Fluid–Solid Coupling Numerical Analysis of Pore Water Pressure and Settlement in Vacuum-Preloaded Soft Foundation Based on FLAC3D

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.