Transiently Loaded Bucket Foundations in Saturated Dense Sand - Demonstration of the Boot Effect

The mono bucket foundation is a cost-effective foundation for offshore wind turbines. During a storm, these foundations are exposed to large wave loads of short duration. This paper investigates the effect of increased loading rate on the bearing capacity of two mono bucket foundations installed in dense sand inside a pressure tank. The foundations had aspect ratios (skirt lengths [L] relative to the diameters [D]), L/D, of 0.5 and 1.0, respectively. Foundations were brought to failure with varying loading rates, resulting in drained, partly drained, or undrained behavior. Increases in bearing capacity were observed as the loading rate increased. This behavior was caused by a combination of dilative soil behavior and suction created by upward movement, known as the boot effect. For mono bucket foundations, the boot effect resulted in an increased bearing capacity that was 18 to 25 times higher than the drained capacity. Furthermore, the boot effect also led to an increase in stiffness. The stiffness of the partly drained response is measured up to four times higher compared with drained behavior.