Piled foundations subjected to cyclic loads or earthquakes

Zheming Li, Cambridge University
Geotechnical Engineering Group



Piled foundations are often designed against static loading. However, numerous piled foundations are subjected to significant cyclic axial and lateral loads due to the wind and wave actions in their fluid environment or rocking and swaying motions of superstructures during earthquakes, and many have failed catastrophically. Unfortunately, the performance of piled foundations subjected to cyclic loads or during earthquakes is not yet fully understood.
This dissertation offers an understanding of cyclic response of piled foundations in dry dense sand, and is divided into three areas of research: single piles subjected to cyclic loads, pile groups subjected to cyclic loads, and pile groups during earthquakes. Centrifuge modelling was mainly used for this research. Different pile installation methods were carried out in centrifuge tests. Displacement-controlled and forcecontrolled loading operations were conducted in cyclic loading tests. Forces and displacements of piles could be obtained using different measurement devices.
Additionally, numerical analysis, including load transfer method and finite difference method, was conducted to compare with centrifuge experiment results.
The single pile tests illustrate that a jacked pile generally has a better performance than a bored pile, with smaller accumulated permanent displacements and larger cyclic secant stiffnesses during pseudo-static load cycling. Additionally, significant permanent pile lateral displacements accumulate during one-way cyclic lateral loading but not with twoway cyclic loading.
The pile group cyclic loading tests show that the interaction effects in a pile group with a pile spacing of four times the pile diameter are significant. It is found that leading piles have larger pile head secant stiffnesses and carry higher loads than trailing piles. During lateral load cycling, leading piles take more and more lateral loads, while trailing piles lose some force. Moreover, cyclic lateral loads would induce rocking motions of pile groups. Individual piles were thus subjected to cyclic axial loads, causing large accumulated permanent settlements.
p-y curves for the laterally loaded piles were obtained in the finite difference analysis, and the relationship between p-y curves and shear stress-strain curves of soil was discussed.
The earthquake tests reveal significant differences in the performance of pile groups compared with pseudo-static cyclic loading. The incremental rate of permanent settlement, caused by rocking motions of pile groups during seismic shaking, is much larger than that induced by pseudo-static cyclic lateral loads with a similar force amplitude.