Cyclic Axial Load Testing of Pipe Piles in Sand-Paper

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The study examines how cyclic axial loading and aging affect the tensile capacity of open-ended pipe piles in sand, relevant to offshore wind jacket structures. Reduced-scale piles were installed and tested at sites at the University of Rhode Island (URI) and the University of Texas at Austin (UT). Tests included monotonic tension, low-frequency tension cycling (0.125 Hz), and, at UT, high-frequency compression cycling (40 Hz). Short-term tests were conducted roughly 0.1–9 days after installation; longer-term testing was planned.<br /><br />At both sites, measured short-term static capacities were substantially below predictions from the CPT-based UWA-05 method. The authors suggest that installation-related friction fatigue—potentially intensified by high driving resistance and the small pile diameter—contributed to the low capacities.<br /><br />Post-cyclic monotonic tests showed no evidence of capacity degradation from cycling. At URI, a pile subjected to more than 10,000 low-frequency cycles had a markedly higher capacity than comparable monotonic-test piles. At UT, capacities after cycling were broadly similar to monotonic capacities, although one pile’s capacity increased from approximately 16 to 19 kN. The results suggest that cycling may have enhanced capacity through an aging or setup effect, rather than causing degradation. UT results also indicated capacity gain with time after installation.<br /><br />Overall, the preliminary findings show that short-term pile capacities can be lower than standard predictions and that cyclic effects are not necessarily detrimental: under the tested conditions, cycling produced no clear capacity loss and may have promoted capacity recovery. The authors note that site variability complicates comparisons and that analysis of long-term tests is ongoing.

Keywords

axial loading

cyclic loading

aging effect

tensile capacity

open-ended pipe piles

sand

offshore wind jacket structures

friction fatigue

CPT-based UWA-05 method

capacity recovery