Observations of shallow skirted foundations under transient and sustained uplift

Abstract

Skirted foundations, or bucket foundations, provide an economically attractive foundation solution offshore to support or anchor structures for the oil and gas industry. A key benefit of skirted foundations lies in their potential to mobilise uplift resistance due to negative excess pore pressures developed between the soil plug and the base plate. Current industry recommendations (DNV, 1992; API, 2009; ISO, 2003) are based on classical bearing capacity theory (Terzaghi, 1943; Brinch-Hansen, 1970; Vesic, 1975) and provide a conservative estimate of undrained collapse loads for skirted foundation systems, particularly due to ignoring the beneficial effect of suction beneath the foundation during transient uplift or overturning. Although industry guidelines and recommended practices acknowledge that temporary suction caused by dynamic loads may allow greater capacities to be mobilised, since the effect is temporary they advise that suction should not be accounted for in the design unless substantiated by appropriate analysis or experimentation. To date there is no formal guidance regarding the timescale over which negative excess pore pressures may be sustained.