Development of two optical fiber sensing technologies and applications in monitoring geotechnical structures

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Development of two optical fiber sensing technologies and applications in monitoring geotechnical structures


Author: Xu, Dong-sheng
Title: Development of two optical fiber sensing technologies and applications in monitoring geotechnical structures
Degree: Ph.D.
Year: 2014
Subject: Optical fiber detectors.
Geotechnical engineering.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Civil and Environmental Engineering
Pages: xxv, 299 p. : ill. ; 30 cm.
Language: English
InnoPac Record:
Abstract: The recent advances in optical fiber sensing techniques have inspired researchers to explore potential applications in geotechnical engineering due to the inherent merits that optical fiber sensors possess such as higher accuracy and stability, small size, light weight, and resistance to electromagnetic interference (EMI) and corrosion. However, optical fiber sensors have limitations for applications in geotechnical engineering since these sensors are fragile, susceptible to the harsh environment and long-term stability, costly, and little matured products available. This thesis focuses on the development, implementation, and evaluation of two advanced optical fiber sensing technologies, i.e. Fiber Bragg Grating (FBG) and Brillouin Optical Time Domain Analysis (BOTDA), by overcoming their limitations for applications in geotechnical engineering. The first part of the thesis is devoted to explore the applications of the FBG sensing technology for measuring small strains of soil specimens and lateral displacements inside the soil mass. Firstly, a new FBG based local displacement transducer (FBG-LDT) is designed, fabricated and installed in a modified triaxial apparatus (MTA) to measure the local small strains of a soil specimen. The MTA is modified to measure soil strains ranging from very small to large strains by incorporating it with bender element sensors, FBG-LDT, and external linear variable differential transformers (LVDT). It is found that the FBG-LDT performs satisfactorily up to a maximum small strain level of 0.3%. The data provide evidence that the FBG-LDT is capable of measuring small strains with greater accuracy as compared to electrical circuit based external LVDT. Secondly, a flexible FBG sensing beam has been designed, fabricated and verified by using a shaking table test to measure the dynamic lateral displacements inside the soil mass. The sensing beam was calibrated with laser displacement sensors (LDS) and validated in a shaking table test. The results have proved that the FBG sensing beam is fairly accurate for measuring lateral displacements.
The second part of the thesis aims to examine the application of FBG sensors in the harsh environment such as in the asphalt pavement structures with high temperature and pressure. By considering this circumstance, an asphalt strain sensor has been designed based on a robust nylon bar protected by high temperature resistant resin with transverse stainless steel anchors at each end. In addition, a sensing beam sensor has been designed to measure the settlement by using quasi-distributed FBG sensors array. A physical model was constructed with instrumentation of two sensing beam sensors, six asphalt strain sensors and four sets of distributed strain sensors. Static loading tests were carried out and the results have indicated that the FBG sensors and distributed strain sensors can provide data for accurate and efficient determination of the strains and deformations in the asphalt pavement. A 3D finite difference model has also been established to examine the pavement response for the optimization of the pavement design. The final part of the thesis is focused on the development of the novel BOTDA technique for distributed strain and temperature sensing. Two field application projects are presented by using distributed optical fiber sensors to measure local strains and deformations of bored piles and GFRP soil nails, respectively. In the first research project, three bored piles were instrumented with distributed strain sensing fibers, temperature sensing fibers, and conventional inclinometers. For each bored pile, two sets of optical fibers were installed in diametrically opposite sides to determine the distributed axial strain and bending strain profiles by the averaging and differencing two strains, respectively. The behavior of the bored pile wall under excavation was analyzed based on both the measured results and finite element (FE) models. The FE model was further used to examine the effect of several parameters such as stiffness of the bored pile, excavation depth, embedded wall length, and undrained shear strength of the soil. In the second project, the distributed optical fiber sensors were used for quality evaluation and performance monitoring of GFRP soil nails in an excavated slope in Hong Kong. The optical fiber sensors and the installation method were calibrated and verified with commonly used strain gauges in the laboratory. Then, the distributed optical fibers were used to instrument three typical GFRP soil nails in which one was examined in a load-displacement test and other two were monitored during excavation stages. The measurement results have been compared and validated against field observations and finite element analysis results. It can be opined that the distributed optical fiber sensors have a great potential in quality evaluation and performance monitoring of geotechnical structures. Research work reported in this thesis has made contributions to (i) design and manufacturing of special optical fiber sensors for geotechnical applications, (ii) development and utilization of calibration devices and methods for the newly developed sensors, (iii) development and verification of new methods for converting wavelength changes (or strains) to other engineering parameters (displacements for example), and (iv) innovative applications of the two types of optical fiber sensors for wide applications in laboratory tests and field geotechnical projects.

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