Evaluation of Rotational Stiffness for Uplifted Wind Turbine Gravity Foundation in Clays

Design for the widely used gravity foundations supporting wind turbines often requires rigorous rotational stiffness analysis in addition to the conventional foundation design requirements such as overturning stability, bearing capacity, and settlement. The rotational stiffness is related to the shear modulus of the bearing soils and the foundation size. The size may vary depending on the experience and skill of engineers while the stiffness is subject to degradation under cyclic loads. With the trend of using larger wind turbines and reducing the construction cost of the foundation, allowing the foundations to uplift during turbine operation is gradually adopted in the wind energy industry which was not previously allowed according to some practical guidelines. Since an uplifted foundation reduces the foundation’s effective bearing area and increases the potential for soil degradation under cyclic loads, it is imperative to develop a method to judge the risk associated with the uplift ratio. Aiming to this goal, this paper performed a study on the failure probability of rotational stiffness of circular gravity wind turbine foundations in clays. The foundation sizes for given wind loads and site conditions were assumed normal distributions, and the uplifted ratios corresponding to the diameters of the foundations were obtained for different load conditions. Numerical simulations with Plaxis 3D for these uplifted foundations using a soil-hardening with a small strain constitutive model were carried out to obtain excess pore water pressure and degraded stiffness. The stiffness degradation was interpreted with shear-strain and excess pore water pressure ratios, and the probability distributions were fitted. Finally, the failure probability for the rotation stiffness was analyzed utilizing Monte Carlo simulations based on performance functions, which were built based on ACP (2021) and empirical equations recommended by DNV.