A calibration test of karst collapse monitoring device by optical time domain reflectometry (BOTDR) technique

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A calibration test of karst collapse monitoring device by optical time domain reflectometry (BOTDR) technique

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Title:
A calibration test of karst collapse monitoring device by optical time domain reflectometry (BOTDR) technique
Alternate Title:
NCKRI Symposium 2: Proceedings of the Thirteenth Multidisciplinary Conference on Sinkholes and the Engineering and Environmental Impacts of Karst
Creator:
Guan, Zhende
Jiang, Xiaozhen
Gao, Ming
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University of South Florida
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English

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Conference Proceeding
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Brillouin Optical Time Domain Reflectometry (BOTDR) is a distributed fiber optic strain sensing systems based on Brillouin scattering. This technique may potentially become a useful tool to monitor and predict karst collapse, especially for linear infrastructure such as roads, highways, and railways. This paper introduces a calibration device which is used to establish the relationship between fiber deformation and underlain soil -cave dimension. Based on the deformation characteristics of the sinkhole collapse, the mechanical relation between soil body and sensing fiber is analyzed, and a simplified model of collapse is proposed for testing design. The experimental tests are carried out through the designed equipment to investigate the effect of the sinkhole's size and the overburden stratum's thickness on embedded optical fibers. Firstly, the sinkhole formation process was stimulated with the orderly changes in load on the optical fiber. Secondly, the impact of the changes of sinkhole size on the sensing fiber monitoring was analyzed. It shows from the experiment results that the strain change in the sinkhole formation process can be monitored by distributed optical fiber sensing technology and the sinkhole size can be reflected through the optical fiber strain range. Besides, the sensibility of coated optical fiber in sinkhole collapse monitoring tests varies between different types of optical fibers. Due to the effective response of the distributed optical fiber sensing technology to sinkhole forming and evolving, it can be adopted in the monitoring for potential sinkhole collapse. -- Authors
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K26-00120 ( USFLDC DOI )
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Description
Brillouin Optical Time
Domain Reflectometry (BOTDR) is a distributed fiber optic
strain sensing systems based on Brillouin scattering. This
technique may potentially become a useful tool to monitor and
predict karst collapse, especially for linear infrastructure
such as roads, highways, and railways. This paper introduces a
calibration device which is used to establish the relationship
between fiber deformation and underlain soil -cave dimension.
Based on the deformation characteristics of the sinkhole
collapse, the mechanical relation between soil body and sensing
fiber is analyzed, and a simplified model of collapse is
proposed for testing design. The experimental tests are carried
out through the designed equipment to investigate the effect of
the sinkhole's size and the overburden stratum's thickness on
embedded optical fibers. Firstly, the sinkhole formation
process was stimulated with the orderly changes in load on the
optical fiber. Secondly, the impact of the changes of sinkhole
size on the sensing fiber monitoring was analyzed. It shows
from the experiment results that the strain change in the
sinkhole formation process can be monitored by distributed
optical fiber sensing technology and the sinkhole size can be
reflected through the optical fiber strain range. Besides, the
sensibility of coated optical fiber in sinkhole collapse
monitoring tests varies between different types of optical
fibers. Due to the effective response of the distributed
optical fiber sensing technology to sinkhole forming and
evolving, it can be adopted in the monitoring for potential
sinkhole collapse. --
Authors



