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Applicability and limitations of time domain reflectometry bridge scour monitoring system in general field conditions
(1970-01-01)
Wang, Kai
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Lin, Chih-Ping
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土木工程學系
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Department of Civil Engineering
A real-time and durable system for scour process monitoring with sufficient spatial precision is in pressing need for bridge safety management. In light of this, an innovative bundled time domain reflectometry sensing cable was recently proposed to enhance the time domain reflectometry technique for scour monitoring. However, current development only dealt with the construction of bundled sensing cable and the corresponding new data reduction method. Before it can be put into practical use, issues related to the effect of hydrological conditions, long-distance measurement, and actual field implementation are yet to be investigated. This study used both numerical simulations and laboratory experiments to examine the time domain reflectometry signals in response to both scour and deposition in different water-level conditions. As a result, the first guideline of waveform classification and interpretation is newly proposed to validly determine scour depth under various field conditions. Since field measurements often come with significant signal attenuation from resistance loss of long cable and dielectric and conductive loss in the sensing section, numerical simulations and a series of full-scale experiments were also conducted to assess the time domain reflectometry scour measurement range. The maximum measurement range of the latest time domain reflectometry scour sensing cable was found to be about 6 m. Within this range, the maximum error of scour estimation is within 0.2 m. Considering the new findings, a field time domain reflectometry scour monitoring system using the bundled time domain reflectometry sensing cable was designed accordingly and implemented at a bridge for the first time. The monitoring system successfully captured the scour process during a storm event and revealed some practical issues for future improvement as well.
A comprehensive framework of TDR landslide monitoring and early warning substantiated by field examples
(2019-11-28)
Chung, Chih-Chung
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Lin, Chih-Ping
;
土木工程學系
;
Department of Civil Engineering
Due to the climate change and hazardous geological environment in Taiwan, real-time landslide monitoring for safety of adjacent tribes or infrastructures is much conspicuous. Time domain reflectometry (TDR) is considered as valuable technology for continuously capturing dynamics and locations of shear deformations with high temporal and spatial coverages. In this study, a comprehensive framework was proposed by integrating up-to-date procedure of installation guide, data acquisition, sliding deformation identification, and early warning processing. The advantages of the proposed framework were then examined and substantiated via field cases. The first case is at a large colluvium slope in Lushan, central Taiwan. The colluvium is still active after heavy rainfall events, but limited evidence of sliding characteristic was collected previously by manual inclinometer logging. Therefore, the TDR framework was deployed and multiple landslide surfaces at great depths were timely observed. Movement of colluvium mass was thereby interpreted as a buckling fold. Additionally, creep occurred among the colluvium layer during the monsoon season because of strength weakening influenced by pore pressure fluctuations. Monitoring results of the second landslide case in Jufan, northern Taiwan, reported the dynamics of a thin sliding plane captured with TDR high temporal resolution. This unique behavior was further applied for devising of a novel early-warning strategy in terms of time-variation of TDR response, validating that the comprehensive framework of TDR landslide can effectively contribute to early warning.
3D and Boundary Effects on 2D Electrical Resistivity Tomography
(2019-08-01)
Hung, Yin-Chun
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Lin, Chih-Ping
;
Lee, Chin-Tan
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Weng, Ko-Wei
;
土木工程學系
Electrical resistivity tomography (ERT) is one of the most widely used geophysical methods in geological, hydrogeological, and geo-environmental investigations. Although 3D ERT is now available, 2D ERT remains state-of-the-practice due to its simplicity in fieldwork and lower space requirements. 2D ERT assumes that the ground condition is perpendicular to the survey line and outside the survey line is homogeneous. This assumption can often be violated in conditions such as geologic strikes not perpendicular to the survey line and topographic changes or buried objects near the survey line. Possible errors or artifacts in the 2D resistivity tomogram arising from violating the 2D assumption are often overlooked. This study aimed to numerically investigate the boundary effects on 2D ERT under various simplified conditions. Potential factors including resistivity contrast, depth and size of buried objects, and electrode spacing were considered for the parametric studies. The results revealed that offline geologic features may project onto the 2D tomogram to some extent, depending on the aforementioned factors. The mechanism and implications of boundary effects can be drawn from these parametric studies.
