Development and evaluation of new fiber optic sensors for geotechnical applications

Abstract

Geotechnical instrumentation plays an essential role in performance evaluation of geotechnical structures and real-time monitoring of geological hazards. Since 1930s, conventional instruments have already been widely used in this aspect. However, they have several inherent drawbacks in engineering practice, such as poor durability, sensitivity to electromagnetic interference, signal loss in long distance sensing, and a large number of cables for multi-point measurement. In recent years, fiber optic sensing (FOS) technology, especially fiber Bragg grating (FBG) sensing technology, has attracted great attention of engineers due to its unique characteristics. However, only limited studies have extended the applications of FOS technology to geotechnical structures and geological hazards. Therefore, this study aims to develop and evaluate a variety of new fiber optic sensors based on FBG sensing technology for geotechnical applications. In the first part of this study, a new fiber optic monitoring system is elaborately designed to monitor the process of geophysical flows impacting on a flexible barrier. Its primary tasks are to monitor (a) dynamic response of a flexible barrier under impact loads and (b) basal stresses generated by geophysical flows. Three instruments based on FBG sensing technology are specially developed to fulfil the requirements. Firstly, new FBG mini tension link transducers are first designed for measuring the forces between rings of a ring-net barrier. A flexible barrier monitoring system is consequently formed for monitoring dynamic response of a flexible barrier under impact loads. The FBG mini tension link transducers were calibrated through the laboratory tests and the flexible barrier monitoring system was examined by the single boulder impact tests conducted in a large-scale physical model. Secondly, new FBG basal friction transducers are designed for monitoring basal friction stress generated by geophysical flows. Calibration tests, including calibration of axial strains and calibration of friction stress, were carried out in a large-scale shear box. Thirdly, new FBG large total pressure cells for monitoring basal normal stress generated by geophysical flows are developed based on the thin plate theory and calibrated in laboratory. Finally, the developed fiber optic monitoring system, consisting of the specially designed instruments, high-speed cameras, high-frequency data-loggers, and computers, was established in the large-scale physical model. A dry granular flow impact test and a debris flow impact test were performed in the instrumented physical model. The results demonstrate that the fiber optic monitoring system has good performance in dynamic monitoring of geophysical flow impacts, which greatly helps understand the dynamic behavior and impact mechanism of geophysical flows on flexible barriers. In the second part of this study, a novel FBG effective stress cell for direct measurement of effective stress in saturated soil using only one diaphragm without measuring pore-water pressure is designed, fabricated, and calibrated. Physical model tests were performed to evaluate the workability and accuracy of the FBG effective stress cell. The measurement results obtained by the FBG effective stress cell is compared with the effective stress calculated from the measured total stress and pore-water pressure by conventional transducers. It is demonstrated that the FBG effective stress cell has good accuracy for direct measurement of the effective stress in a saturated soil with high reliability. The main contributions of this study include: (1) development of special fiber optic sensors for geotechnical applications; (2) development and verification of new methods for converting the shifts of wavelength of FBG sensors into other parameters (i.e. force, friction stress, and pressure); (3) special design of a fiber optic monitoring system for real-time monitoring of the process of geophysical flows impacting on a flexible barrier; (4) development of an effective approach for measuring the forces between rings of a flexible ring-net barrier and thus monitoring dynamic response of the flexible barrier under impact loads; (5) development of a simple method for calculating the impact force on the flexible barrier based on the forces between rings measured by the FBG mini tension link transducers; (6) development of a novel FBG effective stress cell to directly measure the effective stress in saturated soil. It can be concluded that the new findings of this study are of great significance for geotechnical applications.