Ground Penetrating Radar Model Test And Finite Element Forward Simulation Of Cavity In Box Girder Prestressed Pipeline

Ground penetrating radar(GPR)is a geophysical exploration method for detecting the distribution of medium in shallow underground target body through transmitting and receiving ultra high frequency pulse electromagnetic wave.It has the characteristics of convenient operation,high detection precision,nondestructive and wide detection range,and is widely used in the fields of highway,railway,machine field and tunnel engineering.At present,the detection methods used in prestressed bridges include ultrasonic pulse method,impact echo method and ground penetrating radar method,but the application of ground penetrating radar in bridge is relatively small,especially for the detection of cavities in the prestressed pipe of box girder.Therefore,this paper applies GPR in this field,and studies the hole in the pipeline through physical model test,radar detection,atlas analysis,numerical simulation and so on.It provides a reference for the interpretation of the actual project.The main contents and results of this paper are as follows:1 the basic principle of ground penetrating radar and the propagation characteristics of radar wave are introduced,and the physical model test of the cavity in the prestressed pipe of the box girder is made.The different size holes are arranged in the pipe,the hole in the pipe is detected with the SIR-3000 ground penetrating radar,and the Randa7 software is used to filter,gain and convolution of the collected two-dimensional profile.Processing.generating a three-dimensional diagram.2 through the analysis of the image,it is shown that the reflected wave signal and the distance between steel bars can be clearly identified in the two-dimensional profile,and the reflected wave signals can be detected under the reinforcement network.For small holes,the reflected wave signal is "hollow" shape,and the large cavity reflected wave signal is"parabola" shape.The reflection wave signal,position,spacing of steel bar and the reflected wave signal and position of the cavity can be identified in the three-dimensional map.It is known that the smaller the cavity,the less the reflected wave signal,the larger the cavity,the more obvious the reflection wave signal,and the three dimensional view ratio through different depth.It is observed that the reflection signal of the cavity changes with depth.3 from the Maxwell equation,the characteristics and solving steps of the finite element method are introduced.The finite element wave equation of the ground penetrating radar is derived.The Sarma boundary.the improved Sarma boundary and the transmission boundary conditions are introduced,and the calculation equation is loaded.The finite element forward simulation program of the GPR is written in MATLAB,and the experimental model is carried out.The numerical simulation is carried out.Three boundary conditions are loaded to the rectangular model,and the grid and time history analysis are drawn.The signal characteristics of the cavity reflected waves under the three boundary conditions are analyzed.4 through the analysis of the grid and time history maps,it is shown that the void reflection signal in the rectangular model is better under the three boundary conditions,in which the rectangular model which improves the boundary condition and the transmission boundary condition of the Sarma,the reflected signal of the electromagnetic wave at the boundary is weakened,and the interference signal is reduced effectively;the cavity reflection in the rectangular model is the cavity reflection.The effect is related to the size of the cavity.The reflection effect of the small cavity is not obvious and is not easy to identify.The reflection signal of the large cavity is obviously parabolic.5 contrast and analyze the images obtained by the numerical simulation and the test model,and show that the reflected wave signals of the cavity are all the "parabola" shape of the opening downward,which verifies the feasibility of the numerical simulation method and provides some reference for the image interpretation in the actual engineering.