Enhanced lateral capacity of hybrid helical-skirted foundations in clay for offshore wind turbines

Helical piles present a promising solution for resisting uplift loads in offshore wind turbine (OWT) foundations, due to the additional vertical capacity provided by the helical plate compared with conventional driven piles. However, their capacity to resist horizontal and moment loads is inherently limited by the slender shaft and its low flexural stiffness. To overcome this limitation, this study proposes a novel hybrid foundation system comprising a single-helix helical pile combined with a skirted foundation. The undrained lateral behaviour of this foundation system in clay was investigated using three-dimensional finite element modelling, covering a range of geometric configurations and soil strength profiles. The results indicate a significant increase in lateral capacity and stiffness compared with standalone helical piles. A parametric study was performed to examine the effects of helix embedment ratio, skirt length, and skirt diameter on the lateral response. Load-sharing analysis revealed that, at small displacements, the skirted foundation carries the majority of the horizontal load, whereas at larger displacements, the helical pile contribution becomes more significant, with relative contributions dependent on the soil profile. Furthermore, the effect of the soil-foundation interface was evaluated, identifying gap formation behind the skirt as a critical factor affecting lateral capacity.