This paper reviews information on axial cyclic loading of piles installed in clays and in sands, with emphasis on offshore foundations. Primarily, it presents results from a study of an example North Sea Jacket, where the wave-induced cyclic loading regime of the supporting pile groups was investigated using interactive jacket/foundation analyses. For the jacket considered, the majority of the piles were found to experience one-way cyclic loading. The pile penetration. assessed on the basis of static capacity criteria, was found adequate to resist the imposed cyclic loads.
The cyclic loading imparted to an offshore structure by the ocean environment, also results in the platform foundations, experiencing cyclic loading. For safe design therefore. such effects must be taken into account. In evaluating the capacities of the pile foundations and for design purposes, it has been customary, at least for conventional jackets, to use analytical procedures which explicitly recognise cyclic loading effects and the potential deterioration in strength in the pile lateral response but not in axial capacity and stiffness. The pile penetration required to satisfy the ultimate axial capacity requirement is selected based on static criteria, often on the basis of the average group load. and is calculated in accordance with procedures and factors of safety given in Codes (e.g. API RP2A). However, only in limited cases is a check made to assess whether the pile cyclic resistance is acceptable. The pile group is configured to yield the required factor of safety against the maximum leg load, and the individual pile in a group is designed to have a factor of safety with respect to the largest axial load. This latter factor of safety is not specified in codes but it has been the normal practice to use values no less than 1.2. In effect, this margin of safety is intended to cover for various uncertainties such as in loads and foundation strength.
