Seismic Response of Shallow Foundation on Liquefiable Sand

Eight centrifuge model experiments were conducted to investigate the mechanism of liquefaction-induced settlement of a shallow foundation, as well as the effectiveness of sand densification by vibrocompaction under the footing. Two series of model tests were performed with a surface circular footing placed on a medium-dense saturated sand layer overlying an impervious rigid base. The base of each model was excited in flight by horizontal shaking simulating an earthquake; in all cases this shaking liquefied the sand in the free field. The first series of tests (series C) focused on the effect of the depth of soil compacted under the foundation on the footing acceleration and settlement. It was found that, as the compaction depth increased and approached the total thickness of the soil deposit, the footing acceleration during shaking increased and its settlement decreased. In the second series of tests (series G), the soil was not densified; this series focused on the effect of soil permeability on pore pressure buildup and footing settlement. It was found that, as the soil becomes more impervious, significant negative excess pore pressures develop under the footing during shaking, and the contribution of postshaking foundation settlement increases, while the total foundation settlement does not change very much. The paper includes a discussion of building settlements observed after two earthquakes in liquefied areas of Niigata, Japan in 1964, and Dagupan, Philippines in 1990, as well as comparisons between these field observations and the centrifuge results.