Research On Propagation Law Of Radar Signal In Tunnel Segment And Analysis Of Radar Signal From The Void

The construction of tunnel has been developing rapidly in recent years,ground penetrating radar(GPR)has been widely used in the quality detection of tunnel engineering with the advantage of high resolution,fastness and nondestructive.In this paper,we do some research on the topics of tunnel shield grouting layer distribution and the identification of void behind the tunnel lining.Some results are obtained:(a)We use the time domain reflectometry to test the electrical parameter of soil and the grouting in subway station.The results shows that the electrical parameter of soil is rather stable,where the water content of the soil is in 27%-42%,and the relative electric parameter between 14.3 to 27.7.The dielectric constant increases with the increase of soil moisture content.The dielectric constant of the grouting varies with age,but the single grouting and the double grouting are more stable in the first 4 hours,and the dielectric constant is large,the single slurry is 52.5 while the double grouting is 80.The relative dielectric constant of the grouting slurry and the soil is quite different,and the interface reflection coefficient between this two material is large.(b)Based on the measurement of the electrical parameters of the grouting material and the soil,the system loss,propagation loss,ohmic loss and numerical solution of the interface loss are obtained by numerical calculation.Electromagnetic model is established by simulation to obtain the scattering loss.And finally we get the sum of loss curve of the shield tunnel model.Based on the FDTD method,the electromagnetic simulation experiment of three-dimensional shield tunnel segment is carried out.The actual antenna detection of grouting layer is simulated and the reflected signal of the grouting layer is successfully extracted.It is proved that the ground penetrating radar signal can penetrate the shield segment and the grouting layer,we recommend to use the 400 MHz center frequency of antenna.(c)The electromagnetic simulation model boundary conditions are set separately PEC and PMC,making the simplified model instead of full model simulation,and it can save a lot of time of simulation.The influence of the size of the rebar mesh and the spacing of the rebar layer on the transmission of the ground penetrating radar signal is discussed with the standard of S21 parameter.Simulation results show that below the 200 MHz low-frequency signal components difficult to pass through the rebar mesh,while the high-frequency components easier to penetrate single-layer rebar mesh.The smaller the grid,the more difficult the radar signal is to penetrate the rebar mesh.The larger the spacing between the two layers,the resonance point in the S21 result of the simulation model will increase,and the frequency of the "resonant point" will become smaller.(d)Finite difference time domain(FDTD)method is used to simulate the GPR data from two kinds of voids,i.e.air-filled and water-filled.Then,we carried out laboratory experiments on a reinforced concrete wall model,in which two kinds of voids are buried,using a GPR system with a 900 MHz and a 2.5 GHz antennas,respectively.Both the numerical and laboratory experimental results show that the reflected signal from the upper boundary of the void would be out of phase when it is water-filled compared with that from the air-filled void.The reflection signal from the lower boundary of the water-filled void is largely delayed and its instantaneous frequency is downshifted due to the wave propagation inside the filled lossy water.We conclude that time-frequency analysis help interpreting GPR data for identification of the characteristics of the subsurface voids.