Abstract: Underwater two-way vacuum preloading not only reduces more excess pore water pressure comparing to conventional vacuum preloading with vacuum acting only on the top face, but also has the advantage of utilizing the effective load from the overlying water. However, two-way vacuum preloading is currently only applied in the treatment of dredged soil, thus the mechanism and performance of consolidation of two-way vacuum preloading are hardly studied. To explore the consolidation characteristics and efficiency of underwater two-way vacuum preloading, centrifuge modelling has been performed to simulate soft soil subjected to underwater two-way vacuum preloading in collaboration with group sand drains and single sand drain. The finite element method is also used to analyze and compare the results with those from centrifuge modelling test. Variation of pore water pressure and development of deformation are compared. Meanwhile, changes in total head of pore water and stress path of soil element, as well as degree of consolidation are discussed and evaluated. It is found that more effective load can be obtained from underwater two-way vacuum preloading comparing to conventional vacuum preloading, and the rate of consolidation is larger and the ultimate settlement can be reduced in group sand drains zone comparing with single sand drain zone. The stresses for soil in the center of treated zone follow a path close to K₀ line. Under the combination effects of vacuum and gravity, the reduction of pore water pressure at the bottom layer is larger than that at the top layer, and it is found that a lower water head exists at the bottom at final consolidation stage in this experiment. These findings may enhance the understanding and practical application for two-way vacuum preloading.

Key words: two-way vacuum preloading, underwater ground, centrifuge modelling, numerical analysis, consolidation