ABSTRACT
Detecting cavities in the backfill grout behind TBM segment linings is crucial because they can create pathways for groundwater, causing the lining to separate from the surrounding ground. Since grout defects, including cavities, cannot be evaluated visually, non-destructive testing techniques are necessary. This study proposes an ultrasonic tomography method for detecting cavities behind the segment lining. Ultrasonic tomography creates images of a target’s interior using ultrasonic signals reflecting off the materials’ boundaries with different acoustic impedances. The laboratory experiment was conducted to validate the assessment of grout condition using ultrasonic tomography. The experiment consisted of a small-scale model simulating a segment lining and backfill grout. Cavity detection accuracy in the grout was evaluated by measuring defect signals in the B-scan images obtained from the laboratory experiments, with accuracy evaluated above 90%. Additionally, it was found that the presence of rebar in the tunnel lining minimally impacted the effectiveness of cavity detection. The findings of this study suggest that ultrasonic tomography can significantly improve the accuracy of evaluating backfill grout behind TBM segment linings.
