Vacuum Preloading for Enhanced Uplift Performance of Suction Buckets in Soft Clay: Model Tests and Hydro-Mechanical Finite-Element Analysis

Suction bucket foundations provide an efficient foundation solution for offshore wind turbines, but improving their uplift resistance in soft clay remains a practical challenge. Vacuum preloading has been an effective means to enhance soft clay engineering properties, yet its offshore application is constrained by the difficulty of maintaining a reliable underwater seal. This study proposes soil-plug vacuum preloading, in which the impermeable skirt and lid of an installed suction bucket serve as a natural sealed boundary for post-installation soil improvement for enhanced uplift performance. Model tests on a bucket equipped with a central prefabricated vertical drain (PVD) were conducted to examine vacuum transmission, soil consolidation, and uplift behaviour. Two vacuum-preloaded cases with prefabricated vertical drain lengths HPVD = L and HPVD = 2L, where HPVD denotes the PVD length and L denotes the bucket skirt length, were compared with an untreated case. Coupled hydro-mechanical finite element analyses were performed to interpret the observed responses. The tests showed that ultimate pullout capacity increased by 197% for HPVD = L and 288% for HPVD = 2L. The maximum negative pore pressure beneath the lid increased by 89% and 154%, respectively, while the remaining non-suction resistance also increased markedly due to vacuum-induced consolidation. Prototype-scale numerical analyses of a double-walled bucket were further conducted to investigate the effects of the bucket length-to-diameter ratio (L/D = 0.6, 1.0, and 1.5) and the applied vacuum pressure (0 to −70 kPa) on the uplift response. Among the analysed cases, the largest increase occurred for L/D = 1.5 under an applied vacuum pressure of −70 kPa, where the predicted uplift load at a displacement of 0.20 m increased by up to 42% compared with the no-vacuum condition. These results provide proof-of-concept evidence that soil-plug vacuum preloading may offer a potentially feasible post-installation approach for improving the uplift response of suction buckets in soft clay, although further experimental and field-scale validation is required.