Influence of foundation type on seismic response of low-rise structures in liquefiable soil

Abstract

The 2010–2011 Canterbury Earthquake Sequence (CES) caused extensive damage to low-rise structures in the city of Christchurch, New Zealand, mainly due to liquefaction-induced effects including settlement and angular distortion. This paper will present the results of dynamic centrifuge tests comparing the effects of liquefaction on the seismic performance of isolated structures with different types of shallow foundations (strips or a raft), and the effect of being situated adjacent to a heavier neighbouring structure of the same foundation type (i.e. considering structure-soil-structure interaction, SSSI). Performance will be evaluated under a sequence of successive earthquakes from the 2010–2011 CES and 2011 Tohoku Earthquake, Japan, to permit study under ground motions and aftershocks generating full liquefaction either extensively or to only a limited depth below ground level. The results show firstly that lower intensity ground shaking occurs at the ground surface when liquefaction occurs and that this can be estimated as a function of the degree of liquefaction using a simple estimation method. When subjected to these ground motions, using strip foundations for isolated structures can result in a reduction in structural demand, especially when the soil is extensively liquefied. When a neighbouring structure with the same foundation type is present, the effects of SSSI within liquefied soil result in changes to natural period and damping such that raft-founded structures exhibited lower structural demands. In either case (isolated or adjacent), a reduction in structural demand is accompanied by an increase in post-earthquake permanent foundation deformation.