Vacuum–surcharge preloading is widely used in soft soil foundation reinforcement projects. To study the characteristics of the changes in pore water pressure and settlement of the soft soil foundation as a result of the effect of vacuum–surcharge preloading, in this paper, a consolidation model was established for large–sized sand–drain fluid–solid coupling along the depth direction while taking the sequence of the actual surcharge and vacuum into consideration by using FLAC^3D, and a soft foundation’s consolidation process was calculated and analyzed in detail. The results showed the following: (1) the sand drain could be used as a load boundary condition and a vacuum load could be realized by adjusting the pore pressure at the nodes. (2) For the effect of a single vacuum, the settlement and pore water pressure were evenly distributed on the horizontal plane, and the maximum calculated value of the surface settlement was 90.92 cm, but the phenomenon of a “pot–bottom shape” of the settlement and pore pressure gradually became obvious with the increase in the surcharge, and the maximum calculated value of the surface settlement was 153.01 cm. (3) In the numerical calculations, it was found that the pore pressure and settlement distinctly fluctuated once each level of surcharge was applied. The changes in the settlement were basically consistent with the measured values after the adjustment of positive and negative pressure, and the variation trend of the pore pressure was consistent with the measured value. (4) The depth affected by the vacuum could reach at least 16 m. All of these findings are expected to provide references for the calculation of consolidation in vacuum–surcharge preloading.

There are few calculation methods for the design and construction of vacuum preloading to strengthen soft foundations. Based on the FLAC3D, a calculation model was established for the vacuum preloading project of the Beijing–Shanghai high-speed railway. Through calculation and comparison of measured values, the following results were obtained: (1) The top surface of the reinforcement area and the sand drain can be regarded as the load boundary, which can be realized by assigning the node pore water pressure. (2) After 30 days of vacuum action, the settlement rate at each depth decreased significantly and the deformation gradually became stable. It is reasonable to design the vacuum preloading time as 2–4 months. (3) The negative pore water pressure has different transmission times and uneven distribution, which makes the consolidation time and degree of soil on the same level uneven. After 30 days of vacuum action, this uneven phenomenon will be transformed into a uniform phenomenon. (4) The change time of pore water pressure under vacuum preloading is about 30 days. After that, the pore water pressure at different depths will tend to have different constant values. The influence depth of vacuum preloading can reach 16 m. These works can make up for the deficiency of vacuum preloading calculation methods.