Cyclic monopile behaviour in very dense sand: pile geometry and loading sequence effects

This paper presents the setup, execution, and key findings of a comprehensive centrifuge testing programme investigating the cyclic lateral response of monopiles in sand. As part of the Monopile Improved Design through Advanced cyclic Soil modelling (MIDAS) project, a series of centrifuge tests was conducted using instrumented model piles embedded in both dry and saturated sand under monotonic and cyclic loading, with particular emphasis on very dense, dry sand conditions. The programme examined the influence of pile geometry, load sequence, mean load ratio, and soil density on the accumulation of monopile tilt. Laser displacement sensors, strain gauges, and total pressure sensors were used to measure, or derive, ground-level rotation, rotation point depth, and soil stress redistribution along the pile. The results show that while the maximum accumulated pile rotation is primarily governed by the peak cyclic load, the sequence and asymmetry of loading significantly influence irrecoverable deformation. In very dense dry sand, rotation often stabilised or even decreased with continued cycling, contrasting with previous experimental findings typically associated with looser, though often still dense, sands. The discussion is accompanied by a publicly available dataset, which has supported the development of the MIDAS cyclic soil reaction model for monopiles in sand. This dataset, together with the insights presented herein, provides a valuable resource for researchers and engineers developing or validating design models for offshore foundations subjected to cyclic loading.