Fiber optic monitoring and performance evaluation of geotechnical structures

Abstract

Although health monitoring of civil infrastructures using fiber optic sensors has drawn increasingly attention, the potential of fiber optic monitoring in geotechnical applications is still not well investigated. The advantages of fiber Bragg grating (FBG) sensors in accuracy and reliability make them exceptionally attractive for monitoring strains, temperatures, displacements, etc. of geotechnical structures. This research has therefore commenced to develop and apply FBG based sensors for geotechnical structures in both laboratory and field conditions, together with conventional transducers. Firstly, a variety of FBG based sensors including surface glued sensors, tube packaged sensors, embeddable sensing bars, in-place inclinometers, and settlement tubes, were developed and fabricated. Calibration test results show that the FBG based sensors are reliable for measuring strains, temperatures, and displacements with high accuracy. Secondly, the FBG sensors have been installed on steel and glass fiber reinforced polymer (GFRP) soil nails for measuring strain distributions along nail lengths during field pullout tests, respectively. Typical test data indicate that the pullout resistance has an empirical relationship with N value in standard penetration tests (SPTs). The pullout-displacement relationships before failure can be fitted by hyperbolic functions. Based on the test results, a simplified pullout model for soil nail is proposed. Thirdly, newly developed FBG sensing bars have been embedded in two physical models in laboratory for monitoring internal displacements. A two-dimensional (2-D) model of Wudu Dam was overloaded to failure in laboratory. The monitoring results from FBG sensing bars were in good agreement with those from conventional sensors. Based on the experimental and numerical results, the failure mechanism and overloading factor of safety of the gravity dam are studied. A three-dimensional (3-D) physical model has been built to simulate the performance of Shuangjiangkou Cavern Group during excavation. The model was instrumented with FBG sensing bars, multi-point extensometers (MPEs) and a digital photogrammetric system for displacement measurements. The monitoring results indicate that during the whole process of underground excavation, the displacements in the surrounding rock masses were in a considerably small range. After the excavation, the in-situ stresses were raised gradually and significant cracking and collapses were observed. The comparison between experimental and numerical results verifies the reliability of these sensors and leads to some conclusions on the deformation and overall stability condition of the cavern group. Finally, FBG based sensors have been applied for monitoring geotechnical structures in the field. An FBG based monitoring system consisting of various FBG sensors has been installed in a mat foundation site during construction. Implications on the current design assumptions are discussed on the basis of the monitoring results. For a newly stabilized slope, slope movements, strains in a soil nail and two soldier piles were measured by an FBG based monitoring system during and after slope stabilization. The long-term monitoring results show that the slope movements fluctuated with time, indicating that rainfall infiltration was a main factor affecting the slope stability. The strains and stresses in a soil nail had the same tendency as the rainfall magnitude. The soldier piles at the slope toe came to play a role in resisting the potential sliding after the slope movements towards downhill accumulated to certain values. Summary and findings are presented in Chapters 3 to 8. In Chapter 9, the original works and major conclusions of this research are summarized. The further research work related to this research topic is also suggested in this chapter.