Conceptualising the lateral response of monopiles in sand to monotonic and cyclic loading

Abstract

The design of monopile foundations for offshore wind turbines is often governed by lateral tilt under monotonic and cyclic loading, commonly assessed using distributed (spring-based) soil-reaction models. While these models are well established for monotonic loading, their extension to cyclic conditions – and their acceptance in industry – remains limited. As part of a broader research programme on cyclic soil-reaction modelling, this paper supports such developments through non-linear 3D FE analyses of monopile response in sand under monotonic and cyclic loading. The SANISAND-MS constitutive model is employed to capture the drained cyclic behaviour of the sand, including ratcheting. Extensive parametric studies are performed to investigate the combined effects of geometric, geotechnical, and loading conditions. The results confirm well-known aspects of monopile mechanics while also highlighting less-studied features, including the relative contributions of different (cyclic) soil-reaction components (within the PISA framework) and the quantitative influence of relative monopile-soil stiffness on both monotonic and cyclic response. Finally, the study proposes interpolation laws synthesising the numerical findings, which can aid in conceptualising monopile lateral behaviour, guiding future experimental campaigns, and, very importantly, informing the development and validation of simplified soil-reaction models.