Group effect on bearing capacities of tripod bucket foundations in undrained clay

• Group effect of tripod bucket foundations in clay was analyzed by 3D FE analysis.
• The vertical and horizontal capacity factors reach constant values at S/D≥2.
• The moment capacity factor increases linearly with S/D ratio.
• Capacities of tripod buckets can be calculated by using group efficiency factors.
• Moment capacity can be calculated by the moment arm and the single bucket capacity.

Tripod bucket foundations are being used as alternative foundations to resist large horizontal and moment loadings from offshore wind turbines. The bearing capacity of the tripod bucket foundation differs particularly from that of the single bucket foundation due to the interaction among individual buckets of the tripod. This paper investigated the group effect of tripod bucket foundations in clay by three-dimensional finite element analysis. Parametric studies were performed varying the spacing between each bucket foundation, embedded depth of the bucket foundation, and loading directions. Normally consolidated clay under an undrained condition was modeled using a linear elastic-perfectly plastic model obeying the Tresca failure criterion. The undrained shear strength and Young's modulus of clay were assumed to linearly increase with depth. The group efficiency factor, which is the ratio of the bearing capacity of the tripod bucket foundation to that of the single bucket foundation, was evaluated based on finite element analysis for vertical, horizontal and moment loadings. Results showed that the efficiency factors were functions of the bucket spacing and embedment depth of the foundations.