Review of state-of-the-art in structural health monitoring of tunnel engineering

• The application of structural health monitoring systems in tunnels was systematically reviewed, summarizing insights from 50 projects worldwide. • The commonly used monitoring indicators was categorized, establishes selection criteria for different working conditions. • The main monitoring and measurement technologies were summarized, and the methods for preprocessing and analyzing large-scale monitoring data was introduced Structural health-monitoring (SHM) systems are essential for ensuring the safety, reliability, and longevity of tunnel structures. This paper reviews the application of SHM systems in tunnels based on examples from 51 projects worldwide that have implemented such systems. Commonly used monitoring indicators and their classifications are outlined, and the selection criteria for these indicators under different operating conditions are clarified. Additionally, it summarizes the main monitoring and measurement technologies and introduces methods for preprocessing and analyzing large-scale monitoring data. The current limitations of SHM systems are discussed and future research directions to address these gaps are proposed. The objective of this study is to provide a complete guide for enhancing and innovating SHM systems in tunnel engineering.