Browsing by Author "Wu, Po-Lin"

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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.
Applications and challenges of near surface geophysics in geotechnical engineering
(2015-08-01)
Lin, Chih-Ping
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Lin, Chun-Hung
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Wu, Po-Lin
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Liu, Hsing-Chang
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Hung, Ying-Chun
Geophysical exploration methods have been applied to geotechnical engineering problem since their early developments. However, the results often do not live up to engineers\' expectations. Works still need be done before we see the widespread use of geophysical methods in engineering practice. This study provides an overview of newer developments and applications of near surface geophysical techniques in geotechnical problems. More importantly, the limitations and challenges of current geophysical methods in this context are identified and possible countermeasures are proposed. Near surface geophysical techniques, such as travel time velocity tomography, electrical resistivity tomography (ERT), and multi-channel analysis of surface wave (MASW), have advanced significantly in the last couple of decades within the scientific community. The applications of these methods in Taiwan\' s geotechnical problems are first examined, including assessment of liquefaction potential, evaluation of dam safety, investigation of soil and groundwater contamination, and quality control and assurance of ground improvements. The seismic travel time tomography was selected to examine the integrity of a concrete dam in terms of P-wave velocity. ERT was used to investigate abnormal seepage in earth dams and soil and groundwater contamination. Shear-wave velocity profiles non-destructively obtained by MASW are relevant to many traditional geotechnical problems, in which the quantitative assessment of liquefaction potential and ground improvements were particularly presented. The effectiveness of these applications is discussed from an engineer\'s perspective, and the associated challenges and practical countermeasures are systematically addressed. The velocity imaging of the concrete dam was quite successful and promising, allowing the engineer non-destructively "CT scan" the strength of the dam body. ERT works in a similar fashion for water-related problems. However, the results on abnormal dam seepage and groundwater contamination were less conclusive since the resistivity depends both on pore-water properties and geological factors. So it\'s important to integrate geological background and results from geotechnical investigation or monitoring. In addition, time-lapse geophysical measurements together with geotechnical monitoring reveal additional information and are valuable for geotechnical process control, such as groundwater remediation and ground improvement. Shear-wave velocity, which has a stronger link to geotechnical stiffness property, is now readily measured by MASW. Its applications on assessment of liquefaction potential and ground improvements were quite effective, at least qualitatively. However, MASW is basically a 1-D method and does not provide S-wave velocity image with high spatial resolution. Many limitations and potential pitfalls of geophysical methods exist but are not apparent to end users. They are systematically discussed from an engineer\' s perspective. The non-uniqueness nature and weak link to engineering parameters are common problems of geophysical methods. Reasonable inversion results should be obtained with sufficient a priori information and proper initial models. More conclusive or quantitative engineering interpretation can be achieved by data fusion, time-lapse measurements, and physics-based quantitative modeling. Different assumptions and limitations of investigation depth and spatial resolution are inherent in each geophysical method. They are summarized and made clear to avoid overpromise and over-interpret geophysical results. Some examples of practical countermeasures are illustrated. Finally, researches towards the standardization of geophysical methods are suggested to ultimately promote their widespread use in engineering community. Although successful case studies and innovative applications have strengthened the contribution of new geophysical developments to geotechnical problems, several challenges are identified for more common practice of geophysical surveys in engineering applications from an engineer\'s perspective. These include the lack of standard in data reduction, non-uniqueness of data inversion, limitations of exploration depth and resolution, field conditions violating model assumptions, and the weak link between geophysical parameters and engineering parameters. Relevant researches and practical countermeasures regarding these issues are partially discussed herein. More rational and widespread use of geophysics may be realized through the understanding of the limitations and potential pitfalls of geophysical techniques and researches to overcome them.
Monitoring of Sediment Transport in a Reservoir using Time Domain Reflectometry
(2013-01-01)
Chung, Chih-Chung
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Lin, Chih-Ping
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Wu, Po-Lin
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Hsieh, Sheau-Ling
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Wu, Ching-Hsien
Due to geological weathering and climate change, soil erosion in watershed is becoming a serious problem in Taiwan. Large amount of sediment transports to reservoirs during typhoon events and affects the water quality. Density currents in reservoirs have high sediment concentration. The accumulated sediments have significantly reduced the reservoir capacity. Monitoring of sediment transport in a reservoir plays an important role in reservoir management. The existing techniques for suspended sediment concentration (SSC) monitoring, such as optical and acoustic methods, are sensitive to particle size or limited in measurement range. Furthermore, these techniques may not be cost effective for field monitoring due to the required maintenance and spatial coverage. A new technique based on time domain reflectometry (TDR) was recently introduced. It possesses several advantages including particle-size independence, low-cost transducers, durability, and cost-effective multiplexing. This study modified the TDR technique for better field applicability and demonstrated how it can be deployed as a full monitoring program for reservoir management through a case study in the Shihmen reservoir, Taiwan. Monitoring stations at upstream riverbank and outflow channels were installed with fixed protective structures to provide inflow and outflow sediment-discharge records. To capture the characteristics of density currents, multi-depth monitoring stations were designed and deployed on floats in the reservoir. Some of the data collected during Typhoon Fung-Wong are presented as an example to demonstrate the effectiveness and benefits of the monitoring station.
Performance of 2-D ERT in Investigation of Abnormal Seepage: A Case Study at the Hsin-Shan Earth Dam in Taiwan
(2014-06-01)
Lin, Chih-Ping
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Hung, Yin-Chun
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Wu, Po-Lin
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Yu, Zen-Hung
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交大名義發表
There is a great demand for effective non-destructive methods to examine the interior of reservoir structures, such as dams. The present study was aimed at assessing the performance of electrical resistivity tomography (ERT), a popular non-destructive testing method, to investigate leakage at an earthen dam. Several abnormal leaks appeared on the downstream face after the dam was reconstructed to raise the maximum reservoir water level. Three 2-D ERT surveys were deployed on the left abutment, dam crest, and downstream shell. Periodic measurements were also collected on the downstream shell for time-lapse measurements. To gain confidence and avoid over interpretation, the 3-D effects on 2-D ERT were examined and the results of the 2-D ERT were appraised by forward modeling and synthetic inversion. Integration of ERT results with geotechnical monitoring data revealed the likely mechanism of abnormal seepage. Relational interpretation of time-lapse measurements further supported the hypothesized mechanism. By considering possible 3-D effects, the use of time-lapse measurements, and integration with geotechnical monitoring data, this case study provided new engineering perspectives on how 2-D ERT can be effectively used for seepage investigation.