Expressway embankment health monitoring by DAS measurement

Large earthquakes and heavy rains can cause expressway embankment collapse and large deformation. Possible causes include the presence of a high groundwater table in the embankment and the deterioration of the strength of the fill materials over time. Therefore, it is important to evaluate the current health level of the embankment and predict its future level to prevent future embankment failures. The authors have shown that active and passive surface wave explorations can be used to spatially and continuously measure the mechanical conditions of an embankment immediately after its completion and after a number of years. However, the suggested methods require traffic restrictions because the exploration should be performed on the highway surface. Therefore, we tried DAS measurements using optical fibers buried in the ground. This measurement could be carried out from the electrical room or road management office, and did not require traffic restrictions. The results of comparing the dispersion curve obtained by linear microtremor array measurement with the curve obtained by DAS measurement showed that the two were almost identical. This means that DAS can describe the low-velocity area of the embankment, where the road surface continues to settle and where deformation is observed on the embankment slope. DAS measurement can be applied as a screening technique to randomly select low-velocity ranges (weak areas) of the expressway embankment.