Study On Fiber Bragg Grating Sensing Theory And Key Technology For Bridges And Tunnels Engineering Safety Monitoring

With the deepening of the modernization construction in China, Bridge and Tunnel Engineering constructions develop rapidly, and engineering field transfer to those areas with more complex geological conditions gradually. Under this situation, more and more geological disasters and safety accidents occurs during the construction periods or service periods in Bridge and Tunnel Engineering, which lead to disastrous consequences. Study on safety monitoring theory and technology based on different disasters and accidents provide an effective solution to Bridge and Tunnel Engineering safety problem. Relying on the advantages in materials, accuracy, capacity of multiplexing, Fiber Bragg grating (FBG) sensing technology is regarded as one of the most feasible methods for the safety monitoring of Bridge and Tunnel Engineering. However, there are some drawback exsits in Bridge and Tunnel safety monitoring areas. Firstly, little research focuses on smart geotextiles embedded within FBG. Additionally, the investigation of sensing characteristics of new-type fiber grating elements are relatively onefold, as most research only study the uniform fiber Bragg grating sensing characteristic under equally axial strain. Thirdly, the design of FBG sensors with both small size and high precision is imperfect. Therefore, currently FBG sensing technology cannot meet the requirements of real-time monitoring in many applications in Bridge and Tunnel Engineering.In response to above problems, detailed research concerning fiber Bragg grating sensing theory and key technology for Bridges and Tunnels Engineering safety monitoring are carried out in this dissertation. Based on the application of model tests and engineering projects, FBG axial strain sensing model is analyzed firstly, followed by the optimal design of a series of precision and miniature FBG sensors based on Finite Element Method (FEM) according to the requirement of measurement. And then, online embedding technologies which combine FBG into Geogrid are studied, and smart Geogrid embedded within FBG are developed to realize reinforcement and self-sensing. Deformation sensing algorithm based on discrete curvature is utilized to realize2-Dimensional or3-Dimensional deformation field sensing. In order to explore application of new-type fiber grating as well as realize complex spatial load precision measurement, the spectral characteristics of chirped fiber Bragg grating and phase shifted fiber Bragg grating under complex spatial load are studied systematically. Finally, applications of fiber Bragg grating technology for real tunnel as well as model tests are introduced, and missing data repair methods based on support vector machine (SVM) is used to improve intelligence and reliability of FBG sensing system. The major research is organized as followings:1. According to the needs of sensor with small-size and multi-parameter, precision and miniature FBG sensors for different measurands are designed and optimized by Finite Element Method simulation, including surface-mounted FBG strain sensor with less strain transfer loss, FBG crack/displacement senor with high-precision, miniaturized FBG soil-pressure sensor which has better match characteristic with soil medium; a target-type FBG flow velocity sensor which is suitable for both fracture and pipe flow velocity monitoring. On the basis of FBG strain sensing principles, all the sensing model and sensing properties of those sensors are studied theoretically and experimentally.2. In response to the deficiency of smart Geosynthetics, the Geogrid which is originally used for structure reinforcement in civil engineering is adopted as carrier material to embed optical fiber Bragg grating in. By studying online embeding technologies of FBG into Geogrid, the smart Geogrid which can be used for reinforcement and monitoring is developed, self-restore FBG sensing network are studied for the FBG sensing system in smart Geogrid to improve the reliability. And then, the stretching experiment of smart Geogrid is implemented to indicate its stretching property. Additionally, the deformation sensing algorithm based on discrete curvature is studied as the methods to realize2-Dimensional or3-Dimensional deformation field sensing, and feasibility of the method are verified by both numerical simulations and lab experiments. The research fills the blank of smart Geosynthetics with spatial deformation self-sensing capability in Bridges and Tunnels Engineering.3. Under non-uniform load stress, the spectrum of uniform fiber Bragg grating is prone to be distorted, which make it is not only impossible to realize accurate measurement of complex spatial load, but also could not obtain axial strain. According to this problem two non-uniform fiber Bragg grating-chirped fiber Bragg grating and phase shifted fiber Bragg grating-are selected as new-type fiber grating sensing elements. The spectral characteristics of these two kinds of fiber Bragg grating under complex spatial load stress including axial uniform and non-uniform stress, radial load with different values as well as distribution angels are studied systematically. Variation of spectral shapes, central wavelength, bandwidth, and reflectivity under complex spatial load stress are acquired. Relative research results explore application of new-type fiber grating as well as provide theoretical foundation for complex spatial load measurement based on non-uniform fiber Bragg grating.4. According to the defects that the traditional sensing technology used Bridges and Tunnels Engineering is low in accuracy and poor in mechanical matching characteristics with structure to be measured, the FBG sensing system based on the optimal designed FBG sensor has been constructed and used in engineering projects and model tests to monitor the variation of key physical parameters in real-time. Regarding to different disasters and accidents, the tunnel support deformation, shear displacement of bridge’s segment joints, fracture flow velocity in process of dynamic water grouting, and pressure of surrounding rock are monitored by FBG sensing system to study the response of these physical parameters to tunnel deformation, bridge deflection, water inrush disaster, and tunnel stability.5. As data acquired by FBG sensing system might miss due to FBG spectrum distorted or FBG sensor failure in the process of monitoring, the missing data repair method based on SVM is studied. Firstly, based on correlation between the broken sensor and other sensors or relative factors, the nonlinear function relation model between broken sensor’s data and other relative factors are established. Then the function, which utilizes other sensors’ data and other relative factors as input, is adopted to calculate the failed sensor’s data. Finally, the calculated data will be treated as missing data for analysis. This method further improved the intelligence and the reliability of FBG sensing system.