The horizontal-moment capacity of embedded foundations in undrained soil

The bearing capacity of embedded foundations subject to combined horizontal and moment loading in undrained soils is of interest particularly to the offshore geotechnical engineer. These loadings can be experienced by monopod foundations and offshore manifolds and often take the form of a horizontal load applied at a lever arm height above the foundation base. Previous design methods using the failure envelope approach have suggested that peak moment capacity of a foundation is mobilized with positive horizontal loads, but that peak horizontal load is mobilized with no additional moment loading. This paper reports numerical work specifically investigating the effects of the embedment ratio on the horizontal-moment foundation capacity under no vertical load in both uniform strength and "normally consolidated" undrained soil. It is shown that the embedment ratio significantly affects the eccentricity of the M–H failure envelope so that the shape varies when normalized by the pure moment (M0) and pure horizontal load (H0) capacity. The reasons for the eccentricity of the failure envelopes are understood by investigating soil deformation mechanisms calculated during FE analysis and by accompanying upper bound plasticity analysis. This mechanistic understanding is used to suggest load reference point translations that ease curve-fitting of the complex failure envelopes for use in design.