Experimental Study on the Behavior of Bucket Foundations under Sustained Uplift Loading

Bucket foundations can be subjected to uplift loads caused by factors such as wind and wave action on the structure, loop currents, or the buoyancy of the floating structure they support. Current design approaches for bucket foundations loaded into tension tend to allow for frictional resistance along the skirt/soil interface, and neglect the additional tensile capacity generated from negative excess pore pressure, due to uncertainties on the duration over which this component of tensile capacity can be sustained. This paper addresses this uncertainty through an experimental study that investigates the behavior of bucket foundations in lightly overconsolidated clay when subjected to sustained uplift loading. The data indicate that the sustained load is resisted by a combination of skirt friction and the development of negative excess pore pressure—or “suction”—at the underside of the foundation. The experiments indicate that the duration over which a sustained load can be applied safely to a bucket foundation is dependent on both the load magnitude and the foundation skirt length, with practically infinite durations associated with loads that are lower than the frictional capacity provided by the bucket skirts. The data are presented in dimensionless forms and fitted by algebraic equations that provide a first-order approach to estimating both the magnitude of upwards vertical displacement associated with a sustained uplift load and the duration over which this load can be safely applied.