Evaluation of soil models for improved design of offshore wind turbine foundations in dense sand

Abstract

In order to perform optimised and safe design of foundations for offshore wind turbines (OWT), it is important to have calculation tools that describe the key features of water-saturated soil subjected to complex and irregular loading over a wide range of strain levels. Soils subjected to cyclic loading are prone to strain accumulation. The accumulated (plastic) volumetric strain may result in excess pore pressure or stress relaxation, which will reduce the effective stresses, stiffness and strength of the material. Strain accumulation in dense sand is a complex mechanism of deformation and it is challenging to describe it properly. Four different soil models to describe the stress–strain relationships of dense sand are evaluated in this paper: two implicit models that follow the actual stress history and two explicit models that calculate the accumulated strains as a function of number of cycles. These models are first evaluated on the basis of their theoretical framework and back-calculations of laboratory tests specifically carried out for the design of OWT foundations in dense sand. Second, the models are implemented in finite-element analyses and evaluated on the basis of the analyses of an OWT monopile subjected to different loading conditions.