Intelligent GPR Detection of Backfill Grouting Quality and Adjacent Soil Cavities in Shield Tunnels: A Case Study

Abstract

During the construction of shield tunnels, backfill grouting behind segmental linings is a crucial technique for filling the shield tail gap resulting from over-excavation by the cutter head. Direct observation of grouting thickness distribution is challenging, necessitating nondestructive testing (NDT) technologies like ground-penetrating radar (GPR). This study introduces a loaded-to-frame (LTF) device designed to automate the collection and intelligent analysis of GPR data, enabling rapid and intelligent detection of grouting thickness distribution and adjacent soil cavities in the excavation area. Focusing on the complex geological and environmental conditions of Xiamen Metro Line 3, the research highlights the critical role of grouting quality and cavity detection in ensuring tunnel construction safety and surface stability. Time domain reflectometry was employed to assess the electrical properties of the grouting materials, revealing a 12-h relative dielectric constant of 24.08 and conductivity of 4.29 mS/m. These properties significantly differ from those of the surrounding soil, confirming the suitability of GPR for detection. The LTF device, combined with its intelligent analysis system, can complete automated and efficient detection of single-ring shield tunnels within 5 min, providing dynamic feedback on grouting quality control. Furthermore, the study validates the feasibility of using the LTF device to detect adjacent soil cavities during tunnel excavation. This research advances the automation and intelligence of backfill grouting and cavity detection, supporting the safe and efficient progress of shield tunnel construction.