Dynamic soil-pile-interaction effects on eigenfrequency and damping of slender structures

Single pile foundations have been widely used as a support solution for offshore wind turbines (OWT), where the design has been driven towards large diameter monopiles in order to satisfy the deformation limitations in the superstructure. The aim of the current study is to illustrate the effect of the dynamic soil-pile-interaction on the natural vibration characteristics of the flexibly supported structure. For this purpose a two-step iterative procedure has been developed based on two analytical solutions. The frequency dependent dynamic stiffness and damping coefficients are taken into consideration after a rigorous solution of horizontal soil – pile vibration, while the modified SSI eigenperiod and damping are calculated accounting for the cross coupling stiffness and damping terms of the soil – pile system. Disregarding the off diagonal terms is considered inappropriate since it results to non-conservative overestimation of the eigenfrequency and underestimation of damping especially for small slenderness ratios and high flexibility factor of the soil – pile system (short, rigid piles). The observed trends become even more prominent as the height of the slender structure increases. The effect of the monopile foundation properties on the natural vibration characteristics was also examined. The influence of the frequency dependent impedances was proven significant, since the modified SSI eigenfrequency decreased substantially, when the structural eigenfrequency was set between the first and the second eigenfrequency of the soil layer.