Seismic Response of Wind Turbines on Caisson-Type Foundations in Soft Clay

Increasing interest in offshore wind turbines internationally has led to an increasing demand for seismically robust designs. Where subsoils are predominantly soft clay, an effective foundation option for resisting the design loads is a suction caisson but there is limited experience of such foundations in earthquake loading. Previous research has suggested that shallower foundations attracted less seismic load and were therefore preferable from an earthquake design perspective, but the wider applicability of this was unclear. In the present study, the finite element code PLAXIS is used to create a 3D representation of a caisson type foundation in a soft clay, modelled using the HSSmall model to capture nonlinearity and hysteretic soil behaviour. The results are validated against the original physical model. The study is then widened to four foundation geometries and four strong motions. Response spectra recorded on the foundation at ground level differ greatly from surface response spectra but are very well matched with response spectra generated at the depth of the caisson base. This is seen to be because the caisson behaves as a rigid object controlled by accelerations at its deepest level. It is consequently recommended that the spectra used for tower designs on such foundation can be estimated based on response of free field soil at the depth to which the foundation will reach.