Axial Cyclic Degradation of Marine Piles: A Strain-Based Fatigue Limit-Paper

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The paper proposes a strain-based method for estimating the cyclic fatigue limit of axially loaded piles in sand, with potential application to offshore wind turbine foundations. Repeated loading can densify soil in the narrow shear band around a pile, reducing lateral stress and shaft resistance and increasing pile movement.<br /><br />The method uses a static pile load-transfer (t–z) analysis to estimate shear strains at the pile–soil interface. It accounts for nonlinear soil behavior by adjusting the shear modulus according to strain, then compares calculated strains with the threshold shear strain below which cyclic loading is expected to cause no permanent soil changes. For sand, the cited threshold range is approximately 0.01%–0.03%.<br /><br />The approach was applied to full-scale static and cyclic tests on driven piles in sand at Dunkirk, France. The t–z model reproduced the measured static load–settlement response particularly well at small movements. In the cyclic tests, piles classified as metastable or unstable had modeled strains exceeding the threshold throughout the pile. The stable test pile had below-threshold strains along roughly the bottom third, consistent with some resistance to degradation.<br /><br />For this test site and pile configuration, the analysis indicated that limiting the maximum cyclic tensile load to about 10% of the static tensile capacity could keep interface strains below the threshold along the pile and prevent cyclic degradation. The authors present this as a promising way to assess long-term, low-amplitude loading, but emphasize that further research is needed to refine and validate the method.

Keywords

cyclic fatigue limit

axially loaded piles

sand

strain-based method

pile-soil interface

t-z analysis

shear strain threshold

offshore wind turbine foundations

cyclic loading

shaft resistance