Research on the Effect of Permittivity on the Result of GPR Detection of Grouting Material behind Shield Tunnel Segment

摘要

In recent years, grouting behind the lining of shield tunnels is an important way to control the settlement of tunnels in areas of soft soil. One of the main concerns is how to evaluate the quality of this grouting. When ground penetrating radar (GPR) is used to detect the quality of grouting behind segment at the tail of shield tunnel, it is very important to ascertain the velocity of electromagnetic wave (EMW) to obtain accurate results. In this study, a network analytic instrument is initially used to detect the permittivities of different grouting materials, as this could affect results of the GPR. Permittivity is exponentially related to detecting frequency, for different mixture ratios and curing periods of grouting materials. From the results, we find that permittivity varies with detecting frequency, moisture content, and curing period. In particular, moisture content is the most important factor affecting dielectric character. Model and in-situ tests have been carried out. This study demonstrates that it is important to adopt different permittivity values according to different choices of detecting frequency. It is also important to more accurately interpret the grouting effect behind the lining of shield tunnels for GPR.