Improvement of the undrained capacities of foundation as a result of preloading, has received attention in offshore engineering only recently. It offers the benefit of optimising foundation design and reducing footprint and cost. This paper investigates the preloading performance of skirted circular foundations in clay using 3D coupled finite-element analyses. The increase in the ultimate undrained uniaxial vertical, horizontal and moment capacities is provided for non-dimensional groups of foundation skirt length, in situ undrained shear strength heterogeneity, magnitude of vertical preload and normalised consolidation time. An exponential relationship between preloading gain in capacity and normalised consolidation time is established, with the maximum preloading gain expressed as a function of the three other dimensionless groups. An approach to estimate the post-preload undrained ultimate uniaxial capacities of skirted circular foundations on normally consolidated clay is ultimately proposed. The influence of foundation geometry is discussed via comparison with the existing solutions.