Dielectric Spectroscopy Using Dual Reflection Analysis of TDR Signals
(2019-03-02)
Ngui, Yin Jeh
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Lin, Chih-Ping
;
Wu, Tsai-Jung
;
土木工程學系
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Department of Civil Engineering
Time-domain reflectometry (TDR) has been a powerful tool for measuring soil dielectric properties. Initiating from apparent dielectric constant (Ka) measurement up until apparent and complex dielectric spectroscopies, the embedded information in the TDR signal can be extracted to inspire our understanding of the underlying dielectric behaviors. Multiple full waveform inversion techniques have been developed to extract complex dielectric permittivity (CDP) spectrum, but most of them involved prior knowledge of input function and tedious calibration. This rendered the field dielectric spectroscopy challenging and expensive to conduct. Dual reflection analysis (DRA) is proposed in this study to measure CDP spectrum from 10 MHz to 1 GHz. DRA is a simple, robust, model-free, and source-function free algorithm which requires minimal calibration effort. The theoretical framework of DRA is established and the necessary signal processing procedures are elaborated in this study. Eight materials with different dielectric characteristics are selected to evaluate DRA's performance, by using both simulated and experimental signals. DRA is capable of measuring non-dispersive materials very well, whereas dispersive materials require the assistance of a long-time-window (LTW) extraction method to further extend the effective bandwidth. The DRA approach is suitable for field applications that can only record a limited amount of data points and in-situ dielectric spectroscopy.
Investigation of non-unique relationship between soil electrical conductivity and water content due to drying-wetting rate using TDR
(2019-03-26)
Chung, Chih-Chung
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Lin, Chih-Ping
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Yang, Shi-Hui
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Lin, Jhe-Yi
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Lin, Chun-Hung
The characterization of the relationship between volumetric soil water content (theta) and bulk electrical conductivity (EC) (or electrical resistivity) is of high interest in fields of engineering, fundamental petrophysical, hydrological model parameterization, and quantitative hydrogeophysical interpretations. Based on the relationship, for instance, the broad-area image using electrical resistivity tomography (ERT) can be transformed into theta tomograms as initial/boundary conditions of the slope stability analysis. However, the relationship is affected by the hysteresis, leading the inconsistency in drying-wetting circles. Therefore, this study focused on the factor of soil drying-wetting rate using the proposed time domain reflectometry (TDR) method and related sensors for such characterization. A modified pressure plate apparatus enhanced by TDR was first employed with a relatively low rate in which EC and theta were measured simultaneously, without evident non-unique relationship. A TDR penetrometer was then designed for acquiring the temporal variations of theta and EC in the field. On the basis of certain assumptions, observations showed an apparent dependency of the slope of the theta-EC rating curve on rates, but no hysteresis pattern. Finally, a laboratory controlled fast wetting drying cell was used to increase the response time, proving that both the hysteresis and slopes of the theta-EC rating curves were obviously affected by the soil drying-wetting rate, as a significant finding to the related topics.
A bundled time domain reflectometry-based sensing cable for monitoring of bridge scour
(2019-05-01)
Wang, Kai
;
Lin, Chih-Ping
;
Chung, Chih-Chung
;
土木工程學系
;
Department of Civil Engineering
Scour monitoring is crucial for providing early warning of bridge safety and extending the knowledge of scour process. Traditional methods that rely on underwater instruments encounter difficulties in installation and operation in harsh fluvial environment. Time domain reflectometry (TDR)-based scour sensing wire was recently introduced as a promising technique to improve field deployment and durability. This study focused on further improving the concept of TDR sensing wire for better sensor package and performance. An innovative bundled bottom-up sensing cable was proposed to improve sensor durability and avoid the adverse effect of sensor fouling. The sensor durability is enhanced by twisting two sets of steel strands (as two opposing electrodes for the waveguide) around a coaxial cable into a composite sensing cable. The inner coaxial cable is introduced and armored by the sensing steel strands to direct TDR pulse to the bottom end before connecting to the sensing waveguide formed by steel strands for bottom-up sensing. Furthermore, taking advantage of a new data reduction method by using time-lapse differential waveform, robustness of system calibration and scour estimation is further enhanced. Experimental results revealed the optimal design of bundled TDR sensing cable, effectiveness of the bottom-up sensing approach, and better performance of the differential waveform method.
Multiple Reflection Analysis of TDR Signal for Complex Dielectric Spectroscopy
(2018-11-01)
Lin, Chih-Ping
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Ngui, Yin Jeh
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Lin, Chun-Hung
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交大名義發表
;
National Chiao Tung University
Most dielectric spectroscopy techniques require careful system calibration, tedious measurement, specially designed probes, precise input source, and some even involved complicated inversion models. This paper proposed a rapid, robust, and model-free multiple reflection analysis (MRA) of time-domain reflectometry (TDR) signals to measure the complex dielectric permittivity (CDP) spectrum. The key to MRA approach is to decompose the first top reflection and the subsequent multiple reflections from TDR signal and to compare their spectral ratio (MRA ratio). This ratio was theoretically derived from the transmission line theory and found to be independent of source function and impedance mismatches in the leading sections. Based on this theoretical formulation, the CDP spectrum can be uniquely inverted from the measured MRA ratio through optimization and an iterated initial guess method. Numerical evaluations and experimental verifications had proven that MRA is a reliable algorithm for measuring CDP spectrum covering 10 MHz-I CHz. Factors influencing the reliable frequency region were discussed and recommendations on enhancing CDP measurement was proposed for highly dispersive materials. The MRA approach enables dielectric spectroscopy to be conveniently conducted in both laboratory and field, without complicated system setup and calibration.
