Scour depth estimation in a balanced cantilever bridge with deteriorated central hinges based on natural frequencies: field measurements, methodology for estimation and verification

A scheme based on field measurements of vibrations is proposed for estimating the scour depths of a multi-span balanced cantilever bridge resting on foundations embedded in soft soil. Field measurements of ambient vibrations using velocity sensors indicated elongated periods of vibration in the transverse direction at the pier tops due to scour underneath. The modal properties were identified using the Eigensystem Realization Algorithm. The unknown spring constants of the central hinges were determined to represent the deteriorated performance of the bridge superstructure under no-scour benchmark conditions from a finite element model incorporating soft soil conditions. A single set of spring constants compatible with the measured natural frequencies was thus identified for the transverse movement of the central hinges. The strong dependence of the transverse bending mode of the pier and weak dependence of the longitudinal bending mode of the pier on scour phenomena were clearly observed. The observed relationships between the scour depth and natural frequency were sensitive to soft soil conditions in both modes. The estimated spring constants representing soft soil were used to assess the unknown scour depths in the pier foundations suffering from scour. Independent bathymetric surveys verified the general applicability of the proposed scheme for estimating the scour depths around bridges of this type within reasonable limits.