A plasticity model for spudcan foundations in soft clay

Abstract

A plasticity model for predicting the load displacement behaviour of a typical spudcan foundation for offshore jack-up platforms under combined vertical, horizontal, and moment loading in soft clay is presented. Results from geotechnical centrifuge experiments of a spudcan embedded vertically to 0.7, 1.0, and 1.45 footing diameters are described. Augmented with finite element results, these centrifuge experiments are used to evaluate the plasticity model components. As a result of soil backflow on top of the spudcan, enhanced combined bearing capacity was measured and this is reflected in increased yield surface size. A tensile vertical load capacity is also incorporated. The excellent predictive capabilities of the model are demonstrated by retrospectively simulating a selection of centrifuge tests. Résumé :Cet article présente un modèle de plasticité servant à prédire le comportement en chargement-déplacement d'une fondation avec caisson de support typique pour des plates-formes auto-élévatrices en mer soumises à des charges combinées verticales, horizontales et en torsion. On présente les résultats d'essais en centrifugeuse géotechnique d'un caisson de support enfoncé verticalement à 0,7, 1,0 et 1,45 fois le diamètre de semelle. Ces essais en centrifuge, combinés avec des résultats par éléments finis, sont utilisés pour évaluer les composantes du modèle de plasticité. En raison d'un retour de sol sur le dessus des caissons de support, une amélioration de la capacité portante combinée a été mesurée, et ceci est reflété par une augmentation de la taille de la surface de rupture. Une capacité de charge verticale en tension est aussi intégrée. Les excellentes capacités de prédiction du modèle sont démontrées par une simulation rétrospective d'une sélection d'essais en centrifugeuse. [Traduit par la Rédaction]Mots-clés : modèle de plasticité, argile molle, retour, caisson de support, chargement combiné, surface de rupture, centrifugeuse géotechnique.

On the instability of offshore foundations: theory and mechanism

“…This yield surface only accounts for the contribution from the underside of the spudcan while ignores the backflow at large embedment depths. Zhang et al [44] conducted centrifuge tests to characterize the yield surface in VHM space at various embedment depths and proposed a yield surface formulation as:…”

Jack-up push-over analyses featuring a new force resultant model for spudcans in soft clay

“…Employing a combined loading apparatus developed for a geotechnical drum centrifuge (Zhang et al, 2013), the VHM bearing capacity surface of a typical spudcan was investigated at various embedment depths (up to 1.45D) in normally consolidated kaolin clay in the enhanced gravity field of the centrifuge (Zhang et al, 2014a). To expand the experimental database for wider ranges of soil properties and embedment depths, a numerical approach with a large deformation finite element (LDFE) simulation of the spudcan installation and subsequent small strain finite element analyses to define the VHM bearing capacity surface was proposed (Zhang et al, 2014b).…”

“…The plastic flow rule is primarily based on results from centrifuge experiments (Zhang et al, 2014a). The plastic potential Table 1 Yield surface parameters (for a buried spudcan: has the following form:…”

“…tensile vertical load is allowed, which implies significant combined bearing capacity at low level of and tensile vertical load. This has been based on experimental evidence of kaolin clay in a geotechnical centrifuge (Zhang et al, 2014a), though caution should be taken to the level of tensile capacity provided β 1 ¼ 1.0, β 2 ¼1.0 and e is independent of vertical load level h 0 and m 0 values are listed in Table 1 where K ep is the elasto-plastic stiffness matrix, which needs to be defined. The total displacement δε consists of elastic displacement, plastic displacement and coupled displacement due to change of elastic stiffness, as represented respectively by the three parts of the right-hand-side of the following equation…”

A macroelement framework for shallow foundations including changes in configuration

“…These works brought about a paradigm shift in the study of non-linear SFI: it was acknowledged that soil specimens and foundational systems share, at different scales, many behavioural features, that can be modelled similarly. Since then, a number of ME models were developed in the plasticity framework for a very wide range of SFI problems, including, for example, offshore jack-ups and wind turbines (Martin & Houlsby, 2001;Houlsby & Cassidy, 2002;Byrne & Houlsby, 2003;Nguyen-Sy, 2005;Cassidy et al, 2006;Byrne, 2013;Foglia et al, 2014;Zhang et al, 2014), historical towers (Marchi et al, 2011;Pisanò et al, 2014), buried pipelines (Zhang et al, 2002;Calvetti et al, 2004;di Prisco et al, 2004;Cocchetti et al, 2008Cocchetti et al, , 2009aCocchetti et al, , 2009bTian & Cassidy, 2008;Cheuk & White, 2011), rock boulders impacting granular soil layers (di Prisco & Vecchiotti, 2006) and, lately, even piled foundations (Li et al, 2015). ME formulations for cyclic/dynamic-seismic SFI problems are also available in the literature, such as those proposed by Paolucci (1997), Crémer et al (2001Crémer et al ( , 2002, Shirato et al (2008), Chatzigogos et al (2009), Grange et al (2009Grange et al ( , 2011 and Figini et al (2012) -the interested reader is also referred to di Prisco & Pisanò (2011b) and di Prisco et al (2012) for quite recent overviews on this subject.…”