# Shaking Table Testing of Liquefied Soil Layer Located in the Bottom Slab of a Subway Station
## 3 Analysis of Test Results of Foundation Soil
### Analysis of Pore Water Pressure  
The pore water pressure sensor in the soil is denoted by the symbol \"P1-P8\". The pore water pressure sensor in the soil is shown in Figure 11.  
The pore pressure ratio time history curve is shown in Figure 12. Due to the failure of point P2, there is no pore pressure ratio time history curve for this point. The pore pressure ratio at each position exhibits a positive correlation with the rise in seismic intensity. Each point in the study observed a quick increase in pore pressure with the arrival of the peak of the seismic wave, followed by dissipation once the peak of the seismic wave has passed. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Points**} & \multicolumn{4}{c}{**Main Frequency (Hz)**} \\ \cline{2-5}  & **FD-2** & **FD-3** & **FD-4** & **FD-5** \\ \hline A4 & 6.968 & 7.216 & 7.220 & 5.126 \\ A5 & 6.968 & 7.216 & 7.220 & 1.467 \\ A6 & 6.968 & 7.216 & 1.468 & 1.477 \\ A7 & 1.464 & 1.476 & 1.468 & 1.477 \\ A8 & 1.464 & 1.476 & 1.468 & 1.477 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Primary frequency of each measurement point under different working conditions. Point P1 is located at the interface between the saturated and unsaturated sand. During an earthquake, the pore water pressure accumulates within the saturated sandy soil and gradually dissipates into the unsaturated sandy soil. This dissipation occurs together with the self-weight of the unsaturated sandy soil. Consequently, the pore pressure ratio at point P1 is significantly reduced. Figure 11: Arrangement of pore pressure sensors. Figure 12: Time history course of pore pressure ratio at each point under the action of the Beijing Hotel wave. (**a**) Input 0.3 g; (**b**) input 0.4 g; (**c**) input 0.5 g; (**d**) input 0.6 g.  
Point P3 is located at the interface between saturated and unsaturated sand, and point P4 is located in the saturated sand 200 mm below point P3. During seismic loading, the increase in pore water pressure at point P3 dissipates into the unsaturated sand. However, at point P4, there is no way for dissipation, resulting in a higher pore pressure ratio compared with point P3. Points P5 and P7 are located at the interface between the saturated sand and the bottom plate of the station and points P6 and P8 are located in the saturated sand 200 mm below the bottom plate of the station. Regardless of the intensity of the seismic wave, the pore pressure ratio at points P5 and P7 consistently exceeds that at points P6 and P8. The observed phenomena can be attributed to two primary factors. The first of these is the absence of suitable conditions for the dissipation of pore water pressure at the bottom plate of the station. Second, the gravitational force exerted by the subway station is comparatively lower than that exerted by an equivalent volume of sandy soil. Consequently, the subway station fails to generate sufficient counterforce to balance the increasing pore water pressure. When the peak acceleration of the seismic wave input reaches 0.5 g, the pore pressure ratio between points P5 and P7 is 0.5. However, when the peak acceleration of the seismic wave input increases to 0.6 g, the pore pressure ratio at point P5 approaches 1, while the pore pressure ratio at point P7 exceeds 1. This observation suggests that the soil at the bottom plate of the station undergoes complete liquefaction. Points P1, P3, P5, and P7 are located at the same height as the bottom plate of the station and are all located at the interface between unsaturated and saturated sand. The distribution of the peak pore pressure ratio at the interface is shown in Figure 13. When the peak acceleration of the input seismic wave is below 0.5 g, there is minimal variation in the peak pore pressure ratio at any one location. The pore pressure ratio abruptly increases at points P5 and P7 when the seismic wave's peak acceleration reaches 0.5 g, which indicates that the soil below the station's bottom plate has clearly manifested liquefaction.