Soil-structure interaction of a on-shore wind turbine from long term cyclic loading

Abstract

Onshore wind turbines are very tall and slender structures that often have shallow foundations. This type of structure is prone to rocking under applied cyclic loading and must withstand millions of cycles over a 20 year design life from the wind and rotation of the blades. Wind loading is complex and highly variable in frequency and magnitude, with storm events causing large increases of horizontal load. In addition to rocking, uplift often occurs under these larger design loads. A prototype wind turbine was designed for numerical and experimental modelling to investigate the behaviour of an onshore wind turbine with shallow foundations in a slightly overconsolidated kaolin soil, subject to cyclic loading at varying wind speeds. The loading patterns applied were designed to model normal operation of the turbine interspersed with storm events. This allowed investigation of the performance of the structure during and after periods of rocking and uplift. The model was considered to be a two-dimensional (2D) scenario, as the cyclic loading was applied in one plane only. The experimental modelling was conducted at the University of Dundee, with five cyclic tests conducted at 1g and one cyclic test at 50g in the 3m beam geotechnical centrifuge. The experimental modelling presented three main behavioural characterised determined from the reaction of the structure, describing changes in the force reaction, foundation profile and foundation-soil contact. The three main behaviours are amplification, constant and recovery, these behaviours express the long term behaviour of the soil-structure interaction and the ability for the structure to regain strength after large cyclic events. The soil-structure interaction was shown to change though recording of the natural frequency of the structure. The natural frequency decreased in all 1g tests, displaying a softening of the soil structure interaction. Numerical modelling was carried out using 2D finite element analysis in which the soil response was described by the Modified Cam-Clay constitutive model, calibrated using data from experimental testing. The numerical simulations captured many of the mechanisms governing the response of the turbine and allowed for prediction and visual representation of the accumulated deviatoric strain and displacement in the soil body. The findings presented in this thesis show that on-shore wind turbines foundations can be slightly under designed using less resources. Where current practices see foundation uplift as a failure of the design, it can be used as structural protection against cyclic loads that can cause structural degradation.