Modal analysis of subway tunnel in soft soil during operation

Modal parameters are of great significance in civil engineering because they can characterize the properties of structures and be used for vibration control and structural health monitoring. Subway tunnels are long linear truss structures combined with the mutual coupling of the surrounding soil. Therefore, the operational modal analysis of a mutual coupling tunnel is complicate, as is the modal identification of shield tunnels in a time–frequency domain, and these are hot civil engineering topics. Using the shield tunnel of Shanghai metro line No.12 project as a case study, we carried out the vibration response monitoring of a subway tunnel during operation and presented methods to identify structural modal parameters. The modal parameters of lower vibration modes were estimated using response measurements. Modal frequencies and shapes were identified with high precision and accuracy using the orthogonal polynomial clustering algorithm under hammer excitation conditions and the autoregressive-moving-average model under ambient excitation conditions. The dynamic behavior of a mutual coupling tunnel presented obvious low frequency characteristics, and the first 9th order mode frequencies were less than 100 Hz. The diagonal values of the modal assurance criteria were all greater than 0.85. The modal parameters can be used for the health monitoring of operational subway tunnels.