Failure mechanisms of skirted foundations in uplift and compression Divya S. K. Mana MTech Centre for Offshore Foundation Systems, University of Western Australia, Perth, Western Australia, Australia Susan M. Gourvenec PhD Centre for Offshore Foundation Systems, University of Western Australia, Perth, Western Australia, Australia Mark F. Randolph PhD Centre for Offshore Foundation Systems, University of Western Australia, Perth, Western Australia, Australia M. Shazzad Hossain PhD Centre for Offshore Foundation Systems, University of Western Australia, Perth, Western Australia, Australia Kinematic soil failure mechanisms around skirted foundations, embedded in lightly overconsolidated clay and subjected to undrained compression and tension, have been investigated through digital image analysis of drum centrifuge tests and compared with predictions from finite-element analyses. Analysis of images captured in the centrifuge tests showed that rather different kinematic mechanisms govern failure in tension and compression. In tension, a reverse end bearing mechanism involving a bulb of soil beneath the foundation was mobilised even for a skirt depth to foundation diameter ratio as low as 0?1. Bearing capacity factors from centrifuge tests for a selected embedment ratio were similar in compression and uplift despite the difference in associated failure mechanism. Comparison of the failure mechanisms observed in the centrifuge tests with those predicted by finite-element analyses shows some marked differences, in spite of close agreement of bearing capacity factors. 1. Introduction Skirted foundations are a type of shallow foundation used offshore that comprise a top plate and peripheral skirt and sometimes internal skirts. The skirts confine a plug of soil and are beneficial in transmitting foundation loads below the mudline to deeper and often stronger soil. This enhances the bearing capacity and reduces displacements compared with a surface foundation. A particular attraction of skirted founda- tions arises as negative excess pore pressures can be generated between the foundation top plate and the confined soil plug (e.g. Dyvik et al., 1993) during undrained uplift allowing reverse end bearing capacity to be mobilised (provided a good seal between the top cap and skirts can be maintained). The major uncertainties in reliance on reverse end bearing are what minimum skirt depth to foundation diameter embedment ratio is required to generate negative excess pore pressures under the top cap; and over what duration these negative excess pore pressures can be sustained. As a result, uplift resistance is typically calculated as the lesser of external and internal frictional resistance of the skirt/soil interface or external skirt friction and the weight of the soil plug. The conservative assumption of a simple pull-out mechanism (either of the foundation alone or foundation with soil plug) may result in an uplift capacity of an order of magnitude lower than if reverse end bearing were mobilised. The study presented in this paper addresses the first of the key uncertainties that currently prevents reliance on reverse end bearing, by investigating the kinematic failure mechanisms mobilised during relatively rapid (i.e. undrained) uplift and comparing with those observed in undrained compression. Application of particle image velocimetry (PIV) in geotechnical engineering (e.g. GeoPIV; White et al., 2003), has provided a valuable tool for investigating soil flow mechanisms under various load or displacement conditions and for various geotechnical structures, including a range of foundation systems and pipelines (e.g. Dingle et al., 2008; Hossain and Randolph, 2010; White et al., 2005). Previous studies have, however, not considered the soil flow mechanism around skirted foundations under compression and tension. In the study presented in this paper, the PIV technique is used with half models of skirted circular foundations with various embedment ratios to observe the kinematic mechanisms governing undrained failure in vertical compression and tension. The results are compared with kinematic mechanisms predicted by finite-element analysis (FEA). The influence of the observed failure mechanisms on the vertical bearing capacity of the foundation is also investigated through a set of full model centrifuge tests and FEA. International Journal of Physical Modelling in Geotechnics Volume 12 Issue 2 Failure mechanisms of skirted foundations in uplift and compression Mana, Gourvenec, Randolph and Hossain International Journal of Physical Modelling in Geotechnics, 2012, 12(2), 47–62 http://dx.doi.org/10.1680/ijpmg.11.00007 Paper 1100007 Received 26/04/2011 Accepted 29/08/2011 Keywords: centrifuge modelling/failure/footings/foundations ICE Publishing: All rights reserved 47