Research On Radar Detection Technology And Migration Of Hidden Defects Behind The Shield Tunnel

The subway tunnels built by shield method is in the underground hidden environment.Thus,the voids and other concealed defects(void,non-compacting grouting etc.)inside and/or behind the tunnel lining pose critical threat to the safety of the operating metro tunnels.Ground penetrating radar(GPR)is a non-destructive geophysical technique by transmitting electromagnetic(EM)waves and receiving the reflected signals,which is the preferred method of tunnel concealed defect detection.However,the voids behind the shield tunnel segment are still hard to be recognized in a GPR image due to the strong scattering clutter from the dense steel bars reinforced inside the concrete lining.In this paper,we analyze the detection technology of GPR and high-precision imaging of hidden defects in radar profile by finite difference time domain(FDTD)simulations,laboratory measurement and field tests combined with the metal-plate-reflection method.The main research contents are as follows:(1)We used the time-domain reflectometry combined with the metal-plate-reflection method to measure the relative permittivity of different media in shield tunnels.The experimental data shows that the relative permittivity of segment,grout and soil are 9.4,14.1 and 18.0 respectively.The relative permittivity of grout gradually decreases with the condensation of grout and starts to stabilized gradually after the third day of the condensation period,which is the suitable time to carry out the detection for the low interference with electromagnetic waves.(2)Through theoretical analysis,numerical simulation and physical model test,we studied the propagation law of electromagnetic waves in the shield tunnel.By establishing different spacing steel simulation models and reinforced concrete simulation models,the paper studied the penetration ability of GPR in the shield tunnel.Combined with the metalplate-reflection method,the simulation model test results are verified by physical model tests to quantitatively determine the best frequency band range,the polarization direction and the maximum distance from the ground for GPR detection.The research results show that indicate that GPR system operating at a center frequency of 600 MHz has a good penetration through the densely reinforced concrete lining;the penetration strength of HH polarization mode is about 2 times to 3 times of HH polarization mode,which is more effective to the detection of shield tunnel;the closer the antenna is to the surface,the stronger the penetration of the antenna;and the distance between the antennas and the surface of shield tunnel segments should be less than 0.1 m to ensure a good coupling of incident electromagnetic energy into the concrete structure.(3)To image the geometric features of the void behind the segment,reverse-time migration(RTM)is applied to the simulated GPR B-scan profile,which presents higher resolution results than the results by using the traditional diffraction stack migration.In addition,on-site footprint model with artificially set cavities tests were studied to the simulation results.The test results show that the simulation results of applying migration imaging can the cavity signal to the correct spatial location.The test results show that the simulation results of applying offset imaging can restore the cavity signal to the correct spatial location,and can clearly distinguish between the grouting layer and the soil layer interface;the RTM imaging method has better imaging effect and higher imaging accuracy than the back-propagation migration for hidden defects;the test results of the footprint shield tunnel model apply the inverse-time offset imaging,and can clearly identify the location of artificially designed cavities.(4)In the detection of shield tunnel during construction by GPR,this paper studied and summarize the imaging effect of typical hidden defects behind the shield tunnel wall in the radar profile.The field test results show that the GPR with the optimization of antenna measurement parameters proposed in this paper,can effectively detect the location of the cavities behind the shield tunnel wall;the cavities show obvious reflection oscillation characteristics in the radar profile,among which the water-bearing cavities show strong reflection oscillation characteristics.