DIELECTRIC COEFFICIENT TESTING OF GROUTING MATERIAL BEHIND SEGMENTS IN SHIELD TUNNEL

The estimation of electromagnetic wave velocity is very important for the use of ground penetrating radar(GPR) to detect the existence or the thickness of the grouting slurries behind segments in shield tunnel.Firstly,a laboratory coaxial probe method for measuring dielectric coefficient of dielectric materials are proposed.The dielectric materials,backed by an electric conductor,are placed flush with the coaxial probe.The values of the dielectric coefficients of the dielectric materials are determined from the coaxial aperture reflection coefficients,which are measured by a network analyzer.With this method,a series of experimental measurements are carried out to test the dielectric coefficient of inertia slurry in two different proportions during the first day and the dielectric coefficients of double slurry in a certain proportion in the third and fourteenth days within the frequency of 1 GHz(the slurries are the ones used in shield tunnels in Shanghai).Then the electromagnetic wave velocities according to these slurries under different test frequencies can be obtained.Through the numerical analysis of the testing datai,t is concluded that different testing frequencies,different proportions and ages of the slurries can have influences on the electromagnetic wave velocities.Therefore,to improve the precision of GPR to detect the existence or the thickness of the grouting slurries behind segments in shield tunnel,the accurate determination of the dielectric coefficient of the grouting slurries is necessary.