Effect of strength non-homogeneity on the shape of failure envelopes for combined loading of strip and circular foundations on clay

The capacity of surface foundations on clay under pure vertical (V), horizontal (H) or moment (M) loading may be expressed in non-dimensional form through the use of appropriate bearing capacity factors, with values that will be affected by the shape of the foundation and also any variation of undrained shear strength with depth. A common assumption has then been that the shape of the complete failure envelope in three-dimensional loading space (V, M, H) will be similar regardless of foundation shape and soil non-homogeneity, once scaled to the appropriate apex points. The appropriateness of this assumption has been explored by means of two- and three-dimensional finite element analyses of strip and circular footings, for a simple Tresca soil model where the shear strength varies linearly with depth. With a view to applications involving partially embedded foundations, such as offshore skirted foundations, full suction and 'bonding' with the underlying soil has been assumed. The paper documents the normalised capacities under uniaxial (V, M or H) loading, and compares the shapes of the failure envelopes in the three planes H = 0, M = 0 and V = 0 for a practical range of strength gradients. The broad conclusion is that a single shape does indeed hold in the M = 0 plane, for both strip and circular foundations, but that for the H = 0 and V = 0 planes the overall size of the normalised failure envelope reduces as the degree of strength non-homogeneity increases. Hence the assumption of a failure envelope shape derived for homogeneous strength conditions would be unconservative.