Cyclic Laterally Loaded Medium Scale Field Pile Testing for the PISA Project-Paper

Paper

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The PISA project conducted medium-scale field tests to investigate how offshore wind turbine monopile foundations respond to repeated lateral loading. Tests were carried out at Cowden, UK, in stiff over-consolidated glacial till, and Dunkirk, France, in dense marine sand. Piles 0.762 m and 2.0 m in diameter were tested, including one-way cycles that load predominantly in one direction and two-way cycles that reverse direction. Some tests applied tens of thousands of cycles.<br /><br />The paper highlights two tests in dry, unsaturated sand at Dunkirk. In the one-way test, the pile progressively moved in the direction of the loading bias, a behaviour known as ratcheting. This occurred even at low load amplitudes, while the displacement accumulated per cycle generally diminished as cycling continued. The trend was consistent with a power-law description of displacement accumulation.<br /><br />The two-way test produced a largely symmetric load–displacement response. Stiffness decreased as the load amplitude increased, and the measured cyclic stiffness agreed well with the secant stiffness inferred from a comparable monotonic test.<br /><br />These field observations broadly matched behaviours reported in laboratory studies and provide valuable data for developing and validating cyclic design models for offshore monopiles. The authors note that further work is needed to understand cyclic pile response in saturated sand.

Keywords

PISA project

offshore wind turbines

monopile foundations

lateral loading

field tests

Cowden UK

Dunkirk France

ratcheting

cyclic stiffness

dense marine sand