Study on the performance of geotechnical structures using optical fiber sensors and numerical methods

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Study on the performance of geotechnical structures using optical fiber sensors and numerical methods

 

Author: Pei, Hua-Fu
Title: Study on the performance of geotechnical structures using optical fiber sensors and numerical methods
Degree: Ph.D.
Year: 2012
Subject: Structural analysis (Engineering)
Optical fiber detectors -- Industrial applications.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxx, 244 p. : ill. (some), maps ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2615876
URI: http://theses.lib.polyu.edu.hk/handle/200/6978
Abstract: The performance of geotechnical structures such as dams, slopes, foundations, and tunnels are too complicated to be accurately assessed owing to the complex geological conditions and nonlinear properties of rock and soil materials. To monitor the parameters of geotechnical structures, conventional instruments have been advocated and widely applied in recent decades. However, conventional instruments have several inherent limitations including: electromagnetic interference, a large number of cables for multipoint measurement, signal loss in long distance transmission, low reliability, and poor durability. Since the first Fiber Bragg Grating (FBG) sensor was fabricated in 1978, a significant progress has been made on the commercialization of optical fiber sensing technologies. In the 1980s, a fully distributed sensing technology named Brillouin Optical Time Domain Analysis (BOTDA) has been proposed and developed for measuring strain and temperature. In this thesis project, a variety of optical fiber sensors based on FBG sensing technology have been fabricated and calibrated in laboratory. These newly developed optical fiber sensors were used to measure the strains and displacements of geotechnical structures. In the meantime, the BOTDA sensing technology has also been employed to monitor strain and temperature along three bored piles as retaining walls in a cut slope site in Hong Kong. The major research works and main conclusions are summarized as follows: (a) The BOTDA sensing system was first used for monitoring the strain and temperature along three bored piles during progressive excavation of a cut slope site in Hong Kong. Furthermore, the measured strains were used to calculate the deflection of the bored piles using a previous method. To compare with the calculated deflection, the corresponding simplified finite element models were established. It is found that the trend of simulation results are in good agreement with the trend of measured deflections of three bored piles at initial stages of the excavation based on measured strain data available. (b) A novel inclinometer based on classical indeterminate beam theory was proposed and calibrated in laboratory. The FBG in-place inclinometers were fabricated and installed in a slope site for displacement measurement. The preliminary monitoring results of the system show that the slope had gradually developed certain internal movements. It is found that the newly developed inclinometers can be multiplexed and used for displacement measurement in slopes instead of conventional inclinometers. (c) The FBG sensing technology was employed for measuring strains along Pretensioned High-strength Concrete (PHC) pipe piles under hydraulic jacking. The axial force and averaged shear stress along four tested piles were calculated and discussed using the measured strains. It is indicated that the axial strains are significantly affected by the end resistance and skin friction of piles. To investigate the variation of residual strains, two reloading/unloading cycles were conducted after installation. It is found that the residual strains increase after the first reloading/unloading cycle. However, no significant increase was observed after the second reloading process. It is concluded that the FBG strain sensors can be multiplexed and used for measuring the strains along pipe piles under hydraulic jacking. In addition, the axial strain and the averaged shear strain can be calculated using the measured strains. (d) To investigate the performance of soil nails, both field monitoring and laboratory pullout tests were conducted by using FBG sensing technology. In the field monitoring, vibrating strain gauges and FBG strain sensors were mounted on the soil nails for long term monitoring. The preliminary monitoring results show that FBG strain sensors can be used to replace the conventional strain gauges. A Glass Fiber Reinforced Polymer (GFRP) bar soil nail pullout test was conducted to investigate the performance of soil nail after installation. It is found that the pullout force decreases from the nail head to the nail tip.
In summary, the technical merits of the research project are in the followings: (a) Compared with conventional sensors, the FBG sensors developed in the research project have advantages such as multiplexability, long distance measurement, immunity to electromagnetic interference, high resolution, and long-term reliability. (b) The BOTDA sensing technology was used for monitoring geotechnical structures for the first time in Hong Kong. This technology has inherent advantages including fully distributed measurement, low input power, high sensitivity, and large measurement range for both temperature and strain. (c) The field monitoring of geotechnical structures, such as bored piles, soil slope, PHC pipe piles, and soil nails can be used for performance investigation and safety evaluation of these structures. (d) In addition, the results of FEM simulation have been compared with field monitoring results. In this way, these FEM models are validated and can be used for better simulation and understanding of the performance of geotechnical structures in the field. It can be concluded that the outcomes of the research project have great potential applications in geotechnical engineering.

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