Delivered by ICEVirtualLibrary.com to: IP: 130.95.140.155 On: Fri, 17 Sep 2010 00:16:06 Centrifuge modelling of circular shallow foundations on sand L. GOVONI*, S. GOURVENEC{ and G. GOTTARDI* Drum centrifuge tests on circular footings resting on medium dense silica sand subjected to combined vertical, moment and horizontal loading are reported and compared with existing data from similar studies carried out at 1g. Vertical loading tests and displacement-controlled horizontal and rotational swipe tests were carried out; the results are reported in terms of load–displacement relationships and yield surfaces in (V, M/D, H) load space. The centrifuge test results from this study are well described using the existing theoretical framework of strain-hardening plasticity applied to surface footings on sand, developed on the basis of experimental data from 1g tests. Attention is given to the embedment effect, which can be especially pointed out by centrifuge testing. doi: 10.1680/ijpmg.2010.10.2.35 KEYWORDS: foundations; geotechnical engineering; models BACKGROUND Bearing capacity of shallow foundations is widely con- sidered by using the classical formula providing the uniaxial vertical bearing capacity of a surface strip footing, adjusted by multiplicative correction factors to account for foundation shape, embedment and load inclination, with load eccentricity provided for by an effective area. More recently interaction diagrams have been introduced to overcome the linear superposition of effects and to consider the actual interaction among loading components (Gottardi & Butterfield, 1993), especially in relation to the relevant factor of safety, which is strictly load path dependent (Butterfield, 2006). Subsequently, an alternative and effective approach has been developed, in which plasticity-based analyses are constructed in terms of the force resultants (vertical loading V, moment loading over footing diameter M/D, horizontal loading H) acting on the footing and of the corresponding footing displacements (vertical displacement w, rotation multiplied by footing diameter Dh, horizontal displacement u). The approach, first suggested by Roscoe & Schofield (1957) and more fully developed by Butterfield (1978; 1980), enables the prediction of the complete load– displacement response of the soil–footing system for any applied loading path. Several experimental studies at small scale have system- atically investigated the footing response within this context, either at 1g (Nova & Montrasio, 1991; Gottardi & Butterfield, 1993; 1995; Gottardi et al., 1999; Martin & Houlsby, 2000; Byrne & Houlsby, 2001; Bienen et al., 2006) or in an enhanced gravity field (Tan, 1990; Dean et al., 1992) and successfully led to the development of strain- hardening models for the soil–footing system (Nova & Montrasio, 1991; Dean et al., 1997; Martin & Houlsby, 2001; Houlsby & Cassidy, 2002). Determination of the components of a strain-hardening model for the soil–footing system is a sophisticated task and suitable experimental procedures are needed. Small- scale testing at the normal gravity level has always played a key role towards the development of these procedures, since a very sophisticated rig can be used – enabling, for example, the six-degree-of-freedom loading of shallow foundations to be investigated (Bienen et al., 2006) – thus ensuring close control over the testing details and a close response to the prototype. To gain further confidence when employing these models in engineering practice and to take into proper considera- tion the scale effects that might affect the outcome of 1g tests (especially for frictional materials, whose behaviour is strongly stress dependent), several experimental works, similar in their main features to previously performed 1g tests, have been recently carried out at an enhanced gravity field (Punrattanasin et al., 2003; Yun & Bransby, 2003; Cassidy et al., 2004; Govoni et al., 2006; Bienen et al., 2007; Cassidy, 2007) and relevant results have been so far remarkably consistent. However, particularly when (with progressing penetra- tion) the effect of the footing embedment tends to become significant, 1g small-scale models have been shown to provide a conservative response. The aim of this paper is to provide further experimental evidence to support the applicability of the existing strain-hardening framework, used to describe the behaviour of footings on sand and based on 1g tests, to the behaviour of footings at prototype scale, especially when a properly scaled self-weight stress distribution is crucial. OBJECTIVE OF THE EXPERIMENTAL WORK The experiments were intended to examine the drained response of shallow footings on medium dense sand under prototype stress conditions and were designed and con- ducted to be interpreted in a strain-hardening plasticity framework. The key assumption of this approach is that after a given penetration of the footing a yield surface is established in the (V, M/D, H) space. If the footing is penetrated further, the yield surface expands. A convenient way to identify the yield surface experi- mentally is to perform a swipe test (Tan, 1990; Gottardi et al. 1999; Martin & Houlsby, 2000) – a displacement- controlled experimental procedure that allows the yield surface corresponding to a given vertical penetration of the footing to be identified in a single experiment. Manuscript received on 8 March 2010; revised manuscript accepted 10 June 2010. Discussion on this paper should reach the editor by 15 February 2011. * DISTART Department of Structural, Transport, Hydraulic, Survey and Territory Engineering, University of Bologna, Italy. { Centre for Offshore Foundation Systems, University of Western Australia, WA 6009. Govoni, L. et al. (2010). International Journal of Physical Modelling in Geotechnics 10, No. 2, 35–46 35