Development of Grouting Quality Evaluation System for a Shield TBM Tunnel Using the Impact-Echo Method

Effective backfill grouting is crucial for the stability of Tunnel Boring Machine (TBM) tunnels. Insufficient grouting can lead to severe consequences, including groundwater seepage, surface settling, and cavity in the ground. This study introduces an innovative evaluation system using the impact-echo (IE) method to ensure the quality of backfill grouting behind segment linings. Through in-depth analysis of IE signals, three key indicators—Geometric Damping Ratio (GDR), Dominant Resonance Duration (DRD), and the number of post-peak amplitude Counts above 10% of the peak value after 1 millisecond (N10AMP)—have been identified, effectively capturing grouting quality in the near-surface zone. Despite the current limitations of solenoid hammers in terms of structural thickness applicability, our enhancements in the diversification of size, impact force, and contact time extend their utility, promising greater flexibility and precision in structural evaluations. This aligns with the evolving demands of modern tunnel engineering projects, ensuring compatibility and efficiency. The optimized automated IE approach was then applied to a real construction site segment, showcasing its superiority over conventional methods. Successfully deployed in two field locations, the system reliably assessed backfill grouting quality. This impact-echo system streamlines data acquisition with an automated solenoid, proving its field-ready capability. By ensuring proper backfill grouting, this novel IE system promotes safer, more efficient TBM tunnel construction with enhanced stability, safety, and reduced maintenance costs.