Fully assessing foundation failure envelopes under combined loads in spatially variable clay

Soil spatial variability is a common situation in marine geotechnical engineering, while most studies have primarily focused on evaluating the uniaxial vertical (V), horizontal (H), or moment (M) bearing capacity of foundations in spatially variable clay. Whereas the process of combined bearing capacity analysis by probe method is complex, resulting in low efficiency. As a result, the simplified modified swipe (SMS) method is proposed in this paper, which can represent the capacity in any direction of the load space accurately and takes just 6.5 times the duration of a single probe loading for a half-failure envelope. Based on the established three-dimensional random finite element models, the effectiveness of this method is demonstrated by analyzing the combined bearing capacity of five kinds of foundations in spatially variable clay. Furthermore, Monte Carlo simulations revealed that the spatial variability of the soil cause various failure envelopes, and the response of the combined bearing capacity is not identical in all directions. The VHM failure envelopes with different probabilities are given and could be found that the safety factors recommended by the guidelines are insufficiently conservative for structures susceptible to soil spatial variability. Finally, a strategy for applying different safety factors to both load directions is proposed, which could be helpful for the design of foundations in spatially variable clay.