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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 71 The most effective means to avoid geological disasters is prevention. Therefore, monitoring and early warning of karst collapse are particularly important. Current water or air pressure changes in underground karst collapse abnormalities, but due to its strict working environment, limited detection depth, professional monitoring TDR technique has many advantages, such TDR cannot be used to monitor the formation process of karst collapse because it receives only the signal from force, tension or both combined. Monitoring water and air pressure changes in underground karst system can only forecast the collapse risk of the karst fracture around the monitoring points. But it cannot point out Therefore, traditional monitoring methods cannot meet the demand for sinkhole collapses monitoring or system, which can detect temporal and spatial changes continuous basis (Tang et al., 2006). Nevertheless, there are still many problems in the application (Jiang et al., 2006; Li et al., 2005; Meng et al., 2011). According to the deformation characteristics obtained from sinkhole collapse modeling and calibration testing, we analyzed the mechanic relation between the soil and sensing Abstract on Brillouin scattering. This technique may potentially become a useful tool to monitor and predict karst collapse, especially for linear infrastructure such as roads, highways, and railways. This paper introduces a calibration device which is used to establish the characteristics of the sinkhole collapse, the mechanical design. The experimental tests are carried out through the designed equipment to investigate the effect of the sinkhole's size and the overburden stratum's thickness on process was stimulated with the orderly changes in load analyzed. It shows from the experiment results that the strain change in the sinkhole formation process sensing technology to sinkhole forming and evolving, it can be adopted in the monitoring for potential sinkhole collapse. Preface Karst is widely distributed in Southwest China Along of human activities, geological disasters related to karst have become prominent, especially karst collapse A CALIBRATION TEST OF KARST COLLAPSE MONITORING DEVICE BY OPTICAL TIME DOMAIN REFLECTOMETRY(BOTDR) TECHNIQUE Guan Zhende, Jiang Xiaozhen, Gao Ming Institute of Karst Geology, CAGS, Qixing 50 Rd., Guilin, GuangXi 130026 P.R. China, guanzd@karst.ac.cn

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 72 et al., 2006) when the incident pulse wavelength is temperature changes. The experimental temperature variation is less than 5 C, so the temperature effects were not considered. (OTDR) technique. According to the OTDR principle, the scattering position can be determined by measuring the scattered laser echo time. The distance between the ( 2 where: a pulse laser. According to Equation (1), the axial strain distribution Shi et al., 2005). According to Equation (2), the position where strain occurred can be calculated. The karst collapse monitoring model The working principle of BOTDR for collapse monitoring is based on the development of a karst soil void that manifests as deformation of the under the load coming from overlying stratum due to the development of a soil void. The location, scale and development of soil void can be well understood based on the analysis of temporal and spatial variation sinkhole collapse. Monitoring principle of optical fiber sensing technology on three spectroscopic analysis methods including Rayleigh scattering, Brillouin scattering and Raman scattering. Rayleigh scattering is an elastic scattering Brillouin and Raman scattering are nonelastic scattering 2006). Brillouin scattering arises from the interaction between optical and acoustic waves propagating in Brillouin scattering frequency and the temperature or temperature or axial strain can be calculated according to the amount of the frequency drift in the optical a temperature sensor is adopted to offset the drift by temperature change. The relationship between the center frequency drift and (1) where: Figure1. The principle of BOTDR. ( )

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 73 overburden soil; According to Equations (4) and (5): (6) Deformation compatibility of fiber and soil The formation of soil void is the result of varied superimposing collapse factors, which causes overlying soil deformation or potential collapse. The key to the deformation. Reasonable distribution of the sensing is important during soil void monitoring. The placement features of karst collapse in the monitoring region. overburden stratum. During the development of collapse, deformation of the soil mass occurs gradually, and also deformation called compatible deformation. this interaction will be explained by mechanical analysis in the following discuss. (dT) can be demonstrated as below: where: d Thus (Li et al.): ( 4 smaller than maximum static friction, so sliding friction is taken in the analysis. ( 5 where: = = N= .G = h Figure2. The mechanic relationship between soil and optical fiber. As explained in the theory mentioned above, when thickness and bulk density, stress is directly proportional happens while soil is deformed. Simplification of collapse monitoring model As soil void develops, incumbent soil load and void scale are critical to the magnitude and distribution of the stress around the developing void. According to the key monitoring factors and the deformation simulation experiment system was designed. During the formation of soil voids, the friction imposed on optical fiber at the edge of void and its influence area are changeable. Thus, optical fiber fixation should be considered in the model design (Liu et al., 2010; Yao et al., 2005). Intertwist, one of the fixation methods, is adopted which can express the way how friction varies with loads effectively

