship with the applied strain and temperature (Horiguchi and Tateda 1989). This relationship can be expressed by (Bernini et al. 2002) vB ðε; T Þ ¼ vB ðε0 ; T 0 Þ þ þ
∂vB ðε; T Þ ðε ∂ε
∂vB ðε; T Þ ðT ∂T
T0Þ
ε0 Þ ð2Þ
where vB(ε, T) and vB(ε0, T0) are the frequency shift of the Brillouin scattering light before and after the measurement, respectively; ε and ε0 are the axial strain before and after the measurement, respectively; T and T0 are the temperature before and after the measurement, respectively; and the coefficients ∂vB(ε, T)/∂ε and ∂vB(ε, T)/∂T are 0.05 MHz/με and 1.2 MHz/ C, respectively. The measured strain and temperature is expressed as the averaged value
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Fig. 3 Horizontal strain monitoring results of the slope mass subjected to seepage Fig. 2 Photo of the model test for Case 1
over its spatial resolution. Using BOTDA technology, the strain and temperature generated in optical fibers of the length up to 25 km can be measured as distributed in the longitudinal direction (Neubrex Co. Ltd 2011).
Applications to Laboratory Model Tests Case 1: Strain Monitoring of a Soil Slope Under Seepage In order to investigate the influence of reservoir operation on the deformation pattern of adjacent slopes and verify the effectiveness of FBG technology in slope stability monitoring, a small-scale model test of soil slope was conducted in Key Laboratory of Distributed Sensing & Monitoring Technology in Infrastructure Engineering of Suzhou, China. This model test was performed under 1 g condition in a steel chamber with a length of 3 m and a width of 1.5 m. Figure 2 shows the setup of the model test. The model slope has a slope angle of 45 . During model construction, a single-mode optical fiber containing nine FBG strain sensors connected in series was embedded in the slope mass. The FBG sensors were coated using a tight-buffer PVC coating so as to protect them from damage during installation. An sm125 FBG optical sensing interrogator was employed to record real-time data. During testing, the groundwater level inside the slope was varied by changing the water levels in the water chambers at the two sides of the model slope, so that water seepage inside the slope mass was induced. Figure 3 presents the horizontal strain results measured by three embedded FBG sensors during the model test. The monitoring results show that: 1. When the water level inside the slope mass was manually increased, the measured horizontal strains
increased at a considerable high rate. This strain variation corresponds to increased horizontal mass movement, which is normally considered as a sign for slope stability deterioration. 2. When the water level of the reservoir fell suddenly, the horizontal strains accumulated rapidly and reached their peaks. This indicates that the slope movement, especially the horizontal component, is quite evidently affected by the drawdown of the reservoir level. For a slope with marginal stability, seepage-triggered slope instability is likely to occur. 3. After the rapid drawdown of reservoir, water level inside the model slope dropped gradually. There were considerable plastic strains in the slope mass after the seepage circle/water level fluctuation circle. This experimental study verifies that the distributed fiber optic sensors can capture the strain variations within the model slope subjected to seepage, which is important for slope stability monitoring.
Case 2: Strain Monitoring of a Loaded Soil Nailed Slope To verify the effectiveness of the fully-distributed BOTDA technology for slope stability monitoring with surface surcharge, a laboratory slope model test was conducted. The model slope has a slope height of 0.8 m and a base height of 0.5 m. The surcharge loading was applied on the slope crest, as shown in Fig. 4. The BOTDA sensing network was established to monitor strain distributions within the model slope. During model construction, three layers of 0.9 mm-diameter sensing fibers were horizontally embedded in the slope mass. Eight characteristic points were prepared along the fiber in order to facilitate the identification of the boundary of each segment. For every point, a 1-m-long sensing fiber was looped and packaged in a plastic box for temperature compensation.
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Fig. 4 Photo of the model test for Case 2
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34kPa 26kPa
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Figure 5 presents the fiber optic monitoring results. In this figure, the line separating the active and passive zones is the location of the critical slip surface computed by limit equilibrium analysis performed using SLOPE/W. The monitoring results show that the measured strain can reasonably reflect the deformation behavior of the model slope. With the increase of loading, the strains increased as expected. When the critical condition of slope stability was imminent, strain inhomogeneity within the model slope became obvious. This phenomenon indicates that shear strains accumulated within the soil mass and the slope slip surface gradually formed. According to the monitoring results, the strains of the passive zone at a lower slope elevation were large while the strains of the active zone were relatively small. For the slope mass at a higher elevation, the active zone of the slope had larger strains than the passive zone. The strain distribution pattern seems to be dependent on the location of the slope slip surface, which is reasonable.
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In this paper, the working principles of two DFOS technologies and their applications in laboratory-scale model tests have been presented. The following conclusions are drawn: 1. The model test of slope subjected to seepage shows that the embedded FBG sensors can capture the strains induced by seepage within a slope mass effectively. The proposed installation and protection methods are proved to be sufficient and effective. 2. The distributions of horizontal strain measured by the BOTDA strain sensing fibers in the surface loaded model slope indicate the development of potential slip surface in the slope. 3. The capability of the DFOS technology to provide valuable data for performance evaluation of slopes with high accuracy is verified. Slope stability assessment can be implemented based on the distributed strain monitoring results. Acknowledgments The authors gratefully acknowledge the financial support provided by National Basic Research Program of China (973 Program) (No. 2011CB710605), National Natural Science Foundation of China (Nos. 41102174, 41302217), Open Fund of the State Key Laboratory of Geohazard Prevention and Geoenvironment Protection (No. SKLGP2012K011), and Suzhou Science and Technology Development Program (No. SYG201213).
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Fig. 5 Strain monitoring results of the slope mass subjected to increased surcharge loading. (a) Monitoring layer at 0.6 m elevation, (b) monitoring layer at 0.85 m elevation, (c) monitoring layer at 1.1 m elevation
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