This article investigates the integrity testing ability of the sonic echo test by using a field-constructed group pile foundation. The energy of the reflected stress wave is relatively weak when a pile cap is presented during pile testing, and signal processing techniques were generally used to enhance the weak signal. The conventional real wavelet transform is difficult to use to identify the time of the reflected wave of the pile toe. The purpose of this article is to improve this drawback of conventional real wavelet transforms using a complex wavelet transform. This study utilized the amplitude and phase message of a complex continuous wavelet transform to determine the pile length of piles that are grouped with a pile cap by analyzing the time-frequency-phase angle diagram in the wide frequency band. Cases of group piles with two and four piles were studied. The results show that the suggested approach is able to detect the reflected signal from the pile toe, as the testing was performed on a pile cap. The results also show that the measured errors of pile length ranged from 2.5 % to −6.1 % in the cases of this study.
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