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 74 Experiment process respectively. The positions were recorded by the labels at 1068.75m. The loading point deformation and strain loading and unloading. Test data processing and analysis According to its principle, the strain measured by the strain instrument is the integrated strain within 1m starting from the monitoring point (Wu et al., 2005; Yue et al., 2007). Taking the value got from connectivity test difference between the loading test value and the initial value. According to the strain change distribution as shown in be increased to obtain the enough friction The strain Experiment system with the load. This can be simulated by an experiment supporter, dial indicator and vertical loading system. And the formation of soil voids with 0.5~2.5m span under different loads can be simulated as well. Test and analysis Two types of experiments, variable load in certain distance and variable distance under certain load out, respectively. produced by the ANDO corporation of Japan was used to measure the strain distribution in the optical fiber. The main index of the instrument is shown in Table 1. Test under stepwise variable load in certain distance The experiment simulates the load changes of the soil to analyze the changes of the axial strain and the optical Figure 3. Simplified sinkhole model. Technical Index Optional parameter Measure distance 1, 2, 5, 10, 20, 40, 80km Pulse width 10ns 20ns 50ns 100ns 200ns Dynamic range 0.004%(2s) 2dB 6dB 10dB 15dB 8dB 11dB Length resolution 1m 2m 5m 11m 22m Strain test accuracy 0.004%(2s) ( 0.01%) ( 0.005%) Strain test repeatability <0.04% <0.02% Table 1. AQ8603(BOTDR)Technical Index. In the experiment, .004%(2s) strain, 10cm sampling distance and 1m length resolution were adopted. Fiber connectivity was tested before the experiment.

PAGE 5

13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 75 The experiment process experiment simulates the sinkhole span starting from 1 meter to 2.5 meter with 0.5 m interval. Test data processing and analysis strain, there is a mechanical analysis about the certain load experiment (Figure 8). (7) where: point; is suitable to be used for the soil bearing low pressure or unloaded. Following the soil void overburden collapse, the mass this process can be simulated by unloading experiment (Figures 6 and 7). Unloading experiment demonstrates that sinkhole collapse, and the position of the coverboard loading overburden stratum thickness of an incipient sinkhole, the to avoid the elastic modulus value exceeding the test range. Test under variable distance in certain load axial strain changes in different spans of the sinkhole by applying certain load. Figure 4. The loading point strain change (GFRP). Figure 5. The loading point strain change (Ordinary optical fiber). Figure 6. The strain change after unloading (GFRP). Figure 7. The strain change after unloading (Ordinary optical fiber).

PAGE 6

NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 76 and the load. Conclusion In the process of soil void formation and subsequent sinkhole collapse, axial strain and deformation of the technology to monitor the location, size and collapsing process of the void in soil. effects on the strain value, which must be understand and choose appropriately. the deformation coordination between the soil and the response sensitivity. According to Figure 9, the maximal vertical displacement of the loading point is 55 mm and the minimum distance small deformation, the hypotenuse is approximately enough. (8) Not considering the material factor, the relationship the included angle is inverse proportion. For ordinary 1m to 2.5 m (Figure 10). Figure 8. Loading section stress analysis Figure 9. Vertical displacement in different sinkhole span. Figure 10. Strain change in different sinkhole span (Ordinary optical fiber). Figure 11. Strain change in different sinkhole span (GFRP).

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13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 77 to structure health monitoring. China Civil model, it still demonstrates that the strain characteristics useful tool for predicting sinkhole collapse. References of monitoring sinkhole collapse by using BOTDR Li K, Shi B, Tang C, et al. 2010. Feasibility research on soil deformation monitoring with distributed Li Y, Zhu P, Lei M, et al. 2005. Monitoring technique and methods of the karst collapses. Carsologica Liu J, Shi B, Zhang D, et al. 2006. Experimental study fixation technique of fiber based on BOTDR. Chinese Journal of Sensors and Actuators problem in monitoring and predicting sinkhole measurement for tunnel health diagnosis. China Journal of Rock Mechanics and Engineering Tang T, Zhu Y, CAI D, et al. 2006. Experimental sensing. Chinese Journal of Rock Mechanics and study on the measuring characteristics of BOTDR for structure health monitoring. China Civil

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NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE 78


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