Key Technology And Application Research Of Fiber Bragg Grating Strain Sensing In Geotechnical Engineering

In addition to the advantages of fiber optic sensors,Fiber Bragg grating(FBG)sensors have the unique advantages of simple structure,soft material and high sensitivity.They have been flourishing in recent years and have been widely used in geotechnical monitoring.However,due to the complexity of the rock and soil,there are still many uncertainties in the application of this technology.To improve this situation,this paper has carried out a series of theoretical,experimental and simulation studies on three aspects:dynamic strain monitoring,soil shear deformation monitoring of two dimensions and temperature effect,and the following results were obtained:(1)Aiming at the facts that research on FBG in dynamic strain monitoring of rock and soil is insufficient,a series of free-fall ball tests are carried out.By using free-fall ball,impact loads are applied to FBG strain sensors embedded in sandy soil.The strains and their variation of sensors in this process are measured.Based on the analysis of the variation of strain,the soil strain experienced four stages:power law growth,linear growth,power law reduction and residual strain.The concept of peak strain is introduced in this process,and the relationship between falling ball height,FBG depth and soil moisture was studied.By their relations,a method of rapid determination on soil moisture content based on FBG technique is proposed.With this method the soil moisture content can be estimated by an empirical calculation formula.(2)Since there is a little study on soil shear deformation monitoring of two dimensions,an experiment of soil shear-deformation based on FBG and particle image velocimetry(PIV)was carried out.The test results show that the FBG strain on both sides of the soil shear surface is Gaussian;the relationship between FBG strain and soil shear displacement is divided into two linear growth stages,and the soil softening occurs in the second stage.The displacement field and strain field of the soil surface are obtained by PIV analysis technology.Through PIV analysis,the deformation of the sensing fiber inside the soil is predicted.Based on the previous research,two kinds of FBG strain-soil shear-displacement transformation model are proposed.The calculation results of the model show that when the shear displacement is small,the errors of the two models are relatively large;but when the shear displacement increases,the error gradually decreases to less than 20%.(3)To solve this problem,this study is carried out in two aspects.Firstly,it proposes a method which modified the coefficient to improve the temperature compensation system of FBG,and the calibrations of coefficient of two different FBG strain sensors through laboratory experiments verify the feasibility of this method.Secondly,in order to separate the load strain from the thermal strain effectively,a load-temperature strain separation model is proposed.The cross-correlation function is introduced to determine the coefficient in the separation model.This model is applied to analyze the strain monitoring data on a reconstruction project.The results show that the load strain and thermal strain can be effectively separated by this method.This method will provide an important basis for accurate extraction,analysis and prediction of fiber Bragg grating strain data.