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

Abstract

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. Land reclamation projects will produce a large area of ground composed of ultrasoft soil, which has very low strength and cannot be used for construction activities directly on such ground. For such ultrasoft soil ground, the vacuum preloading method is generally used to strengthen the ultrasoft soil. In this study, two different vacuum preloading methods [the vacuum-surcharge preloading method (S-VP) and the air-booster vacuum preloading method (A-VP)] were used to strengthen the ultrasoft soil, and the final reinforcement effect and cost of the two methods were compared. The results show that the final reinforcement effect of the A-VP method was close to that of the S-VP method. Furthermore, the A-VP method saved more than 3 million Chinese currency (CNY) and reduced the power consumption by 80% in this project. In addition, the implementation of the A-VP method does not require a large amount of sand that is difficult to obtain in coastal areas, which is good protection for the natural resources in coastal areas. Therefore, in the ultrasoft soil reinforcement project, the A-VP method is a method worth considering.