Abstract
.
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.