STUDY OF BEARING CAPACITY OF SURFACE CIRCULAR FOOTING ON STIFF-SOFT-STIFF CLAYS

The stratigraphic configurations for the design of offshore shallow foundations usually involve multi-layer soil profiles due to their large sizes. The stiff-soft-stiff clay deposit is one of the most prevalent configurations. Design methods are not available in the current API and ISO standards for shallow foundations to estimate the bearing capacity of a footing on stiff-soft-stiff clays. Based on the "bottom-up" approach recommended for spudcan foundations of jack-up rig,this paper assumes an analytical model that sums the resistances from the punching shear of the stiff-soft layering system and the squeezing of the soft-stiff layering system. The rationality of this predictive model is verified based on comprehensive FE analyses. The comparison between the predictions from "bottom-up" approach and FE analyses indicates that the assumed model is reasonable and realistic. However,the resistance due to squeezing response is significantly underestimated by the current design formula. Detailed investigations are carried out. It is revealed that this is mainly because the effect of the overlying stiff clay on the squeezing mechanism is neglected. Based on the findings,an improved design formula is proposed for the squeezing response and hence for the design of circular foundations on stiff-soft-stiff clays.