Centrifuge Study on Progressive Failure of Shallow Foundations due to Soil Liquefaction

The effect of liquefaction depth on co-seismic and post-seismic settlements of shallow foundation has been studied using three centrifuge test series. The models were constructed in 1/80 scale and subjected to the centrifugal acceleration of 80g. They involved two rigid foundations with two different static surcharges and sufficient spacing to minimize the interaction. Poorly graded sand known as No. 306 sand with a relative density of 55% was used. The model was excited with a 15-cycle sinusoidal base motion having constant amplitude and 2 Hz frequency. In the free-field, liquefaction occurred in the shallower layers first, propagated rapidly to the deeper layers. The full depth of soil profile was liquefied in the strongest event. The liquefied depths were about 2.4 m and 7.2 m for amax=0.04g and amax=0.07g, respectively. Liquefaction caused severe deterioration of soil stiffness resulted in significant decay of accelerations. After excitation ceased, upward seepage from deeper layers enforced the shallower layers to remain in liquefied state for longer time. The free-field settlement commenced immediately after the first cycles and accumulated until excitation ceased. Its rate stopped for a while. The free-field settlement began again and continued up to full EPWP dissipation. Large negative EPWP was observed beneath the foundations, which are attributed to the deviatoric stress induced by their surcharge and soil dilation due to lateral movement of subsoil. Amplification was observed in acceleration time histories within the foundation soil, which is attributed to the negative EPWP generated in this zone. Large horizontal and vertical hydraulic gradient was developed during shaking, causing water flow towards the foundations. Once the water pressure equalized in each level, reconsolidation commenced. The foundations settled linearly with time during shaking with decreasing rate after excitation ceased. The extent of liquefaction had a major impact on the foundation settlement in this period. The higher the extent of liquefaction, the more the foundation settlement occurred. It seems that partial bearing failure and the inertial forces are two dominant mechanisms. The settlement and EPWP time histories can be separated into three different phases: (1) shaking, (2) progressive failure, and (3) reconsolidation. The rate of settlement significantly decreased during the second phase. Previous researchers noted that most of foundation settlement occurs during shaking period, but the results of this research show that most of the foundation settlement occurs after shaking. Foundation settlement continued progressively due to partial bearing failure and strength loss in the foundation soil. It seems that liquefaction extent and soil permeability have major impact on Phase (2). The thicker the liquefied layer or the lower the permeability of foundation soil, the longer time the foundation has to settle. Although the foundation settlement is significant in this phase, it has been neglected in geotechnical designs. The foundation settlement mechanisms are clearly different from that of the free-field. Volumetric-induced deformations are dominant mechanisms in the free-field, whereas, deviatoric-induced strains are the main cause of foundation settlement. It seems that the widely used procedure for the estimation of liquefaction-induced settlements of shallow foundations that is based on volumetric strains might be revised.