Identifying void defects behind Tunnel composite lining based on transient electromagnetic radar method

Void defects behind the linings are typical in most operating tunnels, and effective methods are needed to identify them. This study presents a self-developed non-destructive detection method called transient electromagnetic radar (TER). Measures and algorithms were proposed to improve TER according to the characteristics of the composite lining. The detection depth was increased by adjusting the transmitter current and reducing the primary magnetic field turn-off time. The image resolution was improved using the transient electromagnetic signal enhancement algorithm. Data noise was suppressed using the hybrid algorithm of bipolar superposition and multi-period sampling. The improved TER was subjected to indoor physical model tests and try-outs in a defective railway tunnel to evaluate its actual detection capacity. The results showed that the improved TER had excellent distinguishing capacities on lining-related media and boundaries between different lining layers. It was capable of effectively detecting void defects over 10 cm in size behind the linings. Compared with ground-penetrating radar (GPR), TER is less interfered with by steel bars, and the radar image is more intuitive and clearer. The actual features of the void defects were more accurately reflected with the TER approach, indicating its great superiority in the recognition capacity. This study could help the evaluation of void defects and the subsequent maintenance.