Ground Improvement of Dredged Fills with Two Improved Vacuum Preloading Methods: Case Study

In a full-scale case study, the performance of two methods—the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)—were compared in terms of the dredged soft soil ground of the Xiang'an International Airport pavement reinforcement project in Xiamen City, China. The pore water pressure dissipation, settlement, lateral displacement, and consolidation degree were measured in the zones reinforced using the two methods. The direct shear test and field vane shear test were conducted before and after improvement. The S-VP and A-VP methods were loaded for 308 and 228 days, respectively. The results show that both reinforcement methods significantly improved the shear strength and foundation bearing capacity of the soft soil ground. The S-VP method has about 30% higher final vane shear strength and more uniform settlement compared to the A-VP method. However, the dissipation value of the average pore water pressure and the maximum foundation settlement were 5.6% and 11% higher, respectively, in A-VP than in S-VP. Compared to the S-VP method, the A-VP method decreased the maximum lateral displacements by 15%–20%, and increased the maximum reinforcement depth by 11%. Finally, compared with the use of sand cushion and vacuum pump, the A-VP method was more economical in terms of cost and energy use.