Failure envelopes for offshore shallow foundations under general loading

The interaction of vertical, horizontal and moment (VHM) loads acting on a shallow foundation is complex, and it is becoming increasingly popular to represent ultimate limit states under general VHM loading as failure envelopes in three-dimensional load space. General loading is of particular interest offshore, where harsh environmental conditions lead to large horizontal and moment foundation loads. Shallow foundations with peripheral skirts that penetrate into the seabed are used to resist large lateral and overturning forces. During undrained loading, tensile resistance can be mobilised on the underside of the base plate by suctions developed within the skirt compartment. This paper presents failure envelopes and kinematic mechanisms for undrained ultimate limit states of circular skirted foundations in uniform and heterogeneous deposits under general VHM loading based on finite element results. An approximating method is proposed that permits accurate prediction of ultimate limit states under a full range of general loading.