Experimental Investigation of the Drainage Response around a Large Diameter Monopile

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Offshore Wind Turbines (OWTs) are a critical part of Europes energy infrastructure, and their number and significance are projected to increase rapidly. The monopile is the most common foundation for OWTs. Monopile diameters are currently of the order of 10 m, with future generations projected to increase further. The cyclic response of sands around monopiles has often been thought of as drained. However, as the monopile diameter increases, so does the relevant drainage path, reducing the capacity of the soil to fully drain within a load cycle. Using centrifuge tests performed at ETH Zurich as part of the GEOLAB project, this paper investigates the response of a large monopile in sand, with a focus on the drainage conditions within the surrounding soil. The monopile is subjected to both displacement-controlled harmonic loading and load-controlled storm loading packages. The results reveal that the drained assumption is not appropriate within a load cycle for any of the cases examined. Instead, partial drainage conditions take hold, which affect the effective stresses and thus the system-level response. Large oscillations in pore water pressure around the hydrostatic value occur within a load cycle, with pressure increase in front of the laterally loaded pile and pressure reduction behind it. The average pore-water pressure in a cycle depends on loading history. Loads larger than previously experienced lead to gradual increase in average pore-water pressure and to system stiffness reduction. Overall, the results indicate that drainage effects cannot be neglected in the design of future OWTs founded on large-diameter monopiles, especially when the maximum historical load is exceeded.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

9 - Monopile design to cyclic loads: quasi-static, dynamic and seismic loads