Fiber optic sensor monitoring of mechanical performance and cracking behavior of large-scale prefabricated underground stations

Prefabricated underground stations (PUSs) are widely used for their low-carbon, high-efficiency benefits. The complex construction of large-scale PUS components can lead to stress concentration and cracking. This study analyzes the mechanical performance and crack identification of these components during the construction process. A high-precision monitoring system using optical frequency domain reflectometry (OFDR) was developed to measure the internal strain field of large-scale components. Additionally, a crack identification method based on principal component analysis (PCA) and hierarchical clustering (HC) was established to detect crack initiation and growth during assembly. The main findings of this study are that the monitored data demonstrated the ability of fiber optic sensors to identify areas of highest strain concentration with sub-millimeter spatial resolution. The topsoil backfilling process contributed the most to the bending moment in the roof, accounting for an average of 39.3%, while the foundation trench backfill process had the greatest impact on the base plate and sidewalls, with average contributions of 43.5% and 30.9%, respectively. Additionally, the PCA-HC method proved effective for identifying potential cracks in large-scale assembly components through strain field data measured by OFDR, with cracks observed at the mid-span thin-walled cavities of the roof during hoisting and positioning.