Is under-base filling necessary?

Abstract

In the past decade, suction caissons have emerged as a preferred offshore foundation solution for wind turbines due to their silent installation process and potential for recyclability. However, there has been growing speculation regarding the necessity of under base filling, which involves filling the gap between the top plate of the suction caisson and the seabed. Some experts have suggested that under certain conditions, this under base filling may not be required at all. Furthermore, concerns have been raised about the efficacy of under base filling in achieving full contact between the top plate and the seabed, as it has been observed that gaps may persist even after the filling is applied. Consequently, doubts have been cast on the overall need for under base filling. However, there is limited research focused on understanding the behavior of water plugs in the absence of under base filling, at different loading conditions ( Compression , tension , cyclic etc.). This knowledge gap motivates this thesis study, which aims to investigate the behavior of water plugs specifically in dense sand samples, as sand is considered more critical compared to clay in terms of its variability in drainage conditions that can influence the foundation’s performance. To achieve this, a series of centrifuge tests were conducted on suction caissons that were partially installed and some fully installed. The results of the experiments shed light on the role of under base filling in different loading scenarios. Under monotonic compressive loading at higher rates, it was observed that under base filling played no significant role in the load transfer . Both the caissons with and without under base filling exhibited similar load transfer mechanisms, indicating that filling the gap may not be necessary in such loading conditions. Additionally, under tension loading, it was found that under base filling had little to no effect on the development of tensile capacity. By expanding our understanding of the necessity and effectiveness of under base filling, this study contributes to the ongoing discussion surrounding suction caisson design and installation practices for offshore wind turbine foundations.