This technical note presents a simplified method for quantifying the added mass of the soil that is mobilised as part of the failure mechanism around a foundation during rapid loading, and the resulting additional soil resistance. This note focuses on the solutions for an embedded plate anchor, which is a potential foundation system for offshore floating facilities. In current practice, only the shear strength of the soil surrounding the foundation is considered in calculations of the ultimate bearing capacity. However, the solutions presented in this technical note show that the added mass of the soil involved in the failure mechanism around the foundation can result in a significant increase in ultimate bearing capacity during extreme dynamic wave loading events. These lead to snatch loads transmitted to the anchoring and mooring system, which are high but brief. The technical note provides a general approach applicable to all foundation types, and illustrates the effect of the added mass term and the additional capacity for an embedded plate anchor with typical input conditions.