Experimental study of boundary effects in dynamic centrifuge modelling

Dynamic centrifuge modelling has been established as a powerful tool for studying soil-structure interaction problems under earthquake loading. Increasingly complex models are being tested in centrifuges all around the world in an attempt to understand real structure behaviour under earthquake loading. However, there is a need to model the field conditions correctly in these centrifuge models. In a geotechnical centrifuge, the space available to model real situations is not infinite, and it is necessary to enclose the model within the finite boundaries of a container. The boundary effects of the soil container are important and can lead to inaccurate simulation of a field situation that has infinite lateral extent. The equivalent shear beam (ESB) model container used in dynamic centrifuge testing is built to achieve the same dynamic response as the soil sample to minimise the boundary effects. A series of dynamic centrifuge tests involving loose and dense, dry and saturated models of homogeneous horizontal sand layers has been carried out to investigate the effects of the end walls of the ESB model container on soil behaviour. In both saturated and dry sand models it was seen that amplification of acceleration occurs at the base of the model close to the end wall. In saturated models, as excess pore pressures build up, the response of the centre of the model gets progressively more different from that near to the end wall. This is to be expected as the box stiffness was matched to that of the non-liquefied soil column. It was confirmed that when the relative density of the soil in the model was around 50%, close to that of the design soil layer, the change in behaviour towards the end wall was minimal.