Cause and countermeasures for the symmetric effect in borehole-to-surface electrical resistivity tomography
(2018-12-01)
Wang, Haoran
;
Lin, Chih-Ping
;
交大名義發表
;
National Chiao Tung University
Researches and applications of borehole electrical resistivity tomography (BHERT) were mostly about cross-hole ERT (CHERT). Literatures on single borehole-to-surface ERT are limited, although this type of measurement could be useful when only one borehole is available. The undesirable symmetric effect is an important issue in the borehole-to-surface ERT survey. It has been observed that not only the pure in-hole data but also some borehole-to-surface configurations would induce mirrored images in the inversion model. In this study, all the possible borehole-to-surface configurations were classified and investigated for their sensitivity patterns and imaging resolutions, and the root cause of the symmetric effect was revealed to be the condition where one current pole and one potential pole coexist in the borehole. The configurations without symmetric effect suffer from low resolution. The investigation finally led to seeking a balance between resolution enhancement and symmetric effect reduction. Two countermeasures to suppress the symmetrical mirroring were proposed. The combined data inversion approach is based on the capability of each array to resist symmetric effect, while the weighted model stacking technique (WMOST) is derived from the characteristics of inversion models. Both methods were shown to be effective in suppressing the symmetric effect while maintaining sufficient resolution. (C) 2018 Elsevier B.V. All rights reserved.
Application of geophysical methods in a dam project: Life cycle perspective and Taiwan experience
(2018-11-01)
Lin, Chun-Hun
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Lin, Chih-Ping
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Hung, Yin-Chun
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Chung, Chih-Chung
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Wu, Po-Lin
There is a growing demand for using non-destructive geophysical techniques to internally image dam condition and facilitate the early detection of anomalous phenomena. Near surface geophysical techniques have advanced significantly in the last few decades, and can play a significant role in the siting, construction, and operational safety and sustainable management of dams. Application of engineering geophysics in site characterization during feasibility investigation phase is already part of the standard of practice. This paper introduces newer applications of engineering geophysics during construction phase, dam safety assessment, and sustainable management, including quality control of compacted soils, investigation of abnormal leakage in an earth darn, evaluation of an aged concrete dam, geophysical health monitoring for a newly-constructed dam, and monitoring of sediment transport for sediment management. The applications were presented with more emphases on the needs of dam engineering and adapting appropriate geophysical methods to make assessment more effective and consequential. The collage of these case studies is to broaden the view of how geophysical methods can be applied to a dam project throughout a dam's life cycle and strengthen the linkage between geophysical surveillance and engineering significance at all stages. (C) 2018 Elsevier B.V. All rights reserved.
Quantitative Interpretation of Surface Wave Testing for Assessment of Ground Improvement by Jet Grouting
(2017-01-01)
Lin, Chih-Ping
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Lin, Chun-Hung
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Chang, Yu-Cheng
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Chien, Chih-Jung
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防災與水環境研究中心
Jet grouting technique is special in that the ground becomes highly heterogeneous after installing the improvement columns. The surface wave method was recently shown to hold good promise to assess the average property of the improved ground and evaluate the area replacement ratio. Even though the improved ground was highly heterogeneous along the seismic survey line, the surface wave method worked well and yielded a homogenized shear wave velocity profile. The increase of shear wave velocity after ground improvement was found to be correlated with the area replacement ratio. This study further examined the quantitative relation between the percentage increase of shear wave velocity and the area replacement ratio. The effect of offline sampling (perpendicular to survey direction) was also investigated. 3D spectral element method was utilized for the numerical investigation. Numerical results show that the offline sampling range is about 75%-85% of the wavelength. Hence, it should be noted when planning survey lines. Furthermore, a positive correlation between replacement ratio and velocity increase exist and the difference caused by the velocity contrast between soils and improved columns could be neglected as the velocity contrast becomes greater than about 5. An empirical predictive model between the velocity increase percentage and area replacement ratio was established based on the numerical simulations.

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