Physical modelling of deep screw piles in medium-dense sands for floating wind applications

This paper presents centrifuge testing to investigate the uplift capacity of a deep screw pile installed in medium dense sand whilst varying the advancement ratio (AR) during installation. Although tested as individuals these piles are designed to form part of a group of nine piles designed as a single mooring point for floating wind applications. Previous research on the uplift capacity of large screw piles for offshore application has focussed on shallow failure mechanism where the failure surface propagates towards the soil surface inclined at the dilation angle. However, there is limited research on the deep failure mechanism of large screw piles when subjected to uplift loading. It is thought that deep screw piles may generate larger uplift capacity due to an increase in helix depth and the added contribution (often ignored onshore) of skin friction occurring along the longer pile shaft. Physical centrifuge modelling was used to investigate the uplift capacity of a deep screw pile at various advancement ratios (ARs). The tested screw pile had an embedment ratio of 17.5. The AR was varied below the industrial guidelines known as pitch-matched installation. The results indicated that by reducing the AR (known as over-flighting) to 0.5 and 0.25, the uplift capacity increased by 53% and 40% respectively, whilst also reducing the installation crowd force and even generating pile self-pull-in.