# Application Study of the High-Strain Direct Dynamic Testing Method
## 3 High-Strain Direct Dynamic Test and Static Load Test
### The High-Strain Direct Dynamic Test  
After preparing the field test equipment, the pile hammer was lifted to the designated height and released from rest. Each hammer strike was followed by waiting for the pile to come to a complete stop before the next strike. The testing process strictly adhered to the provisions of the Code for Testing of Building Foundation (DB)/T 15-60-2019) [39]. Using the high-strain dynamic tester and fiber optic demodulator, extensive strain and acceleration data were collected. A Python program was developed to process the input data, yielding time-history curves of velocity, acceleration, and dynamic resistance at various depths of the pile. The ultimate static side resistance for each pile segment was found through this analysis. These values were then added up to obtain the measured bearing capacity values for the four test piles, which can be seen in Table 4. The table also lists the single-blow penetration depths, as determined by the precision level. The table indicates that the single-blow penetration depths for all four test piles meet the code requirements (2-6 mm according to the local code), suggesting that the soil resistance around and below the piles was fully mobilized. Given the soil layer distribution characteristics beneath arch bases 1-4, representative key measurement points were selected among the four test piles. At these points, Figures 7 and 8 present the velocity and acceleration time-history curves, respectively. After the pile hammer's significant impact force, the pile experienced downward acceleration, causing compressive deformation within the elastic range. As the impact force propagated downward, the velocity curves' peak times varied at different depths, and the pile's peak motion velocities gradually decreased. This reflects a reduction in axial force along the pile  
Figure 6: On-site construction drawings. (**a**) Fabrication of steel cage and binding of FBG. (**b**) Pouring concrete. due to the side soil resistance. After the impact stopped, the pile rebounded elastically and eventually returned to a relatively static state. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**File No.** & \begin{tabular}{c} **Maximum** \\ **Impact Force at** \\ **Pile Top (kN)** \\ \end{tabular} & \begin{tabular}{c} **Pile** \\ **Penetration** \\ **(mm)** \\ \end{tabular} & \begin{tabular}{c} **Pile Side** \\ **Resistance** \\ **(kN)** \\ \end{tabular} & \
\begin{tabular}{c} **Vertical Designed** \\ **Bearing Capacity of** \\ **Single File (kN)** \\ \end{tabular} &
\begin{tabular}{c} **Vertical Allowable** \\ **Bearing Capacity of** \\ **Single File (kN)** \\ \end{tabular} \\ \hline
1–5 & 12,103.8 & 4.24 & 6637.7 & 4777.8 & 11,415.5 & \(\geq\)9510 \\ \hline
2–7 & 10,907.1 & 2.13 & 6001.3 & 3732.0 & 9733.3 & \(\geq\)8688 \\ \hline
3–9 & 11,540.5 & 3.44 & 7729.2 & 3059.2 & 10,788.4 & \(\geq\)9402 \\ \hline
4–17 & 13,070.5 & 3.36 & 9012.7 & 3838.6 & 12,851.3 & \(\geq\)11,160 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Measured value of test pile bearing capacity. By analyzing the axial forces at the upper and lower nodes of a pile segment and the segment's average acceleration, the dynamic resistance of the segment at any time can be determined. Figure 9 illustrates the dynamic resistance time-history curve for a 2 m pile segment above a key node. As the relative displacement between the pile and soil increased, the soil resistance on the pile side also increased, leading to a gradual decrease in dynamic resistance and a corresponding reduction in the amplitude of its fluctuations. Eventually, the dynamic resistance reached a steady value rather than returning to zero because of the relative displacement between the pile and soil after the hammer impact. Since the hammer impact duration was very short, the surrounding soil did not undergo significant consolidation settlement, thus remaining relatively static. Therefore, side friction resistance continued to act after the pile's self-vibration ceased.