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The high-cycle accumulation (HCA) model by Niemunis et al. allows the prediction of the mechanical response of sand under millions of load cycles. It has originally been developed with focus on sand under drained high-cyclic loading assuming a constant strain amplitude, which can be periodically updated. In order to apply it for partially drained or fully undrained conditions, an adaptive definition of the strain amplitude is proposed in this work. The proposed extension allows taking into account the influence of rapid changes in soil stiffness on the strain amplitude as encountered, for example, in case of large changes in effective stress during the high-cyclic loading. Two approaches are presented for the update of the strain amplitude: a ‘private’ update in each integration point combined with a nonlocal smoothing algorithm and an update in a separate analysis performed parallel to the simulation using the HCA model. Both approaches are compared to the methodology used in previous work employing so-called update cycles. The importance and advantages of the proposed modifications are demonstrated by the simulation of undrained cyclic triaxial tests and monopile foundations for offshore wind turbines (OWTs) under high-cyclic lateral loading and partially drained conditions.

KEYWORDS: high-cycle accumulation model, high-cyclic loading, offshore wind turbine foundations, par-tially drained, Sanisand, strain amplitude