Soft Soil Foundation Treatment with an Improved Vacuum Preloading Method: A Case Study

The traditional vacuum preloading method for improving soft soil foundation often encounters problems like substantial vacuum pressure loss along the length and clogging. In response, this paper introduces and discusses an improved vacuum preloading method, referred to as flow direct pressurize vacuum preloading combined with surcharge (FDP-S), which integrates flow prefabricated vertical drains, direct and sand-free cushion, intermittent pressurization, vacuum energy-saving technology, and surcharge preloading. To verify the feasibility of the FDP-S method, field experimental research was conducted in a subgrade treatment project along the coast, where the thickness of the soft clay ranged from 15 to 34.8 m. Soil samples were collected from the construction site before foundation treatment, and the properties of the soils were investigated through laboratory tests, which used as the reference to analyze the effect of the foundation treatment. During the treatment process, pore-water pressure, settlement, and lateral displacement were measured. The vane shear strength and foundation bearing capacity were also tested. Simultaneously, numerical simulations were utilized to compare and analyze the treatment effects under different loading conditions. The results indicated that the improved FDP-S method significantly enhanced the treatment effect of soft soil foundation, with an overall increase in the shear strength of the improved soil by 2.2–2.6 times and bearing capacity exceeding 120 kPa. Within this method, the combined action of gas injection and surcharge achieved optimal settlement results, improving by 22.4% and 39.3% compared with loading conditions wherein gas injection or surcharge was excluded individually. The research results provide practical and operable case guidance for similar soft soil treatment projects.