Strain Transfer Mechanism And Layout Technology Of Distributed Optical Fiber For Monitoring Tunnel Deformation

Deformation monitoring of metro tunnels,a significant condition evaluation method,has been widely used in metro tunnels’ disaster warning.With the expansion of metro,a series of engineering difficulties have been triggered,including;the complex and intricate metro lines,the increase of metro depth,and complex and changeable geological conditions have imposed increasing requirements on safety monitoring of metro tunnels.Therefore,it is of great significance to seek new monitoring means in order to prevent and reduce the occurrence of metro tunnels’disasters and accidents.Distributed optical fiber monitoring technology is characterized by several advantages,such as distributed type,high precision,high sensitivity,favorable electrical insulation,anti-electromagnetic interference,corrosion resistance,light weight,and can make up for the deficiency of traditional monitoring methods to some extent.In order to meet the requirements of safety monitoring of metro tunnels,the application of distributed optical fiber monitoring technology in the deformation monitoring of metro tunnels was studied.The thin and fragile optical fiber was combined with the tunnel structure constructed extensively.While the problem concerning the survival rate of optical fiber and the accuracy of monitoring is still to be solved.Taking the section of metro tunnel through CRD method of the Beijing new airport under construction as the engineering background,the research on the effect of optical fiber sheath was conducted and the influence of optical fiber sheath on strain/temperature measurement was explored based on the tests and the features of the different sheathes of strain/temperature cables commonly used in the engineering.The strain limit and elastic property of the optical fiber were obtained by testing its mechanical properties.The principle of pre-stretching layout of the optical fiber and the specific engineering implementation methods were proposed.The mathematical model of fiber optic strain and tunnel arch crown deformation was established and then verified by the model test.The traditional linear layout of the optical fiber was improved to the zigzag layout,the rationality of zigzag layout was theoretically analyzed and verified,and the zigzag layout parameters with different measurement accuracy requirements were obtained through laboratory model experiments.The strain transfer mechanism of the optical fiber with buried arrangement was theoretically analyzed,and the effect of the embedded length of the optical fiber on strain transfer ratio was revealed.The shear performance test of the buried optical fiber was carried out to obtain its shear performance.Model tests were carried out on the different layout modes of the buried optical fiber to obtain the strain test results of the optical fiber under each layout mode.Considering the actual construction conditions,the technology of slotted buried optical fiber layout and the layout technology of binding the optical fiber along the steel-bar cages were applied to the metro line of the new airport in Beijing,and the practical application effect of the layout technology of distributed buried optical fiber was discussed.In view of the above researches,the following results were obtained:(1)The optical fiber sheath effect was proposed,and monitoring functions and application occasions of different types of fiber sheath were obtained through strain and temperature stimulation tests,so as to preliminarily solve the fiber selection and positioning.The ultimate strain value of the strain optical cable was obtained through the stretch-contraction test.It can be considered that the strain optical cable can monitor both tensile deformation and contraction deformation because of its elastic property.(2)The fiber pre-stretching layout mechanism and engineering solution were proposed.Based on the ultimate strain value of strain optical cable,the pre-stretching strain values for monitoring tensile deformation and contraction deformation were suggested;a mathematical model suitable for the deformation of the tunnel arch crown was established;the zigzag layout principle of optical fiber was proposed.Reasonability of zigzag layout was verified through the theoretical analysis and model test.Different zigzag layout parameters were given according to different requirements of monitoring accuracy in the engineering.(3)The effect of embedding length on strain transfer was obtained by theoretical analysis of strain transfer in the buried layout.Through the shear test of buried strain optical cable,the shear performance of three kinds of common strain cable was obtained.Through the single-point loading and three-point loading tests on reinforced concrete beams,the monitoring results and effective strain monitoring ranges of various buried optical fiber layout were obtained,and the monitoring effect was evaluated.(4)Through the application of buried layout of the optical fiber in the actual monitoring engineering of the metro line for the new airport in Beijing,and the results of strain monitoring are compared with total station monitoring,the feasibility of buried optical fiber layout in the monitoring of metro tunnel in CRD construction method was verified,which can provide reference for related monitoring projects.