A probabilistic approach to the ultimate capacity of skirted foundations in spatially variable clay

Skirted foundations are used in offshore applications to resist the large horizontal and moment loads that are characteristic of the ocean environment. The combination of vertical-horizontal-moment (VHM) loading results in complicated stress conditions in the seabed and design is often based on VHM failure envelopes. These have generally been constructed by numerical analysis using a deterministic characterisation of soil properties and disregard the natural spatial variability of marine sediments. In this study, spatial variability is taken into account by coupling a random field model with finite element analysis. The paper presents a probabilistic analysis of the ultimate capacity of skirted foundations in spatially variable undrained clay. The increase of strength with depth typical of a marine clay is included in the modelling framework. Probabilistic failure envelopes are constructed to analyse the effect of spatial variability when skirted foundations are subjected to different combinations of VHM loading. The results show that the probability of failure increases under high vertical loads and at peak moment capacity in the HM plane, suggesting that care should be taken in design at these areas of the failure envelope. The methodology demonstrates a straightforward and effective way of quantifying uncertainty in the ultimate limit state design of offshore geotechnical structures and the results presented provide specific guidance for the design of skirted foundations.