Frequency-Based Vibration Energy Indicators for Assessing Mechanical Degradation in Riverbank Soils

Progressive mechanical degradation in riverbank soils is governed by gradual reductions in small-strain stiffness and concurrent increases in material damping under repeated low-amplitude environmental excitation. These processes modify wave propagation characteristics within the soil mass and result in systematic changes in the frequency distribution of vibration energy. This study evaluates frequency-based vibration energy indicators as physically grounded measures for assessing mechanical degradation in riverbank soils within a soil dynamics framework. Ground vibration records are analyzed in the frequency domain using power spectral density (PSD) techniques to quantify band-limited vibration energy within low-, mid-, and high-frequency ranges. Energy ratios between frequency bands and high-frequency spectral attenuation slopes are derived to characterize stiffness- and damping-controlled components of the dynamic response. The results show a consistent redistribution of vibration energy from high- to low-frequency bands along the riverbank, reflecting progressive softening of the soil skeleton and enhanced energy dissipation at small strain levels. In particular, increases in the low-to-high frequency energy ratio and steepening of high-frequency spectral decay are indicative of mechanically degraded zones. Because the proposed indicators are normalized in the frequency domain, they exhibit reduced sensitivity to excitation amplitude and allow meaningful spatial comparison between measurement points. The findings demonstrate that frequency-based vibration energy indicators provide a non-invasive and mechanically interpretable approach for assessing progressive mechanical degradation in riverbank soils, complementing conventional geotechnical investigations focused on large-strain behavior.