Date of Award
Master of Science in Civil and Environmental Engineering
Civil and Environmental Engineering
Aaron S. Bradshaw
With the emergence of the offshore wind energy sector in the United States with the Block Island Wind Farm, and future commitments to offshore wind development, understanding of the impacts the environmental loads have on these structures is important in predicting their performance. In the case of offshore wind structures with pile-supported jacket systems, such as at the Block Island Wind Farm, the wind, wave, and current loads cause overturning moments that are resisted by the axial capacity of the supporting piles.
These environmental loads, present in the form of cyclic loads varying in magnitude with time, are hypothesized to cause a loss of capacity of the supporting piles through a process known as cyclic degradation. Another factor to consider is the loss of expected pile capacity during installation due to disturbances in the soil strength in a process known as friction fatigue. However, after installation, piles are known to increase in capacity with time due to pile-soil shear band setup and aging. The objective of this investigation is to evaluate the effects of these factors on model piles driven in a coastal environment, and while subjected to cyclic axial loads on the order of 104 0.125 Hz frequency cycles, similar to 0.1 Hz frequency that would be placed on a wind turbine from wave loading (Jardine et al., 2012). The research conducted in this study is part of a larger study involving the United States’ Bureau of Ocean Energy Management (BOEM), the Norwegian Geotechnical Institute (NGI), and the University of Texas at Austin.
In this investigation, a hydraulic load control system was developed, and paired with a standard tensile load frame setup to apply monotonic and cyclic loads. The testing schedule consisted two sets of three pile tests, approximately one-week and ten-weeks after installation, to evaluate the effects of friction fatigue, setup, and cyclic degradation.
It was found during the one-week (short-term) testing that the piles may have encountered “smooth” interface-soil conditions, aligning with maintained post-cyclic monotonic capacities between sets of tests. Furthermore, interface roughness testing on a pile installed 39 days at the site showed considerable increases in roughness in the intertidal zone, which may have produced some “rough” dilative post-cyclic monotonic performance.
In terms of capacities, one-week strength results determined that the test piles surprisingly presented a pure monotonic capacity an order of magnitude lower than predicted amounts from CPT based correlations, most likely due to installation friction fatigue effects. Low-level cyclic loading at a similar time after installation determined a post-cyclic monotonic capacity of three times higher than the pure monotonic capacity, and outside the realm of site variability. This leads to the conclusion that cyclic loading after significant amounts of friction fatigue causes a densification of the shear band towards minimum void ratio levels, producing a stronger dilative post-cyclic monotonic response.
Taking into account the aging response, the pure monotonic long term (ten-week) test results show that the capacity of the site piles increased along a linear scale with respect to logarithmic time. In terms of long-term cyclic test results, the rate of capacity increase over monotonic strength on nearby piles was much lower, indicating aging strengthened the soil-pile system in a manner similar to short-term cycling. It was determined that the maximum rate of capacity increase for the long-term cyclic tests were similar, whether the ultimate loads were presented in cyclic or post-cyclic monotonic form. No cyclic degradation of capacity was noted throughout the tests, except when mobilized displacements had been already produced cyclically.
Keefe, Timothy, "FIELD STUDY OF PILE AXIAL CYCLIC DEGRADATION AND AGING" (2020). Open Access Master's Theses. Paper 1889.