Application of novel fibre optic sensors to monitor geo-structures

Abstract

Failures in geo-structures and infrastructure impose significant financial, environmental, and social costs worldwide. These failures affect many parts of the geo-infrastructure mainly in form of total or partial collapse of the ground. The ground, as supporting structure for our geo-infrastructure, can be a major geohazard, and its complex yet inadequately understood behaviour poses huge risks to geo-structures and infrastructure. This project, as interdisciplinary research, integrates precise measurements of physical parameters of the ground, such as soil water content, soil temperature, and soil salinity contamination, along with strain monitoring using state-of-the-art fibre optic sensing systems, to enhance our understanding of geo-structures and the ground. The first part of this research focuses on investigating the application of a polymer optical fibre (POF) sensor to measure soil moisture content, temperature variation, and soil salinity contamination. Based on the results, poly(methyl methacrylate)-based optical sensors showed the ability to detect changes, even as low as 0.5%, in soil water content. Comparing the performance of this sensor with the commercial soil water content probe (SM150) and the oven-drying method, the results indicate that the developed sensor's prediction of water content variation is more accurate than the commonly used commercial probe, with a mean absolute error of 1.21%. Additionally, the sensitivity of the polymer optical fibre Bragg grating (POFBG) sensor to salinity concentrations in water and soil environments was assessed and estimated to be 58±2 pm/%, as an average value. In the second part, 3D printing technology is employed to develop a novel packaging for strain sensing systems in geotechnical engineering. This packaging aims to decrease strain transfer dissipation during the measurement process. Initially, direct shear tests are conducted to characterize the interface between the soil and 3D printing materials. Further characterization of the mechanical properties of the 3D printing material for the development of the packaging for strain sensing systems involves numerical modelling and experimental investigations. Through conducting direct shear tests, an optimal packaging design is presented for the application of 3D printing technology in soil strain monitoring. Additionally, based on the mechanical tests, thermoplastic polyurethane (TPU) material is selected to package the strain sensing systems in the soil environment to reduce strain transfer dissipation and achieve more accurate measurements.