# Dynamic Response Analysis of Soil around Curve Section Tunnel under Train Vibration Load
## 3 Analysis of Pore Water Pressure Response to Vehicle-Induced Vibration Loads
### Variation Patterns of Pore Pressure at Different Depths under Vibration Loads  
During train operation, the response of the pore water pressure sensors buried at different depths to the changes in pore water pressure caused by the subway vibration is different. Taking May 3 rd as an example, Figure 7 shows the daily pore water pressure monitoring data at the K3-2 monitoring point, and the train operating schedule. The first train arrived at the monitoring point around 6:07, and the pore pressure was detected by the instrument. The pore pressure detected at this time was static water pressure with a value of 148.869 kPa. The pore pressures detected in the next three minutes were 148.874 kPa, 148.884 kPa, and 148.879 kPa, with excess pore water pressures of 0.005 kPa, 0.015 kPa, and 0.01 kPa, respectively. The excess pore water pressure detected during the first pass at the  
Figure 5: Change in excess pore water pressure at manual monitoring points.  
Figure 6: Change in super-pore water pressure at automatic monitoring point.  
K3-2 monitoring point was 0.015 kPa. After the train passed, the excess pore water pressure slowly disappeared.  
Due to the accuracy of measurement, it is not possible to determine the peak value of excess pore water pressure with certainty. However, by reviewing the entire day's monitoring data at the K3-2 monitoring point, it can be seen that after train operations, there are constantly occurring peaks of excess pore water pressure that are generated and dissipated, and the values are not significant. The maximum excess pore water pressure tested that day was 0.265 kPa. The pore water pressure fluctuated between 148.582 kPa and 149.134 kPa, and gradually decreased in the afternoon period without any apparent accumulation of pore pressure. This is because the time interval between trains allows for the dissipation of excessive pore water pressure before the arrival of the next train.  
As the train vibration load is applied, the pore pressure at K3-2 and K3-3 rapidly increases and reaches a peak in a short time. After the night train stops operating, the pore pressure at K3-2 shows a slow upward trend, but at K3-3, the pore pressure remains in a fluctuating state after the train stops operating, as shown in Figure 8. The pore pressure at K3-4 gradually increases with time, and the groundwater level at this point is also rising. A small downward trend is observed in the morning peak period (8:00-10:30) and evening peak period (20:00-21:00), considering the peak hours and a certain time delay effect, as shown in Figure 9. According to the distribution of each layer in this small radius interval, it can be known that K3-2 is located in the silty soil and silty sandy soil layer, while K3-3 is located in the silty sand layer, and K3-4 is located in the silty soil. This indicates that the silty sand layer is more susceptible to the impact of train vibration.  
Figure 8: Variations in pore pressure for K3-3 on May 3 rd.  
Figure 7: Variations in pore pressure for K3-2 on May 3 rd.  
Based on the above, it can be inferred that the response of the soil surrounding the tunnel to train vibration loading is closely related to the location, and the sensitivity of the response increases the closer the soil is to the tunnel. After exceeding a certain depth range, the disturbance impact of train vibration on the soil gradually decreases, and the soil is more affected by changes in groundwater level and rainfall. The pore pressure in the soil is basically the same as the static water pressure at the same depth, and it increases as the depth increases.