Calculation method for the ultimate torsional capacity of bucket foundations in clay

Bucket foundations are widely used in offshore fixed platforms, wind turbines, and floating structures. They are subjected to complex loads from the superstructure, among which the torsional load has a significant impact on foundation performance. Therefore, this study focuses on the bucket foundation under torsional load to address inaccurate calculations owing to the partial bearing phenomenon. First, the torsional bearing mechanisms of bucket foundations in normally consolidated and homogeneous soils are revealed through numerical simulations, and the limitations of the general equations for calculating the ultimate torsional capacity of bucket foundations are elaborated. Next, the torsional bearing characteristics under diversified conditions are determined considering the soil–bucket friction coefficient, bucket aspect ratio, and soil strength–heterogeneity ratio. A theoretical bucket foundation model under torsional loads is subsequently developed by summarising the observed torsional failure modes, and novel equations are derived to compute the ultimate torsional capacity of the bucket foundation accordingly. Finally, the effectiveness and accuracy of the proposed theoretical model are verified against the results of numerical simulations and its improved efficacy is demonstrated through comparison with the general equations. •The torsional failure mechanisms of bucket foundations are revealed through numerical simulations.•Key factors are investigated to reflect the torsional bearing characteristics of bucket foundations.•A novel theoretical method is proposed to calculate the ultimate torsional capacity of bucket foundations.