Theoretical and experimental investigation of the multiaxial soil response around monopile foundations

Large diameter monopiles are still the preferred foundation method for offshore wind farm developments. The performance of the pile under cyclic lateral load is the leading design criterion for this foundation option. In order to investigate the cyclic response of the soil surrounding the pile, it is simplistically assumed that soil elements located in the direction of pile loading are subjected to axisymmetric triaxial loading, while those on the side of the pile are subjected to simple shear loading conditions. Conversely, by setting up a 3-D finite element analysis for a hollow cylinder driven pile, this paper will investigate the real multiaxial (6-D) stress paths induced in soil elements around the pile. It will be shown how the stress paths change according to (i) the orientation of the soil element with respect to the pile loading direction, (ii) the depth of the soil element from the ground surface and (iii) the lateral distance of the soil element from the pile. It will be found that most of the soil elements (even those in front and on the side of the pile) will follow a multiaxial stress path, with continuous orientation of principal stress axis. This research will then develop guidance on which experimental apparatus and loading conditions can be imposed to satisfactorily approximate the loading history experienced by each soil element. Differently from common practice, the necessity of using multiaxial stress equipment (e.g. the hollow cylinder torsional apparatus) will be demonstrated. Examples of real experimental tests and test results under the loading conditions representative of laterally loaded pile will be also shown.