Acosta-Martinez, H. E. et al. (2012). Ge ´otechnique 62, No. 4, 317–328 [http://dx.doi.org/10.1680/geot.9.P.027] 317 Centrifuge study of capacity of a skirted foundation under eccentric transient and sustained uplift H. E. ACOSTA-MARTINEZ  , S. GOURVENEC† and M. F. RANDOLPH† Environmental and buoyancy forces impart overturning moments and potentially direct uplift to offshore founda- tion systems. Shallow skirted foundations are an attrac- tive design solution, as negative excess pore pressures developed within the soil plug provide temporary uplift resistance. This paper presents the results of beam cen- trifuge tests on a shallow skirted foundation in a lightly overconsolidated clay under eccentric transient and sus- tained uplift. When compared with the response under concentric uplift, the experimental results show a reduc- tion in capacity and an increase in the rate of degrada- tion of capacity under sustained loading. KEYWORDS: bearing capacity; clays; foundations; model tests; offshore engineering; pore pressures; time dependence Des forces environnementales et de portance appliquent des moments de basculement et des soule `vements poten- tiellement directs a ` des syste `mes de fondations en mer. Les fondations a ` jupe peu profonde constituent une solu- tion conceptuelle attrayante, les surpressions interstitielles excessives de ´veloppe ´es au sein du bouchon de sol pre ´sen- tant une re ´sistance provisoire au soule `vement. Cette com- munication pre ´sente les re ´sultats d’essais centrifuges de faisceaux effectue ´s sur des fondations a ` jupe peu pro- fonde dans une argile le ´ge `rement surconsolide ´e soumise a ` des soule `vements excentriques transitoires et soutenus. Lorsque l’on compare les re ´sultats expe ´rimentaux avec les re ´actions sous l’effet d’un soule `vement concentrique, on rele `ve une re ´duction de la capacite ´ et une augmenta- tion du taux de de ´gradation de la capacite ´ en pre ´sence d’une charge soutenue. INTRODUCTION Environmental forces (e.g. from waves, winds and currents) lead to significant overturning moments in offshore founda- tion systems, causing uplift forces across all or part of a foundation. Environmental forces are generally transient in nature, and of sufficiently short duration to be treated as undrained loading events in most soil types. Sustained uplift is imparted to the foundation systems of buoyant facilities, and would typically act in conjunction with a transient moment from environmental forces. A selection of offshore foundation systems that are subjected to uplift from over- turning moments or buoyancy is illustrated in Fig. 1. Shallow skirted foundations are an economically attractive alternative to deep foundations to resist uplift. Skirted foundations comprise a baseplate with a peripheral skirt, and in many cases internal stiffeners, which penetrate the seabed confining a soil plug. When the foundation is subjected to direct uplift and/or overturning moment, from buoyancy or environmental forces, negative excess pore pressures (passive suctions) are developed within the soil plug, providing temporary uplift capacity. With time, the negative excess pore pressures dissipate, and uplift capacity is reduced to the lesser of the combined internal and external frictional resis- tance along the foundation skirts or the external friction and soil plug weight. Industry standards and recommended practices for off- shore shallow foundation design are based on classical bearing capacity theory (Prandtl, 1921; Terzaghi, 1943; Meyerhof, 1953; Vesic, 1975), and uplift capacity due to negative excess pore pressures is not considered (DNV, 1992; API, 2000; ISO, 2003). In order to rely on negative excess pore pressures, a clearer understanding is required of the rate of dissipation within the soil plug, and hence the rate of reduction in capacity under sustained uplift. A key uncertainty that prevents reliance on reverse end bearing during tensile loading of shallow skirted foundations is whether the required negative excess pore pressures can be sustained beneath the base of the foundation. Reverse end bearing is expected for longer-skirted suction caissons (or anchor piles), and has been observed in various experimental studies, provided field stress conditions are appropriately replicated (Clukey & Morrison, 1993; Clukey & Phillips, 2002; Randolph & House, 2002). Limited experimental stud- ies have investigated uplift of shallow skirted foundations: Puech et al. (1993) reported results from centrifuge tests of skirted mudmats with a skirt depth to foundation diameter ratio d/D ¼ 0 . 18, and Watson et al. (2000) described a centrifuge modelling study of skirted foundations with d/D ¼ 0 . 4 and 0 . 5. Both studies showed that significant reverse end bearing was mobilised under concentric un- drained uplift. Uncertainty exists, irrespective of skirt length, as to whether full reverse end bearing can be mobilised, that is, of equal magnitude to undrained compression capacity. Resolu- tion of this uncertainty has proved difficult, either because compression and tension load paths are not investigated in the same study, or because, in cases in which both directions of loading are investigated, the findings from different studies are inconsistent. For example, Watson et al. (2000) reported undrained uplift capacity equal to undrained compression capacity, and Clukey & Morrison (1993) reported a 20% reduction in undrained uplift capacity compared with un- drained compression capacity. Interestingly, the study by Watson et al. (2000) showed that the uplift capacity was significantly degraded if the foundation was first brought to failure in compression, so the loading sequence can clearly influence the conclusion regarding reverse end bearing. It should be noted that the compression tests reported by Watson et al. (2000) involved loading the caisson well beyond failure. Another key uncertainty affecting reliance on negative excess pore pressures to resist uplift loads concerns the Manuscript received 5 March 2009; revised manuscript accepted 22 September 2011. Published online ahead of print 7 December 2011. Discussion on this paper closes on 1 September 2012, for further details see p. ii.  AECOM Australia Pty Ltd; formerly Centre for Offshore Founda- tion Systems, The University of Western Australia. † Centre for Offshore Foundation Systems, The University of Western Australia.