In this paper the results of two and three-dimensional finite element analyses of the combined loading of strip and circular skirted foundations resting on homogeneous and non-homogeneous clay are presented. The results are presented in terms of the shape of the failure envelope in vertical, moment and horizontal loading space and compared with data from similar studies where either soil nonhomogeneity or three-dimensional geometry was investigated.


Much of the existing work relating to failure conditions of shallow foundations pertains to strip footings subjected to predominantly vertical loading resting on homogeneous deposits. In the field of offshore engineering foundations are typically quasi-circular, the selfweight of the superstructure coupled with environmental loading to the superstructure from wind and wave forces cause complex combined foundation loads (Figure 1). In addition, most marine deposits show increasing shear strength with depth, particularly in the soft lightly over-consolidated sediments found in deep water. The conventional bearing capacity equation, originally proposed by Terzaghi (1943), in conjunction with the various modification factors to account for load orientation and non-uniform soil conditions (e.g. Brinch-Hansen, 1970; Meyerhof, 1980) has been shown to be nonconservative for combined loading of strip footings even in homogeneous clay soils (Ukritchon et al, 1998), and its reliability becomes increasingly questionable for circular footings on nonhomogeneous soils subjected to combined loading (Martin and Randolph, 2001).

Skirted foundations

Skirted foundations are shallow foundations, usually circular, with thin skirts (either a single circumferential skirt or multiple concentric skirts) which penetrate the soil vertically such that there is a bearing area on the seabed with a confined soil plug underneath. The presence of the skirt alters the behaviour of the foundation significantly compared to the more conventional type of flat or conical shallow foundations, enhancing vertical, horizontal and moment load capacity.

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