Seismic Response of Shallow Foundation on Liquefiable Sand

Abstract

Investigators conducted eight centrifuge model experiments to study 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 conducted with a surface circular footing placed on a medium-dense saturated sand layer overlying an impervious rigid base. Horizontal shaking simulating an earthquake was used to excite the base of each model in flight; in all cases this shaking liquefied the sand in the free field. The first series of tests concentrated on the effect of the depth of soil compacted under the foundation on the footing acceleration and settlement. Investigators discovered 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. The soil was not densified in the second series of tests; this series focused on the effect of soil permeability on pore pressure buildup and footing settlement. Investigators learned 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 significantly. Building settlements observed after two earthquakes in liquefied areas of Niigata, Japan in 1964, and Dagupan, Philippines in 1990, are discussed. Comparisons are made between the field observations and the centrifuge results.