Influence of initial stress distribution on liquefaction-induced settlement of shallow foundations

During earthquakes, saturated sandy soils may generate significant excess pore pressures and approach a state of liquefaction. Structures founded on shallow foundations above such soils may consequently undergo large settlements. Recent case history analysis has shown that the stress imposed by the foundation is a key factor in the estimation of such settlements. However, the case history data showed that although increasing bearing pressure caused an increase in settlements as expected, this was only true up to a point, and that very heavy structures appeared to settle less than some lighter structures. This work aims to investigate these counter-intuitive results by means of controlled experimental testing using a geotechnical centrifuge. Results of the centrifuge tests show that the trend derived from case histories is correct and that liquefaction-induced settlements peak for a given bearing stress (90 kPa for the models tested) and reduce for greater applied stresses. Further, by analysis of excess pore pressure distributions beneath the foundations it is shown that the main factor inhibiting pore pressure generation beneath the footings is not so much the confining pressure as the in-situ static shear stress around the edge of the foundation. This is supported by element test data from the literature. When this initial static shear stress is so high that the applied cyclic shear stress cannot exceed it (i.e. the direction of shear stress does not reverse) then pore pressure generation is greatly reduced, thus causing the observed reduction in expected settlements.