## 6 Conclusions  
In this study, the application of the wavelet packet transform (WPT) was demonstrated for the efficient noise suppression, identification of the feature parameters, and detection of defects in the integrity testing of piles by impact echo test method. Diagnostic rules, which have been lacking in the literature, were presented to serve as a guide in interpreting decomposed signals and in analyzing various characteristics of peak waveforms that are associated with certain types of defects. In this study, the finite element method was used to simulate the impact echo test of nine cases of defective piles. To verify the proposed scheme, six data sets of the nine cases of defective piles were made, in which a total of 54 piles were analyzed. Using the decomposed signal by WPT after amplification, the characteristics of the velocity reflection responses of the various simulated impact echo test of different cases of piles were clearly identified through their associated geometric and material defects present at the pile. Based on the results of the study, the following conclusions were drawn:  
* Segments with weak concrete were hardly detected using method 2 (nonlinear amplification of the original signal without WPT applied) if its modulus of elasticity was almost the same as the reference modulus of elasticity and were moderately detected if the length and area of the very weak segment were short and small.
* Neck defects and bulge imperfections were hardly detected using method 2 if the defects were small and if the segment with the defect was long.
* Notches and cracks were hardly detected using method 2 if it was located nearer to the top of the pile and if the depth of the defect was shallow.
* Using method 1 (WPT-based method), a very short extent of a weak pile segment might be misinterpreted to be a defect due to a reduction in cross-section area, especially when the measured reflection time at the pile tip was almost equal to the reference time.
* Based on the observations of the velocity waveforms for pile defects due to reduction in the area of the pile section, it was not possible to distinguish among a neck, crack, and void using method 1 when the length of the neck or void was short.
* Using method 1, it might be difficult to distinguish a reduced pile area defect from being a long necking or a long and narrow void.
* Even though the integrity testing on piles might not identify all minor imperfections, the results of the study showed that the identification method based on WPT coupled with the conventional amplification method could be a useful tool in identifying major defects within the effective length  
Figure 19: Comparison of detection performance between method 1 (amplification method and WPT) and method 2 (amplification method only).  
of the pile compared to the method by signal amplification alone, wherein the identification method based on WPT detected defects 87.04% of the time compared to the conventional method, which only detected defects 64.81% of the time.  
Conceptualization, J.L.M. and H.-J.K.; supervision, H.-J.K.; simulation and validation, J.L.M.; analysis, J.L.M. and P.R.D.; writing--original draft preparation, J.L.M.; writing--review and editing, P.R.D.; project administration, H.-S.K.; funding acquisition, H.-J.K. and T.-W.P. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Research Foundation of Korea (NRF), grant number NRF-2017R1D1A3B03034189, and the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant number 20183010025200. The APC was funded by Brain Korea 21 Plus.  
This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) (NRF-2017R1D1A3B03034189) and Brain Korea 21 PLUS.  
The authors declare no conflict of interest.