Study on Local Damage Identification of a Masonry Retaining Wall Based on Wavelet Packet Decomposition

Masonry retaining walls are widely used in mountainous regions but are susceptible to progressive internal damage under environmental and operational loads, which is often difficult to detect through conventional visual inspection. To address this problem, this study proposes a baseline-free vibration-based damage identification method for existing masonry retaining walls. The method combines impulse response function (IRF) estimation with wavelet packet decomposition (WPD) and introduces a scalar damage index, termed the energy ratio standard deviation (ERSD). Unlike conventional WPD energy ratio deviation (ERD) vectors, ERSD condenses multi-band energy redistribution into a single positive scalar for each sensor location, thereby facilitating spatial interpolation and field-level damage localization without modal extraction. The method was validated through four monthly impact hammer tests on a masonry retaining wall in Zhenjiang, China. The results show that non-zero ERD vectors indicate vibration energy redistribution between successive monitoring states, while the spatial peak of ERSD identifies the most likely damage zone. The ERSD maximum occurred at point 5 and was confirmed by post-test visual inspection, which revealed a local crack of approximately 0.8–1.2 mm in the adjacent mortar joint. To avoid overfitting with the limited four-test dataset, the temporal trend of ERSD was evaluated using a linear regression and finite-difference progression rates rather than a high-order polynomial. The proposed method provides a practical preliminary screening tool for field damage localization; however, its quantitative damage severity calibration requires further validation using controlled stiffness-reduction tests and environmental compensation models.