# Transfer Learning-Based Detection of Pile Defects in Low-Strain Pile Integrity Testing
## 2 Background  
Destructive and non-destructive methods are two primary approaches used for pile quality testing, each serving different purposes in assessing the condition and performance of foundation piles [18]. Destructive methods involve physically altering or damaging the pile during testing to evaluate its structural strength, material quality, and failure points. These methods are typically used in load testing scenarios where the goal is to determine the ultimate load-bearing capacity of the pile by applying either static or dynamic forces until failure occurs. While destructive methods provide highly accurate and direct measurements, they are costly, time-consuming, and result in the need for pile repair or replacement, making them unsuitable for widespread application across large projects. Non-destructive methods, on the other hand, assess pile integrity without causing damage. These techniques are designed to detect internal defects, such as bulge imperfection, neck defect, weak concrete, cracks, or broken piles (Figure 1) by analyzing reflectograms collected during testing. In the rest of this section, non-destructive methods are classified according to the theoretical approaches they use [19; 20]. While various non-destructive testing methods exist, LSPIT remains the most widely used method for its cost-effectiveness, ease of implementation, and suitability for detecting structural defects in piles without requiring access to deep layers. (1) _Elastic (seismic) waves:_ This approach is based on the principle of generating an internal body wave and tracking the propagation of the wave along the pile body to identify areas where defects occur. Techniques that use this approach are listed below: \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Method** & **Data Type** & **Defect Types** & **Dataset Size** & **Accuracy** \\ \hline RNN [7] & real & 6 & 25 intact, 300 defective & 98.46\% \\ \hline ELM [8] & real & 2 & 860 intact, 63 defective & 87.50–96.77\% \\ \hline ANN [9] & real & 5 & 40 in total & - \\ \hline ANN [10] & real & 3 & 10 in total & 80\% \\ \hline CNN [11] & real & 4 & 1200 intact, 3600 defective & 98.58\% \\ \hline CNN [12] & synthetic & 5 & 2000 defective & 94.4\% \\ \hline CNN [13] & synthetic & 5 & 1000 intact, 200 defective & 99.17\% \\ \hline CNN (this study) & real & 2 & 246 intact, 82 defective & 98.2\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of the proposed method with existing machine learning approaches. 1. _Low-Strain Pile Integrity Test (LSPIT) [21]:_ The response of the impact created by hitting the pile head with a light hand hammer is recorded by a motion transducer (accelerometer). The pressure wave produced by the impact travels along the pile and is reflected from the pile tip. Anomalies within the pile are detected by analysis of the reflected wave. Due to its low cost and simple implementation, LSPIT is the preferred choice in many practical applications. 2. _Transient Response Method (TRM) [4,22]:_ The low-strain impact method is based on velocity data only. This method requires the recording of velocity and force data. For this purpose, an instrumented hammer is used to measure the force. TRM collects more information than LSPIT, helping to identify defects near the top of the pile. 3. _Bending Waves Method [23]:_ This method is suitable for situations where conventional testing cannot access the pile head, such as in support structures for offshore generators. It involves measuring transient bending waves--generated by lateral impacts on the pile's exposed side and traveling up or down the shaft--using two accelerometers positioned around the shaft to directly capture the surface vibration response. 4. _Parallel Seismic [24]:_ This method is used to determine the lengths of deep foundations where the pile head is inaccessible. A borehole is installed next to the pile to be tested, and a geophone is lowered down the borehole while the pile head is impacted with a hammer.