3D Finite Element Modelling of the Cyclic Behaviour of Large Diameter Monopiles in Sand-Paper

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The paper presents a three-dimensional finite-element method for predicting the cyclic response of large-diameter monopiles in sand during storm loading. Unlike approaches calibrated on drained centrifuge tests, the method models both the generation and dissipation of excess pore water pressure around the pile.<br /><br />Implemented in LS-DYNA, the analysis combines cyclic shear stresses calculated around the monopile with laboratory-derived relationships between stress ratio, cycle count and pore-pressure generation. A coupled consolidation model simulates drainage during a representative storm wave train. The study examines two pile geometries and sand profiles with different relative densities, stiffnesses and permeabilities.<br /><br />Results show that cyclic loading can substantially increase lateral pile displacement, particularly in medium- or low-density sand. For a 5 m pile, the post-cyclic displacement under a 9 MN load was 2.26 times the monotonic displacement at 60% relative density, compared with 1.08 times at 100%. For a 6 m pile in 60%-dense sand, the displacement ratio was 2.08 at permeability of 1 × 10⁻⁶ m/s and 1.55 at 1.5 × 10⁻⁴ m/s. Lower density promotes greater pore-pressure generation, while lower permeability slows dissipation between waves. In very dense sand, cyclic effects were comparatively small.<br /><br />The authors conclude that monopile displacement after a storm depends strongly on both soil density and permeability. Empirical methods based on drained testing should therefore be used cautiously where sands are loose or poorly permeable, as they may not capture significant pore-pressure buildup and its effect on foundation performance.

Keywords

finite-element method

monopile

sand

cyclic loading

pore water pressure

storm loading

LS-DYNA

consolidation model

lateral displacement

soil permeability