Partially-Drained Loading of Shallow Foundations on Sand

Prof. Guy Houlsby and Jan Mangal

This work was sponsored by the EPSRC (support for Jan Mangal)

The development of marginal oil-fields and novel offshore structures has resulted in the use of relatively small shallow foundations, such as suction caissons. These foundations are cheaper and easier to install than piled or large gravity-base systems. They can also be removed from the seabed as part of a decommissioning programme. The use of shallow foundations in the offshore environment has necessitated the development of new design guidelines. This is especially the case with regard to the influence of pore-fluid pressures on the behaviour of shallow foundations on sand, since the response under typical wave loading is partially-drained. Liquefaction and load rate effects can lead to a reduction in integrity. The effects of load rate, and consequent pore-fluid pressures, on the behaviour of a model footing on sand were investigated. The results of this experimental investigation have led to the development of a preliminary theoretical framework for modelling partially-drained behaviour. The risk of liquefaction, even under monotonic loading, was also detailed.

This research is applicable to suction caisson foundations on sand. Although design guidelines exist for estimating the ultimate capacity under fully-drained or undrained conditions, the interaction of load rate and pore-fluid flow is not well understood.

The load:deformation results from an experimental investigation of a 1-g flat-circular footing, on an oil-saturated sand base, will be reported. A definite trend was observed between the load rate and the load:deformation characteristics. This indicated the possible existence of unique partially-drained load:deformation responses. The nature of the pore-fluid distribution on the base of the footing was also an important factor, especially with regard to analysing the liquefaction mechanisms.

This research contains important and topical results from a laboratory investigation into the behaviour of shallow foundations on sand. These results make a significant contribution to the understanding of load rate effects on shallow foundations.