Tunnel lining detection and retrofitting

The underground tunnel structure is important and common in transport infrastructures. With the increasing service time, it is crucial to detect the deteriorations in the ageing tunnel linings and make informed retrofitting decisions to ensure their structural safety and extend their service life cycle. This emphasizes the importance of understanding the framework of tunnel lining detection, evaluation, and retrofitting. However, there is no up-todate review available that covers the entire workflow of tunnel lining detection and retrofitting. This paper provides a comprehensive review of non-destructive testing (NDT) methods, health evaluation methods, and retrofitting methods for tunnel linings. The achievements, challenges, and development trends of these methods are illustrated. Specifically, NDT methods for three representative tunnel lining defects, including cracks, leakage, and voids, are introduced and analyzed to show the corresponding advantages and disadvantages. Based on the data obtained by the defect detection methods, the procedures for lining health status evaluation are also summarized to provide a systematic and quantitative evaluation of tunnel linings. Finally, the retrofitting methods and techniques that are suitable for lining structures are reviewed. This paper provides an insight into the development of structural health monitoring (SHM) and the maintenance of tunnel linings, offering a systematic guide for understanding the framework of tunnel lining detection and retrofitting.