Focusing on the fact that rocking vibration is dominant in structures with rectangular cross-sections, we proposed a method to identify the natural frequencies of structures with high accuracy. We conducted a series of tests on a model caisson-type breakwater to investigate the applicability of the proposed natural frequency identification method to the soundness assessment of a breakwater under several degrees of scouring. Our results indicated that the progress of scouring of the caisson-type breakwater can be detected by microtremor measurements.


Ports and harbors are indispensable forms of social capital for supporting economic activities. However, these structures have been damaged by tsunamis and waves, causing them to tilt and subside owing to scouring of the foundation ground (Arikawa et al., 2012). To restore port and harbor activity earlier, the integrity of the structures post-damage must be evaluated as soon as possible in a quick and simple manner using overwater diagnosis.

Oumeraci (1994) classified the damage from vertical breakwaters as overall global and local failures. The progress of the local failures can lead to overall failures. Therefore, integrity monitoring of the existing caisson structures is necessary. Natural frequency has often been used as an index to evaluate the soundness of structures (Kariyawasam et al., 2020; Lee et al., 2020; Belmokhtar et al., 2021). Natural frequencies of the caisson due to impact excitation by a rubber hammer were investigated at different damage levels of the foundation mound (Lee et al., 2012; Lee and Kim, 2015; Zhang et al., 2016). An attempt to identify experimental modal parameters of wharf structures was also made using microtremors produced by tides, wind, and waves (Boroschek et al., 2011). However, studies on evaluating the vibration characteristics of caissons using microtremors are limited. This is because the vibration level of a caisson is lower than that of a port structure. Microtremor observations are conventionally used as a method that does not require impact excitation. However, they are occasionally ineffective because the peak in the observed Fourier spectrum of the structures is often unclear (Masui et al., 2008).

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