Bucket foundations under lateral cyclic loading: Submitted for the degree of doctor of philosophy

To enable a prosperous development of offshore wind energy, economically feasible technologies must be developed. The monopod bucket foundation is likely to become a cost-effective sub-structure for offshore wind turbines and has the potential to make offshore wind more cost-competitive in the energy market. This thesis addresses issues concerning monopod bucket foundations in the hope of providing tools and ideas that could be used to optimize the design of this sub-structure.

The work is focussed on the behaviour of bucket foundations under lateral cyclic loading. Other related and propaedeutic topics, such as bucket foundations under transient lateral loading and under monotonic lateral loading, are also investigated. All the scientific work is fundamentally based on small-scale experimental tests of bucket foundations in dense water-saturated sand.

The most important scientific documents on bearing capacity and installation of bucket foundations are reviewed and the results from the models found in literature are compared to the experimental results obtained in the current study. Monotonic tests of bucket foundations under lateral loading until failure are compared with existing failure envelopes. A jacked installation test is successfully compared with existing models.

Tests of bucket foundations under lateral loading applied at different loading rates are analysed. As expected, the bearing capacity of bucket foundations under transient lateral loading increases dramatically with the loading rate. Though, there is no difference in the initial stiffness. Pore pressure transducers inside and around the foundations recorded the distribution of the pore water pressure during loading.Horizontal and rotational displacements are not found to be influenced by the loading rate.

A comprehensive experimental campaign of bucket foundations under lateral cyclic loading is interpreted with an existing empirical model that calculates the long-term rotation. The model is calibrated for dense sand. The model calibration reveals that the parameters are significantly dependent on the relative density but not on the embedment ratio. The ultimate capacity of bucket foundations pre-subjected to cyclic loading is found to be larger than the monotonic capacity recorded with standard quasi-static tests.

A macro-element model for bucket foundations supporting offshore wind turbines is developed on the base of monotonic tests. To simulate the cyclic loading response, a boundary surface model is integrated into the macro-model. The model is able to reproduce the monotonic and cyclic experimental results quite well. Nevertheless, a proper strategy on how to evaluate the parameters of the boundary surface model is yet to be established.