| As modernization construction and sustainable development strategy are carried out in China, a large number of tunnels (holes)and other underground engineering are being or are about to be constructed in the transportation, water conservancy and hydropower, mining and other areas. Since the major transportation and hydropower engineering turn to the western mountains and the depth of mining increases, the hydrogeological condition suffered by the underground engineering construction are increasingly complicated. As a result, more and more geological disasters such as sudden water inrush and collapse with high risk occur, which induce to serious casualties and economic losses. However, the technology of monitoring underground engineering geological hazards is relatively backward at present. It has become a key scientific problem to be solved that monitoring and forecasting the geological hazards real-timely. Fiber grating sensing technology provide a feasible approach to solve this problem. Unfortunately, since there are a few kinds of FBG sensors, and the system with simple function can monitor single parameter, in which signals extraction and analysis technology is poor and chirp effects could not be effectively removed, FBG sensing technology can not well satisfied with the needs of real-time monitoring and forecasting underground geological disasters currently.In response to above problems, regarding fiber Bragg grating sensing theory and technology as breakthrough in this dissertation, based on the significant underground engineering geological hazards model test, the influence of FBG intrinsic parameters and strain distribution on the reflective spectral characteristics is analyzed, and FBG parameters reconstruction theory based on central wavelength constraint and optimization method on the basis of improved genetic algorithm are both proposed. Therefore, fast recognition and elimination of FBG chirp effects is realized. The dissertation presents numerical simulation optimization design method of FBG sensors. Consequently, FBG seepage pressure sensors with high sensitivity, new FBG strain sensors, target-type FBG flow velocity sensors, micro FBG displacement sensor and multipoint FBG displacement sensors have been developed. FBG multi-parameter sensing system is also established and the system has been successfully used in the different underground engineering geological hazards model tests. The major research is expressed as followings:(1) According to the basic problem that FBG spectral characteristics is conditioned by multiple parameters, FBG spectral simulation program based on FBG transfer matrix theory and fiber grating strain sensing model has been developed. The influences of FBG intrinsic parameters and strain distribution on the spectral characteristics are disclosed. Especially, the influences of strain distribution on central wavelength, reflectivity, and bandwidth have been revealed. In tuning FBG by two ends fixed compression bar experiment, the reflection spectrums of FBG which is tuned by uneven strain is obtained, and the validity of FBG numerical analysis work is verified. As a result, the research lays a theory foundation for reconstruction of FBG parameters and study on FBG sensors.(2) In response to that traditional FBG parameters reconstruction theory and especial strain distribution reconstruction theory both have poor effect and slow convergence speed, FBG parameters reconstruction theory based on central wavelength constraint and optimization method on the basis of improved genetic algorithm is presented, which solves the difficult problem that the reconstruction theory has poor effect because of neglecting central wavelength and which significantly improves the non uniqueness and the confidence of reconstruction. FBG intrinsic parameters and strain reconstruction program has been developed, and parameters reconstruction numerical simulation experiments are comprehensively carried out. Through tuning FBG by two ends fixed compression bar experiment, the validity, applicability and practicability of the novel reconstructing method have been verified. Consequently, a novel method of accurately reconstructing FBG intrinsic parameters and strain distribution is generated.(3) In the current situation that the design technology of FBG sensor is backward, numerical simulation optimization design technology of FBG sensors based on finite element numerical analysis theory is presented. Optimization design platform for FBG sensors is established based on reconstruction theory and calibration experiments. For there are only a few kinds of FBG sensors which have poor matching ability with ambient medium, technological bottlenecks of measuring seepage pressure and flow velocity using FBG sensing technology are resolved. Based on the optimization design technology of FBG sensors adopting finite element theory, the FBG seepage pressure sensor with high sensitivity and target-type FBG flow velocity sensor are exploited. At the same time, FBG strain sensor based on matrix material of measured object is developed, which has good matching ability with ambient medium. The micro FBG displacement sensors and multipoint FBG displacement sensors are also designed. Ultimately, novel FBG sensor system suitable for monitoring underground engineering geological hazards and identifying information real-timely is constructed.(4) For the problem that FBG sensing system is simple and it can not monitor various parameters, FBG multi-parameter sensing system adopting wavelength division multiplexing and space division multiplexing technology is constructed. The software which can monitor underground engineering disasters real-timely and analysis information on-line is also developed based on Labview and C++Builder. As a result, it realizes synchronous acquisition and real-timely display of multivariate data.(5)According to the defects that the traditional FBG sensors used in underground engineering have low sensitivity and poor waterproof property, the FBG sensing system has been used to real-timely monitor key physical parameters of engineering disasters. Regarding to the typical geological disasters, coal mining water inrush model test, stability of tunnel surrounding rock model test, zonal disintegration model test and other full-scale model test are carried out. It reveals the influence of the parameters such as temperature, strain, displacement and seepage pressure on the damage process of the rock, and the especial response characteristics on the precursory information of hazard. Consequently, judging standard of underground engineering geological disasters precursor information has established, and underground engineering disasters real-timely monitoring and forecasting system based on the fiber grating sensing technology has formed. It has significant theory significance and engineering value to realize real-timely monitoring and forecasting underground engineering disasters. |
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