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 ª , 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 ª 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 ª 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 ª . 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 ª . These charts are compared with existing guidance for spudcan bearing capacity.La présente communication décrit une série de tests sur modèle centrifuge examinant la force portante verticale de fondations plates et circulaires sur le sable, typiques des fondations spudcan des plates-formes autoélévatrices en mer. Il en ressort qu'avec des encastrements peu profonds, le facteur de force portante N ª , mobilisé par une semelle conique, est sensiblement inférieur, parfois d'un facteur de 2, à celui d'une semelle plate. Des solutions de plasticité indiquent que cette différence ne peut être attribuée à des différences théoriques de N ª découlant de la rugosité et de la géométrie du cône : ces différences théoriques sont faibles (<10%). On suggère, par contre, que dans le sable dense, le pré-cisaillement induit par la forme conique de la semelle donne lieu à une forme de détérioration progressive, et qu'en consé-quence on doit appliquer un angle de friction opération-nel inférieur pour les fondations coniques par rapport aux semelles plates. É tant donné qu'il existe des solutions précises pour N ª , l'incertitude clé pour la prédiction de la force portante est l'angle de friction approprié, qui est affecté par ce mécanisme de détérioration progressive. On introduit une contre-analyse, incorporant les corréla-tions de contrainte /dilatance de Bolton dans un calcul itératif de la force portante, dans laquelle on fait ...

Undrained monotonic simple shear response of clayey sands and silts

“…These estimated values are in good agreement with the constant volume friction angle (φ cv ) of 31 • determined for the host fine sand White et al (2008) and the φ cv value of 24 ± 2 • for normally consolidated kaolin soil Lehane et al (2009).…”

Multi-directional force–displacement response of underground pipe in sand

“…White et al (2008) show that the bearing capacity of a shallow foundation at the ground surface in sand is related to shape, with the bearing capacity of a conical circular foundation as low as 50% of the bearing capacity of a flat foundation with the same diameter.…”

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