Effect of Failure-Inducing Cyclic Loading on Pile Capacity in Sand

Although the behavior of pile foundations under cyclic loading is critical in offshore engineering, the underlying mechanisms governing load transfer and the bearing capacity of piles at the ultimate limit state in calcareous sand (a problematic ocean soil) subjected to cyclic compression and uplift remain poorly understood. This study employs six centrifuge tests on single piles subjected to monotonic compression loading, monotonic uplift loading, and cyclic vertical loading in dry calcareous sand and silica sand. The results reveal a transition in the pile failure mode from progressive failure under monotonic loading to abrupt failure after ultimate cyclic loading. Cyclic loading reduces the ultimate uplift bearing capacity of the pile by approximately two-thirds, while enhancing the ultimate compressive bearing capacities of the pile by 143% in calcareous sand and 6% in silica sand, respectively. These trends are corroborated by the energy evolution patterns, where uplifting energy decreases with increasing cycles, while compressive energy exhibits an initial decrease followed by an increase. Such variations are attributed to cyclic compressive and uplift loading, which degrade skin friction on the pile shaft while improving the bearing performance of the sand beneath the pile tip. This result suggests that the end resistance of the pile in calcareous sand should be a focus of attention and can be reinforced by cyclic preload of the pile.