A Macro-Element design method for shallow foundations accounting for SSI effects: characterizing response as failure envelopes

Skirted foundations are increasingly used in the design of offshore wind turbine structures and facilities to withstand combined vertical (V), horizontal (H) and moment (M) loading conditions. Skirted foundations have been used widely for offshore wind energy converters based on their ease of installation, cost effectiveness the avoidance of noise emission compared with deep footings such as monopiles and piled foundations.

Classical design methods and determining separate modification factors could not accurately consider interaction of the effective parameters in the bearing capacity expressions. Nowadays, the conventional bearing capacity approaches of a foundation under combined vertical, horizontal and moment loads are being replaced and most usually expressed by the planes of H:V, H:M, M:V interaction envelopes and sometimes in 3D space through a failure surface. In the interaction diagrams or failure envelopes, the effects of foundation geometry, horizontal load eccentricity and various soil conditions can be interacted.

The capacity of shallow foundations on sand under combined loading will be expressed here in nondimensional form through the use of appropriate failure coefficients that will be affected by the foundation characteristics and stress-dependent stiffness.

Having identical failure envelopes upon scaling with the pure bearing capacities (i.e., which are given by intersections with the axes), the assumption appropriateness will be examined here by means of an alternative Macro-Element model in presence of the soil strength inhomogeneity.