## 4 Analysis of Calculation Results
### Pore Water Pressure Analysis  
Figures 12 and 13 show the change curve of pore water pressure at each depth of the soil mass over time under the concave parabola and convex parabola distribution modes. During the calculation, we assumed that the initial value of pore water pressure at each point in the soil was hydrostatic pressure, i.e., a positive value, as the groundwater level was flush with the top surface of the model. After applying negative pore water pressure due to vacuum preloading to the top surface of the model and sand drains, the pore water pressure at each point in the soil decreased to varying degrees. Between 0 and 20 days, the pore water pressure decreased most obviously within a depth of 18 m. Between 20 and 30 days, the rate of pore water pressure decrease was significantly reduced and gradually became stable. After 30 days, the pore water pressure at each depth was basically unchanged and maintained at a constant value. Figure 11: Asymptotic fitting curve of calculated values. Figure 12: Pore water pressure–time curve under a “concave” parabolic distribution. The pore water pressure at a depth of 2 m under the concave parabolic distribution was \(-\)69.17 kPa, that under the convex parabolic distribution was \(-\)79.59 kPa, that under the uniform distribution was \(-\)80.28 kPa, and that under the linear distribution was \(-\)72.20 kPa. The pore water pressure at a 16 m depth under the concave parabolic distribution was 1.79 kPa, and that under the linear distribution was 5.84 kPa. The pore water pressure at an 18 m depth under the convex parabolic distribution was 10.52 kPa, and that under the uniform distribution was \(-\)5.38 kPa, indicating that the vacuum preloading influence depth was approximately 16 m under the \"concave\" parabola and linear distributions and approximately 18 m under the \"convex\" parabola and uniform distributions. Combining the comparison and analysis results between the settlement calculation and the measured values proved that the influence depth of vacuum preloading was approximately 16 m.  
Vacuum preloading causes a decrease in pore pressure in the soil. The greater the absolute value of negative pore pressure is, the better the consolidation effect of the soil is. However, the impact range of vacuum preloading on soil is limited. The maximum negative pore pressure is one atmospheric pressure that is impossible to achieve. The distribution pattern of pore pressure in sand drains for numerical calculation should be consistent with the actual situation. From the above settlement analysis, it can be seen that the linear distribution mode and the \"concave\" parabolic distribution mode are more in line with the actual situation. Due to the different mean values of the four sand drain pore pressure distribution modes, we can also infer that the decreasing values of pore pressure in soil are in descending order: uniform distribution mode, \"convex\" parabolic distribution mode, linear distribution mode, and \"concave\" parabolic distribution mode. Figure 14 shows the pore water pressure-time change curve in soil under the four distribution modes at a 2 m depth. Figures 15 and 16 show the pore water pressure-time curves of the four models and measured values at 4 m and 6 m depths. The measured pore water pressure curve had a large dispersion, which may have been caused by various factors, such as equipment error, construction activity, and human errors during monitoring. However, this curve can qualitatively explain some of the phenomena. The pore water pressure-time curve obtained by the logarithmic fitting of the measured values was found to be closest to the calculated value of the linear distribution model. Figure 13: Pore water pressure–time curve under a “convex” parabolic distribution. Figure 16: Pore water pressure–time curves of four distribution models and measured values at a 6 m depth. Figure 14: Pore water pressure–time curve under four distribution modes at a 2 m depth.