Research Of Longitudinal Sttlement And Lateral Displacement Monitoring For Shield Tunnel On The Distributed Fiber Bragg Grating Sensing Technique

In the increasing of population and construction intensity of China’s cities, the urban underground rail transit has gradually turned into an unprecedented development stage. In addition, shield tunnel has been one of the best choices to build a long tunnel under city due to its lower labor intensity, lower disturbance to surrounding environment, higher safety and reliability than other kinds of tunnel. It has been shown from Shanghai metro operation monitoring that longitudinal structure deformation has occurred in most shield tunnels because of the inevitable load around the tunnel and is deteriorating with their growing service ages, ultimately it will become a serious threat to the safety of the tunnel operation and the surrounding environment. However, the traditional monitoring methods based on manual observation on some preset uniformly spaced points, already cannot satisfy the overall, high precision and long-term stability requirement of the current tunnel longitudinal structure deformation monitoring. It is hoped that the question will be resolved with introducing high precision of distributed optical fiber sensing technology.Based on the structural and deformation characteristics of shield tunnels, this paper focus on the research of the distributed fiber Bragg grating(FBG) Sensors performance and the algorithm from strain to deformation to investigate the applicability of the sensors in the shield tunnel monitoring and to extend the unidirectional algorithm to the bi-directional algorithm based on the strain distribution. The calculating models are verified by numerical simulations and shield tunnel model experiments in laboratory. The main innovative points in this paper as follows.a) The accuracy and long-term stability of the long-gage distributed FBG sensor is taken into consideration to evaluate the applicability of shield tunnel longitudinal structure deformation monitoring in the paper, according to the tensile testing and environmental testing. The results indicate that the accuracy of long-gauge FBG sensor can achieve 5με in good situation and its linearity, strain coefficient and data-show all remain stable in a variety of environments of the different time.b) The algorithm to achieve the longitudinal settlement and lateral displacement monitoring of shield tunnel at the same time is developed based on the bidirectional curvature separation to complement the unidirectional deformation algorithm. Moreover, the correctness and feasibility of the method is initially examined by the finite element simulation and also is compared and analysed with its error.c) To validate the feasibility of the practical application of the proposed monitoring algorithm, a long steel tube model, which is bonded long-gauge fiber Bragg grating sensors, is deformed by the vertical load, two reverse lateral load and the combination load to inspect the accuracy of deformation monitoring. Results of experiment indicate that the monitoring error of settlement is less than 5%, larger lateral displacement monitoring error is within 15% in unidirectional deformation and the displacement monitoring error of combination deformation can control in 15%. It is illustrated that the proposed algorithm has enough precision in shield tunnel longitudinal structure deformation monitoring.