## 3 Test Results  
### Radial Peak Ground Acceleration and Lateral Earth Pressure  
#### 3.1.1 Open-Ended Pipe Pile Tests  
During pile driving, the surrounding soil was affected by the squeezing and vibration effects simultaneously. To determine the characteristics of the soil after being subjected to striking vibration, the earth pressure and radial peak ground acceleration (referred to as RPGA herein) applied to the pile at the same time were analyzed. Figure 5 compares the RPGA and increment of lateral earth pressure at various depths of the open-ended pipe pile at distances of 40 mm and 120 mm from the pile, where X represents the horizontal distance between the pile and the sensor, H represents the burial depth of the sensor, and \(L_{p}\) represents the length of the soil plug.  
Figure 4: Arrangement of sensors in soil and data acquisition system.  
\begin{table}
\begin{tabular}{c c c c} \hline  & **Type of Sensors** & **Position** & **Index** \\ \hline \multirow{2}{*}{Open-ended} & Earth pressure cell & X = 1D, 4D; & Earth pressure \\  & Accelerator & H = 125, 250, 375, 500, 550 mm & RPGA \\  & Accelerator & X = 1D–5D; H = 0 & VPGA \\ \hline \multirow{2}{*}{Pile with a fixed-length soil plug} & Earth pressure cell & X = 1D, 4D; & Earth pressure \\  & Accelerator & H = 125, 250, 375, 500, 550 mm & RPGA \\  & Accelerator & X = 1D–5D; H = 0 & VPGA \\ \hline \end{tabular}
\end{table}
Table 3: Setups of sensors.  
Figure 5: \(Cont\).  
Figure 5a-d corresponds to a 125 mm and 250 mm sensor embedment, with a minor crest of earth pressure and acceleration occurring simultaneously when the pile tip penetrated to a depth slightly shallower than the burial depth of sensors. Meanwhile, the oblique distance between the pile tip and the sensor was relatively close, and the vibration of the pile tip induced a squeezing effect and vibration responses. As the penetration depth increased, the increment of earth pressure decreased gradually and ceased to change substantially. When the penetration depth of the pile tip exceeded 400 mm, another crest of RPGA appeared that was no longer consistent with the trend of earth pressure variations. As the soil plug formed and adjacent soil was compacted, each strike caused less penetration of the pile as the pile descended, resulting in the conversion of striking energy to vibrational energy that was dispersed into the surrounding area by the energy that caused the pile to descend. Therefore, a crest of acceleration will be generated at a deeper penetration depth. This indicated that changes in wave motion at shallow source locations were not completely synchronized with changes in earth pressure: when the oblique distance between the pile tip and the sensor was relatively close, the earth pressure and acceleration appeared to have small crests, synchronously; with the increase of penetration depth, the cooperative effect of the soil plug, the compacted soil, and the diffusion of striking energy to the adjacent soil resulted in the occurrence of a subsequent, more significant crest of acceleration. In other words, there will be a second wave crest at shallow source locations during the striking process, and the vibration response corresponding to the second wave crest generated at a deep penetration depth will be more pronounced, which should be considered in the practical pile driving process.  
According to Figure 5e,f, the earth pressure and RPGA reached a crest at a depth of 375 mm, followed by a second crest of acceleration at a depth of 400 mm. The difference in amplitude between crests of acceleration was not significant. At depths of 500 mm and 550 mm, the earth pressure and RPGA simultaneously reached their peak values, as shown in Figure 5g-j. At this time, the oblique distance between the pile tip and the sensor was relatively close, and the penetration depth of the pile tip exceeded 400 mm, resulting in the appearance of an acceleration crest.