Date of Award

2020

Degree Type

Thesis

Degree Name

Master of Science in Civil Engineering (MSCE)

Department

Civil and Environmental Engineering

First Advisor

Aaron S. Bradshaw

Abstract

The United States built their first offshore wind farm off the coast of Block Island, Rhode Island officially breaking into the wind energy movement. Future offshore wind development will utilize both fixed and floating structures that need to be anchored to the seabed. These foundations/anchors can be subjected to large uplift loads. Driven piles have been a conventional option but present challenges due to pile driving noise and cyclic degradation. One alternative under consideration is groups of helical anchors, which can be “silently” screwed into the ground. They are also more efficient than driven piles because the resistance is in bearing rather than friction. While cost effective, the use of helical pile groups raises questions about the size of the foundation footprint, how densely the piles can be installed and the effect of installing a pile next to already installed piles. The current recommended spacing for helical pile groups is that the piles be spaced at 5 times the diameter of the helix (5B) or greater to avoid overlapping stress zones. However, due to the limited area that piles can be installed and the required vertical capacity, the piles may have to be spaced less than 5B. There are limited data on the group effects of closely spaced helical piles. The objective of this study is to investigate soil-structure interaction and group efficiencies for helical piles with spacing at or below 5B in sands. The objectives were achieved by performing a field test on single helical piles and groups of helical piles; targeted different soil densities, group sizes and pile-to-pile spacing. A parametric finite-element model study was also completed targeting single helical piles and groups of helical piles; targeting different soil densities, different rigidity indexes, different group sizes and different pile-to-pile spacing. The field test results indicate that group efficiencies increase as the pile spacing decreases and as the group size increase. Thus suggesting that the installation of the piles in a group improved (i.e. densified) the surrounding soil and helped to counteract the overlapping stress zones. The results of the parametric finite-element model results displayed a decrease in group efficiency as the pile spacing decreased. There was also a decrease in efficiency with the increase of group size. Both of these trends were due to the increase in overlap of the stress zones. Higher rigidity indices resulted in a decrease in group efficiency. The vertical loading and displacement disturbed the soil above. The FE modeling was not conducted as a direct comparison to the field test and opposite trends in group efficiencies were seen between the two test setups. This indicates that the soil disturbance during pile installation could play a large role in the failure mechanism and are not accounted for in the “wished in place” modeling configuration.

Available for download on Sunday, December 18, 2022

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