Bearing Capacity of Foundations Under Highly Eccentric Loads

In the case of wind turbines the eccentricity of the loads at foundation level is generally very high and very often the application point of the vertical load falls outside the core of the foundation. In the paper, two types of foundation of a tall wind turbine under highly eccentric loads are considered and designed: A shallow foundation and a pile group foundation. The design of the shallow foundation is based on the requirement of bearing capacity and on the allowable rotation for a safe operation. On the contrary, for pile groups the main design requirement is to avoid a bearing capacity failure, since the rotation limitation at foundation level is usually ensured by the high rotational stiffness of the group. Widespread practice for pile groups design is to consider the achievement of the axial capacity (in compression or in uplift) on the outermost pile as the ultimate limit state of the pile group. This approach neglects the ductility of the foundation system and therefore is uselessly conservative. A recently proposed and more efficient approach for the evaluation of ultimate moment-axial force interaction diagrams for a pile group is applied, and comparison with the design of a shallow foundation is presented.