Effect of previous cyclic axial loads on pile groups

Numerous piles are often subjected to the combination of cyclic axial and cyclic lateral loads in service, such as piled foundations for offshore platforms which may suffer swaying and rocking motions owing to wind and wave actions. In this research, centrifuge tests were conducted to investigate the effect of previous cyclic axial loads on the performance of pile groups subjected to subsequent cyclic lateral loads. Different pile installation methods were also applied to study the different behaviour of bored and jacked pile groups subjected to cyclic loads. During lateral load cycling, it is seen that cyclic axial loads to which pile groups were previously subjected could reduce the pile cap permanent lateral displacement in the first lateral load cycle but do not influence the incremental rate of permanent displacement in the following lateral load cycles. Moreover, it is found that previous cyclic axial loads could improve the pile cap cyclic lateral secant stiffness, especially for the pre-jacked pile group. When rocking motions were induced by cyclic lateral loads, pile groups subjected to cyclic axial loads before have smaller permanent settlement than those without the cyclic axial loading effect. The designers of piles that are intended to resist significant lateral loads without excessive deformations in service may wish to deploy cyclic axial preloading, accordingly.

Cyclic jacking of piles in silt and sand

Jacked piles are becoming a valuable installation method due to the low noise and vibration involved in the installation procedure. Cyclic jacking may be used in an attempt to decrease the required installation force. Small scale models of jacked piles were tested in sand and silt in a 10 m beam centrifuge. Two different piles were tested: smooth and rough. Piles were driven in two ways with monotonic and cyclically jacked installations. The cyclically jacked installation involves displacement reversal at certain depth for a fixed number of cycles. The depth of reversal and amplitude of the cycle vary for different tests. Data show that the base resistance increases during cyclic jacking due to soil compaction at the pile toe. On the other hand, shaft load decreases with the number of cycles applied due to densification of soil next to the pile shaft. Cyclic jacking may be used in unplugged tubular piles to decrease the required installation load. © 2013 Taylor & Francis Group, London.

Centrifuge testing of monopiles subject to cyclic lateral loading

Monopiles supporting offshore wind turbines are subjected to cyclic lateral loading. The properties of the applied cyclic lateral load are known to have an effect on the accumulation of permanent displacement and rotation at the pile head. The results of centrifuge testing on model piles show that certain loading regimes lead to the development of locked in soil stresses around the pile. These locked in soil stresses change the stiffness of the monopile response to cyclic lateral loading and the natural frequency of the pile-soil system. © 2014 Taylor & Francis Group.