Advanced time-lapse GPR method for quantitative monitoring of grout diffusion in tunnel lining

Tunnel grouting is a commonly used measure for mitigating groundwater leakage in tunnels. However, the actual grout diffusion range within the fissure between the secondary lining and the waterproofing membrane is hidden from direct observation. Conventional geophysical methods struggle to provide a quantitative evaluation of grout diffusion, while the most common drilling-based assessment is not only destructive to the tunnel but also unable to reconstruct the overall grout distribution from a few localized borehole observations. To address this problem, this paper proposes a quantitative grout-diffusion monitoring method based on time-lapse ground-penetrating radar. Multiple time-lapse datasets were acquired during grouting, and a wavelet time–energy density based Structure Similarity Index Measure was introduced to characterize subtle electromagnetic response changes induced by grout injection. Attribute slices were then reconstructed via interpolation, and, combined with the recorded cumulative grouting volume, a quantitative method was developed to estimate the extent of grout-enriched anomalies. Field test results show that the proposed method can reveal the spatiotemporal evolution of grout diffusion inside the lining. The quantitatively delineated grout boundary shows good spatiotemporal agreement with the grout leakage observed at the observation holes. The method provides a non-invasive, high-resolution, and engineering-feasible tool for evaluating grouting effectiveness and supporting targeted grouting construction.