Combined loading of spudcan foundations on clay: numerical modelling

The load-displacement response of a spudcan foundation on clay is described by means of an incremental plasticity model with three degrees of freedom (vertical, rotational, horizontal). The model is termed 'Model B' and emplo ys a yield surface and flow rule that are derived from a programme of caref ully controlled small-scale laboratory tests. Behaviour inside the yield su rface is defined by a set of elastic stiffness factors for embedded conical footings, determined from three-dimensional finite element analysis. The h ardening law, which defines the vertical bearing capacity as a function of plastic spudcan penetration, is based on a set of theoretical lower bound b earing capacity factors for embedded conical footings. Care is needed to en sure consistent treatment of partially and fully penetrated spudcans under combined loading. Full details of the elastic and elasto-plastic stiffness matrices needed to predict incremental load-displacement behaviour are prov ided, and the performance of the model is demonstrated by simulating some o f the laboratory calibration tests numerically. While Model B succeeds in c apturing many important features of the observed spudcan behaviour, its pre diction of sudden rather than gradual yielding is a noticeable weakness, pa rticularly in the simulation of cyclic loading events. Practical applicatio ns of the foundation model are discussed with reference to the soil-structu re interaction analysis of independent leg jack-up units.