Study On High-precision Optical Fiber Testing Technology For Axial Force Of Foundation Pit Concrete Support

With the continuous acceleration of urbanization,the limited land resources can hardly meet the needs of social development.While high-rise and super high-rise buildings realize efficient use of land resources,it brings the construction problem of deep foundation pit engineering.How to ensure the safety during the construction of foundation pits has become a hot and difficult point of research for geological and geotechnical engineering workers.The support structure of foundation pit is an important part to ensure the safety of foundation pit,among which,concrete support and underground diaphragm wall are widely used.Therefore,the research on the testing technology of concrete support axial force is of great practical significance.Based on the research results of high-precision fiber optic testing technology of the group,the paper carries out a study on the problem of low testing accuracy and precision of concrete support axial force;analyzes the influencing factors of concrete support axial force testing;proposes a high-precision fiber optic support axial force testing method,combined with indoor tests,and deduces the concrete support axial force considering concrete shrinkage,creep,temperature effect and stress-strain nonlinearity The accuracy of the axial force calculation of concrete support is improved through the correction of the axial force calculation formula;finally,the high-precision fiber optic concrete axial force testing method proposed in the paper is successfully applied to the actual project in combination with the concrete axial force monitoring project in deep foundation pits in Shanghai.The results of the full paper are summarized as follows.(1)According to the current situation of foundation support project,the importance of concrete support axial force testing in foundation support is explained,the research status and disadvantages of traditional monitoring methods are analyzed,the basic principles of optical frequency domain reflection(OFDR)and fiber Bragg grating(FBG)and foundation concrete support axial force testing principles are introduced,and the advantages of these two high-precision fiber optic testing technologies in foundation monitoring are summarized;by analyzing the influencing factors of concrete support axial force calculation,focusing on the importance of temperature effect in concrete support,and finally deriving the formula of concrete support axial force calculation considering concrete shrinkage,creep,temperature effect and stress-strain nonlinearity.(2)The temperature coefficients of distributed optical fibers were calibrated to obtain the temperature coefficients of the four optical cables from 10 to 70°C.The temperature coefficients of bare fiber and steel strand are constant,and the temperature coefficients of 2mm polyurethane sheathed strain-sensing optical cable(2mm)and Hytrel sheathed strain-sensing optical cable(0.9)have a cut-off point at 43°C.Therefore,when the monitoring environment temperature is less than 43°C,the four When the ambient temperature is greater than 43℃,the bare fiber and steel strand with a constant temperature coefficient are more suitable;the strain gauge was used to test the substrates with different linear expansion coefficients,and the results show that the strain gauge can test the linear expansion coefficients and strain coefficients of different substrates well through temperature compensation,which provides a theoretical basis for applying the high-precision optical fiber testing method to concrete support axial force testing theoretical basis for applying the high-precision fiber optic testing method to concrete support axial force testing.(3)High-precision fiber optic testing technology can accurately test the real-time changes of temperature,which solves the pain point of low accuracy or even lack of temperature testing by traditional monitoring means.The monitoring results show that:the strain gauge can accurately monitor the strain change of concrete support and daily temperature value;by eliminating the correction formula of concrete shrinkage,creep,temperature effect and stress-strain nonlinearity to calculate the axial force value,the calculation result is more suitable for the actual value.The high-precision fiber optic testing technology can well monitor the changing state of the axial force of concrete support in the foundation pit,which is an important guide for the subsequent foundation support.