Detection of railway subgrade moisture content by GPR

Abstract

In cold regions, heaving of the railway subgrade is an important security risk to high-speed railways. It is necessary to monitor the moisture content of the subgrade in order to ensure normal railway operation. Common midpoint (CMP) measurement using ground penetrating radar (GPR) was carried out to estimate the moisture content of railway subgrade. As the subgrade is typically a layered medium, the CMP method is suitable for moisture content measurement. Firstly, the CMP data is analyzed through the velocity spectrum to obtain the stacking velocity, from which the vertical interval velocity profile can be calculated by Dix's formula. The moisture content can be obtained by Topp's formula from dielectric constant, which depends on the velocity. However, velocity analysis has lots of problems in shallow and thin layers like rail subgrade. With the help of FDTD simulation by gprMAX, we find that there are many multiples and refracted waves which cause many false results during velocity analysis as the offset is large. Under the guidance of the simulated result, we use optimized data for velocity analysis of the real data. It is found that the final result is good and in accordance with the result of induced polarizability (IP). It is proven that it is indeed feasible to use GPR to detect the moisture content of subgrade. This method can be widely applied in subgrade moisture detection.