## 4 Analysis of Calculation Results
### Settlement Analysis
Figure 9: Settlement–time curve of four distributions at a 0 m depth. Many simplifications and assumptions were applied in the calculation model, such as assuming that the three soil layers were horizontal and homogeneous with the sand drain and smear layer and that the soil parameters remained constant. However, in reality, the soil layer was not horizontal or homogeneous due to its complex geological origin, and the soil parameters changed gradually during stress deformation, which resulted in the calculated curve being smooth, whereas the measured curve was more variable. During the first 5 days, the vacuum effect had little impact on the soil mass, and the settlement at each depth was minimal, resulting in highly consistent values between the four calculated curves and the curve of measured values. Between days 5 and 30, the vacuum negative pore water pressure was gradually transferred from the sand drain to a wider range of soil layers, resulting in rapid settlement of the soil mass at each depth. However, due to the complex structure of the soil mass and the anisotropy of negative pore water pressure transmission, the distribution of pore water pressure at each point in the soil mass was extremely unbalanced, resulting in a large deviation between the calculated value curve and the measured value curve. After 30 days, the calculated settlement values under the \"concave\" parabola and linear distributions gradually became consistent with the measured settlement values, whereas the difference between the calculated and measured settlement values under the \"convex\" parabola and uniform distributions remained at the same level as between days 5 and 30. After 30 days, the unbalanced pore water pressure in the soil gradually reached equilibrium and stabilized at a constant value. The pore water pressure at the same depth within the influence range of vacuum action was basically the same, indicating that the isotropic pressure values at each point at the same depth were consistent. The pore water pressure at different depths tended to have different constant values, and the isotropic pressure values at various points at different depths also differed. From the settlement calculation curve, using the two setting modes: \"concave\" parabolic and linear pore water pressure modes, the calculation results were significantly better than those yielded using \"convex\" parabolic and uniform distribution modes. The linear setting mode was slightly better than the \"concave\" parabolic setting mode. In the numerical calculation of the vacuum preloading reinforcement of soft foundation, the pore pressure in the sand cushion and sand drains under the membrane can rapidly decrease to a constant value in a short period of time, so the sand cushion and sand drains can be regarded as negative pressure boundaries. We can assign and adjust the node pore pressure of the sand cushion and sand drain area to achieve the loading effect of vacuum load. Based on the calculation results of the four negative pressure distribution modes of sand drains in the article, the calculation results using smaller pore pressure distribution modes (linear distribution mode and \"concave\" parabolic distribution mode) are in line with the actual situation on site. Since the consolidation settlement curve in  
Figure 10: Settlement–time curve under four distributions, measured at a 4 m depth. the linear mode is very close to the measured curve (especially the final consolidation settlement), we selected the surface settlement time curve in this mode for asymptotic fitting and obtained the calculation formula and coefficients, as shown in Figure 11.