Evaluating Rate and Time Effects on Cyclically Loaded Offshore Foundations via a Bounding-Surface Viscoplastic Model

Offshore foundations are subjected to cyclic loading from waves, winds, and operations, and their response is inherently time-dependent and strongly influenced by loading rates due to the viscosity of clays. This study develops a simple yet theoretically robust framework for evaluating rate and time effects in cyclically loaded foundations. A bounding-surface viscoplastic (BS–EVP) model is formulated within a total-stress framework, enabling the capture of key time-dependent features of undrained clay under nonmonotonic stress paths. The model is first assessed against laboratory soil element tests, where it reproduces experimentally observed behaviors such as the reversal of creep direction following stress changes and the path dependence of stress relaxation. It is then implemented in a finite-element framework and applied to monopile field tests from the pile soil analysis (PISA) project. The simulations capture essential aspects of foundation response, including increased resistance under faster loading, isotach behavior during stepwise rate changes, progressive displacement accumulation during sustained load-holding stages, and both ratcheting and creeplike displacement growth under cyclic loading. Although some discrepancies remain, particularly due to the exclusion of strain softening and the reduced rate sensitivity observed at high strain rates, the results highlight the value of a practical constitutive model for linking soil-scale viscous mechanisms to nontrivial foundation-scale behaviors of restrengthening, creep, and relaxation across a wide range of loading rates and time scales.