The Impact of Installation Method on Monotonic Loading of Monopiles in Sand

Abstract

Monopiles are the most commonly used foundations for offshore wind turbines (OWTs). To date these are mostly impact driven into the seabed using high-capacity hammers. The noise and vibrations caused by driving create environmental concerns and the increasing size of these foundations has resulted in alternative installation methods being explored. In this paper a comparison is made between piles installed by vibration and driving. The results of full-scale field trials for pile pairs installed in dense sand by driving and vibration are presented. Lateral load tests were performed to determine the effect of installation method on the capacity and stiffness of the piles. In this paper 3D finite element model predictions of the pile response are reported. The soil model adopted was the HS small model in PLAXIS. The measured pile responses are compared to predictions made using soil parameters derived using correlations with Cone Penetration Test, CPT end resistance qc value (known herein as the direct approach) and through correlation with sand relative density. It is seen that the stiffness and capacity of the vibratory installed piles were lower than driven piles. The finite element model predictions using the input parameters based on relative density underestimate the stiffness response at low displacement levels whilst providing good predictions for displacements in excess of 2cm. The direct correlation approach provided good predictions of the initial response whilst over-estimating the stiffness for displacements above 2cm. The influence of sand creep on the field response of the piles and the performance of the numerical predictions is discussed.