Investigation of the Load-Sharing Mechanisms of Suction Buckets during Vertical Cyclic Loading in Layered Soils

Suction bucket jackets have been used as foundations for offshore wind turbines in intermediate water depths where layered soil stratigraphies are often encountered. Although suction installation in layered soils has been studied, experimental data on the in-service response is scarce. During installation in stratigraphies containing a low permeability layer underlain by a high permeability layer, suction is transferred to the underlying layer when the pressure at the lid invert is sufficient to uplift the low permeability plug. This suction-transfer mechanism also affects the in-service response, albeit the load-sharing mechanism is not well understood. This paper presents data from centrifuge tests of suction buckets subjected to constant amplitude and varying amplitude cyclic vertical loading in two stratigraphies - a sand with an overlying clay layer and in a sand with a sandwiched clay layer. These experiments show that tensile stresses exceeding the vented tensile resistance can be withstood without significant uplift of the bucket in both stratigraphies, even under a zero mean stress. Plug uplift was shown to have an important effect on the amount of stress transferred to the skirts, with the load-sharing mechanism depending on the stratigraphy. Additionally, the load-sharing mechanism and the bucket in-service resistance was shown to depend on the effectiveness of the clay in sealing the soil plug within the bucket, with a more effective seal resulting in higher tensile resistance and therefore better performance. A limiting loading condition was not identified in the sand with a sandwiched clay layer, with the data indicating that the suction pressure to cause plug uplift during cyclic loading may be much higher than during suction installation.