A comparison of the bearing capacity of flat and conical circular foundations on sand

Abstract

This paper describes a series of centrifuge model tests investigating the vertical bearing capacity of flat and conical circular foundations on sand, which are representative of the spudcan foundations of offshore jack-up units. It is found that at shallow embedment the bearing capacity factor, N γ, mobilised by a conical footing is significantly lower - by a factor of up to 2 - than that for a flat footing. Plasticity solutions indicate that this discrepancy cannot be attributed to the theoretical differences in Nγ that arise from the roughness and geometry of the cone: these theoretical differences are minor (<10%). Instead, it is proposed that in dense sand the pre-shearing induced by the conical shape of the footing leads to a form of progressive failure, so a lower operative friction angle is applicable to conical foundations than for flat footings. Since accurate solutions for Nγ exist, the key uncertainty in the prediction of bearing capacity is the appropriate friction angle, which is affected by this progressive failure mechanism. A backanalysis that incorporates Bolton's stress-dilatancy correlations in an iterative calculation for bearing capacity is introduced, using recently published definitive values of N γ. This framework is calibrated using centrifuge model test data. The progressive failure mechanism is captured in a simple fashion by varying Bolton's m parameter, which controls the dilatant contribution to peak strength. This back-analysis is used to create simple charts that illustrate the effects of stress level, density and progressive failure on the operative friction angle and Nγ. These charts are compared with existing guidance for spudcan bearing capacity.