Cyclic axial loading of offshore piles-An issue of concern?

Expansion of the global piling industry has been coupled with significant
research efforts investigating the mechanisms controlling pile capacity. Static pile
design has transitioned from total stress approaches toward effective stress
design, with associated improvements in predicting pile performance. As a result,
the available literature is heavily skewed toward static capacity prediction.
However, when considering piles offshore, the majority of loading is not static but
highly variable and cyclic in nature. This paper outlines the additional factors that
require consideration for cyclic pile design, including elevated loading rates,
cyclic displacements and capacity degradation. A review of the literature
identified a number of cyclic pile testing programs which are collated in this
paper. General trends are established through comparison of the tests within the
database and the conditions where cyclic loading is most likely to be a concern
are identified. In particular, loading conditions which involve full plastic shear
reversals are seen to cause the most extreme cyclic damage, with the degraded
capacity as low as 31% of the precyclic static capacity. The underlying
mechanisms which can result in cyclic damage are explored, with particular
attention given to the pore pressure response, radial stresses, shear transfer and
particle reorientation.