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

White, D.J., Teh, K.L., Leung, C. F. and Chow, Y.K. (2008) A comparison of the bearing capacity of flat and conical circular foundations on sand. Geotechnique, 58 (10), 781-792. (doi:10.1680/geot.2008.3781).

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 N_γ 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 N_γ 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 N_γ. These charts are compared with existing guidance for spudcan bearing capacity.

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Contributors

Author: D.J. White

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