Long-Term Cyclic Deformation of Soft Clay: Invariance Behavior and Its Application to Laterally Loaded Monopile Foundations

When subjected to low-amplitude cyclic loading, the cumulative strain (εp) in clay is generally observed to follow a power-law relationship with the number of cycles (N). Analysis of long-term cyclic deformation data on clay accumulated by the authors revealed the cyclic invariance behavior. Specifically, for soils with given basic physical properties, the exponential parameter (B) in the power-law relationship remains independent of both the consolidation stress conditions and cyclic stress paths. Based on this behavior of invariance, a stiffness degradation formula was derived, incorporating a single parameter (i.e., B). When this invariance-informed stiffness degradation formula is incorporated into finite-element (FE) modeling, the long-term cyclic deformation of infrastructure resting on clay can be predicted both efficiently and accurately. The process involves only three steps: (1) sampling the clay and conducting a conventional cyclic test to determine the B value; (2) selecting the Mohr-Coulomb model and running a FE model to establish the static behavior of the infrastructure; and (3) specifying the N value and rerunning the FE model to determine the cumulative deformation of the infrastructure after the Nth cycle. As an illustration, the cumulative lateral deformation of a monopile subjected to cyclic loading was assessed and validated using three centrifuge tests from existing studies.