Tunnel Convergence Monitoring Based On Quasi-distributed Fiber Bragg Grating Sensor

With the accelerating process of national urbanization,the subway,as a public infrastructure that can greatly alleviate the pressure of urban population flow and has little impact on the ground transportation,has become a key construction project of the country.However,with the increase in service time of shield tunnels and the influence of external environmental factors such as rock and soil pressure,geological environment,and third-party construction,various disease problems will occur in the tunnel structure,which has great security risks.Therefore,it is urgent to monitor the tunnel structure in real time.At present,measuring instruments such as laser scanner,static level,total station and vision-based system are mainly used in tunnel structure monitoring.Due to the disadvantages of inconvenient operation,large volume and slow response speed,the conventional technique can not meet the needs of modern tunnel monitoring.Compared with traditional monitoring methods,optical fiber sensors have many advantages,including anti-electromagnetic interference,corrosion resistance,high measurement accuracy,easy networking,small size,and real-time online monitoring,which make them the first choice of tunnel structure health monitoring.According to the needs of shield tunnel monitoring and the characteristics of the tunnel structure,the quasi-distributed fiber bragg grating(FBG)sensing technology is proposed in this thesis,in which the fiber optic sensor based on a transducer structure of micro-bent plate is proposed.Furthermore,the packaging technology and installation methods are introduced and discussed in this thesis.And the feasibility of these models in tunnel convergence monitoring is verified by the equivalent model in the laboratory.In the end,the application of fiber grating sensing device in engineering is explored.The main contents of this thesis are as follows:(1)Proposing a tunnel convergence monitoring scheme based on quasi-distributed fiber bragg grating sensor,which has the advantages of large-scale,real-time,safe and reliable,and intelligent identification.The shield tunnel model is built and the convergence process of the tunnel is numerically analyzed by employing the ANSYS software.The simulation results show that the strain of the sensor is about 20 με-2534 με as the tunnel convergence is from 2 mm to150 mm,which is within the detection range of FBG.The simulation results verify the feasibility of the scheme.(2)Carrying out the experimental study of tunnel convergence monitoring scheme based on quasi-distributed fiber bragg grating sensor.The phase mask method is adopted to the fabrication of the FBG.The copper-based FBG sensor is developed,which is embedded in a 1cm thick ABS board to form a sensing device.A 6:1 scale model of the tunnel is designed to carry out the equivalent experiment.In the experiment,the sensors were installed at the waist and the locations at the angles of 30° and 45° with respect to the waist,which the results show that the sensitivities of the sensors are 8.28 pm/mm,4.42 pm/mm and0.78 pm/mm,respectively.The results provide a reference location for the sensor in the project.The convergence of the model measured by optical fiber is basically consistent with the actual value,whose maximum error is 7.91%,which verify the feasibility of the monitoring scheme.(3)Developing the tunnel convergence monitoring system and exploring the temperature self-compensation package technique of FBG sensor.The FBG interrogation network system is based on wavelength division multiplexing and space division multiplexing,which the multiplexing capacity can up to 320.The tunnel convergence sensor is designed with dual-FBG based temperature self-compensated structure,in which a bent FBG to eliminate the influence of stress is connected with the strain grating in series to compensate the environmental temperature influence.In the experiment,we have fabricated 6 sensors and tested the temperature sensitivity,with the average temperature sensitivity of the strain and temperature-compensated grating are about 31.12 pm/℃ and 10.54 pm/℃,respectively,and the standard deviation is 1.41 and 1.43,which show the designed sensor has a very well stability.Finally,we have developed tunnel convergence monitoring system interface with the tunnel visualization model by Lab VIEW software,in which the software realizes the real-time display of tunnel structure information by processing the collected wavelength data.