Applications of Optical Fiber Sensors in Geotechnical Engineering: Laboratory and Field Implementations Under Various Loading Conditions

The purpose of this study is to explore the feasibility and effectiveness of Fiber Bragg Grating optical sensors (FBG sensors) in geotechnical engineering applications, both in controlled laboratory settings and in real-world field environments. Specifically, the study aims to evaluate the sensors' performance in monitoring key geotechnical parameters—such as strain, temperature and acceleration—under various loading conditions, including static, dynamic, seismic, and centrifuge loads. By assessing FBG sensors across these diverse conditions, the study seeks to determine their potential for improving the accuracy, safety, and reliability of geotechnical monitoring systems. Within this framework, the application of optical fibers was investigated using the one-degree-of-freedom shaking table at the National Technical University of Athens, with the goal of evaluating the sensors' performance during seismic loading. This investigation aimed to provide insights into the behavior of geotechnical physical models under seismic conditions, allowing conclusions to be drawn about the reliability and accuracy of optical fiber sensors in capturing dynamic responses during earthquake simulations. Furthermore, several physical models were constructed and tested using the drum geotechnical centrifuge at ETH Zurich. During testing, optical fiber strain recordings were made possible as gravitational acceleration was increased up to 100g and impact loading scenarios were included. The optical fiber sensors successfully captured the loading conditions, and the recorded data reflected the anticipated behavior of the models, demonstrating the sensors' effectiveness in monitoring geotechnical responses under extreme loading scenarios. Following the laboratory investigation, optical fibers were installed at the Acropolis of Athens Perimeter Wall (Circuit Wall) to record strain, acceleration, and temperature in real-time and remotely. The installation of this monitoring system posed significant challenges due to the limitations of the archaeological site, requiring careful planning and execution. Despite these difficulties, data was successfully gathered over an extended period of two years, providing insights into the structural behavior of the historic site under various environmental and loading conditions. Regarding the aforementioned applications, the test setups and monitoring schemes are presented in detail. This includes an overview of the types of sensors used, the coating materials, and the methods employed for integrating the sensors into both the experimental models and the Acropolis Hill. Additionally, the data gathering techniques are discussed, along with any technical issues that arose during the process and the proposed solutions. Finally, indicative recordings are provided, along with their interpretation. The results align with expectations, demonstrating the effectiveness of optical fibers in various applications and underscoring their vast potential in geotechnical engineering. This highlights the numerous opportunities for incorporating optical fiber sensors in future projects, reinforcing their role as valuable tools for monitoring and assessing structural integrity.