Ground Penetrating Radar Inspection Experiment on a Shield Tunnel Segment and the Backfill Grouting

Defects of the backfill grouting have a strong impact on the shield tunnel engineering and gotten more and more attention from the experts in recent years. To estimate the thickness of the grouting and the position of some objects behind a shield tunnel segment, we conducted a field experiment using Ground Penetrating Radar (GPR). In the paper, a segment was put vertically on the ground with half of its outer surface covered by a wooden box. We injected concrete grout into the box until 80% of the box was filled by it. SIR4000 pulse radar system made by GSSI (Geophysical Survey System Inc.) with 400 MHz center-frequency antenna was used in this experiment. First, static measurements show that the direct wave and the coupling wave were stacked together when the antenna was put on the internal surface of the segment. We, therefore, regard the peak point as the time zero of the signal. Then contrastive analysis reveals that Principal Component Analysis (PCA) performed better than Average Mean Removal (AMR) in eliminating the direct wave and highlighting the effective signals. From the processed radargram of dynamical measurement, we can see that it was easy to distinguish the inner structure and the interfaces of the segment, as well as the grouting and other objects in the back. After a week, we retested it again and found that the travel time of electromagnetic wave passing through the grouting became shorter than the previous one. It may be attributed to the loss of moisture in the solidification process of concrete, which means that the technique we proposed can monitor the change of the grouting layer and provide decision-maker with reliable information of the shield tunnel quality.