| Author: | Lin, Shaoqun |
| Title: | Refined monitoring using the OFDR-based distributed fibre optic sensor : from laboratory tests to site monitoring |
| Advisors: | Yin, Jian-hua (CEE) Yin, Zhen-yu (CEE) |
| Degree: | Ph.D. |
| Year: | 2023 |
| Subject: | Fiber optics Optical fiber detectors Structural stability Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xxii, 230 pages : color illustrations |
| Language: | English |
| Abstract: | The distributed fiber optic sensor (DFOS) based on optical frequency-domain reflectometry (OFDR) has drawn increasing attention due to its high spatial resolution of up to sub-millimeters. It can provide a fine strain field and detect local features. However, its potential has not been fully investigated in both laboratory tests and site monitoring due to the lack of understanding of the strain transfer theories underlying different strain profiles, especially when cracks occur in the host material. In addition, the identification of local defects is critical for assessing the performance of underground structures. The capability of OFDR-based DFOS for local feature detection holds promise as a potential solution, but it has not been extensively studied thus far. This research has therefore been initiated to validate the OFDR-based DFOS technique and investigate the issues mentioned above. Firstly, the reliability and accuracy of OFDR-based DFOS were validated by a series of laboratory tests. During this, a novel sensor layout and corresponding installation process were proposed to measure both axial and lateral strains of cylindrical specimens simultaneously using a single optical fiber. The strain transfer between the DFOS and the specimen was investigated to solve the boundary effect in measurements and to explore crack opening displacement (COD) quantification. The experimental results indicated that OFDR-based DFOS can capture the development of strain localization and failure sequence of a sandstone specimen and quantify the propagation of CODs on the surface of a granite specimen. Secondly, the capacity of OFDR-based DFOS to quantify COD under both micro and macro cracks was further investigated. The interfacial shear stress-slip relationship between the bare fiber and its coating layer before and after interfacial debonding was obtained through a splitting test. The relationship was simplified and utilized to establish an interfacial-fracture-energy-based analytical model for COD quantification. The performance of the proposed model was validated using experimental data for both single and multiple cracks. Thirdly, site monitoring using OFDR-based DFOS was performed after laboratory validation. Fibre optic cables were embedded into three diaphragm wall panels of a twin-circular shaft excavation. The development of the vertical curvatures, hoop forces and circumferential bending moments during various construction phases was investigated through the monitoring results. The release of internal forces in wall panels due to partial structure demolition was detected, thanks to the high spatial resolution. Three-dimensional numerical modelling with different construction settings highlighted the importance of monitoring the development of circumferential forces and bending moments in walls supporting circular or multi-cell excavations. Finally, efforts were dedicated to the detection of local defects in underground structures using the measurements of OFDR-based DFOS. Strain spikes induced by cracks and cavities were obtained from a four-point bending beam test and a loading test of a reinforced concrete specimen with cavities, respectively. Strain spikes measured in diaphragm wall panels and a bored pile were also preliminarily interpreted based on the characteristics of spikes with different causes. To effectively process the vast amount of data obtained, a support vector machine (SVM)-based method was introduced to automatically classify three different types of strain profiles, including crack-induced spike and non-crack induced spike, and non-spike strain profile. |
| Rights: | All rights reserved |
| Access: | open access |
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