The MIDASclay project (Monopile Improved Design via Advanced Cyclic Soil Modelling in Clay) aims to evaluate the cyclic lateral behaviour of monopiles in clays and clay-sand mixtures, focusing on developing a cyclic soil reaction model that captures cyclic ratcheting and operational stiffness in cohesive soils. Together with the MIDAS project – aimed at cyclic behaviour of monopiles in sand, it seeks to establish a comprehensive framework for modelling the cyclic lateral response of monopiles across diverse soil conditions. To address knowledge gaps, the project employs centrifuge testing of monopiles under cyclic lateral loads, aiming to produce a soil reaction model tailored to cohesive soils. This paper outlines the project setup, research plan, and initial findings. Early results from an auxiliary centrifuge test highlight the preparation of highly over-consolidated samples and provide insights into the influence of clay properties on monopile behaviour. MIDASclay is expected to support and advance design methodologies for monopiles in cohesive soils.

Offshore monopiles accumulate permanent tilt under long-lasting cyclic environmental loads. Accurate prediction of monopile tilt is key to assessing their serviceability, and requires a fundamental understanding of loading history effects. While both experimental and numerical studies are shedding light on this matter, this work uses step-by-step implicit 3D FE modelling to investigate loading history effects in the response to cyclic lateral loading of monopiles in sand and to identify links between local soil behaviour and relevant features of global pile behaviour. For this purpose, the recently developed SANISAND-MS model is adopted to achieve a reliable simulation of sand's cyclic ratcheting. In particular, the validity of an up-scaled Miner's rule for monopile tilting under multi-amplitude cyclic loading is assessed based on the results of 3D FE parametric analyses, with emphasis on the role played by the engineering idealisation of random environmental loading. The validity of such a rule has been numerically investigated both in terms of local soil element response and global foundation behaviour — for the particular case of a large-diameter monopile. In respect, the effect of the loading history idealisation is presented, and it is concluded that Miner's rule does not always rigorously apply to all the cases considered herein. The translation of irregular loading histories into a regular version with loading packages sorted in ascending amplitude order is shown to be a reasonable approach, at least when the possibility of cyclic pore pressure build-up is disregarded.