Even though there is no consensus on the best approach to assess its effects, cyclic axial loads effects have long been recognized as one of the most significant actions that pile foundations of offshore structures have to withstand during their useful life. In this paper, the CPT-based effective stress ICP (Imperial College Pile) method was used to evaluate the post cyclic axial resistance under a given load history. The first step in the procedure followed was to characterize the soils involved in terms of the reduction of effective stress observed in Cyclic Direct Simple Shear tests under constant amplitude of shear stress. Then the evolution of cyclic axial resistance was computed from the local reduction of the effective radial stress along the pile shaft. This reduction is variable along the pile shaft and develops progressively during the load history. In order to evaluate it, the piles were discretized with elastoplastic degrading t-z curves and the load history was divided in a sequence of blocks of cycles of axial load. The degrading effect of the variable amplitude history of local shear stress in each t-z element is computed from the constant amplitude cyclic DSS laboratory tests by an effective stress reduction accumulation procedure. Two alternative methods proposed in the literature for the computation of this accumulated reduction are discussed. The results are summarized in terms of degradation factors for the t-z curves and cyclic interaction diagrams. The sensitivity to several input parameters is also presented. This procedure was applied to several cases of piles driven in a variable soil profile in the Gulf of Mexico area.
Jose G. Parra and Mariajose Guevara
Keywords: cyclic axial load, cyclic interaction diagrams, cyclic degradation
Publisher: DFI