Investigation of Cyclic Loading of Aged Piles in Sand

David Igoe,Kenneth Gavin

doi:10.1061/(asce)gt.1943-5606.0002451

Abstract

In recent years, there has been a significant push to develop optimized pile designs in the offshore industry, driven by the growth of offshore wind. One of the largest remaining uncertainties governing axial pile design in sands is the influence of ageing effects and how an aged pile responds under axial cyclic loading. This paper presents results from a long-term campaign of field tests on pile ageing and axial cyclic loading in sands, undertaken at the Blessington geotechnical test site in Ireland. Five open-ended driven steel piles were subjected to a total of 33 static and cyclic tension pile load tests undertaken over a 3-year period. The tests were planned and coordinated in order to accurately quantify the effects of ageing and cyclic loading on the pile shaft capacity over time. The tests showed that significant gains in pile shaft capacity occurred over time as a result of pile ageing, in line with results presented by other researchers. Piles subjected to cyclic tension loading remained stable under cyclic loading at load levels much larger than their 1-day capacity. The 1-day capacity appears to offer a lower bound for the degraded pile capacity following tension cyclic loading to failure. The reduction in capacity caused by cyclic loading to failure was related to the pretest increase in capacity caused by ageing. The findings from these field tests in addition to reinterpretation of previous testing indicated that previously published cyclic interaction boundaries may be overly conservative.

Highlights

As the offshore wind industry expands and moves into less favorable sites with deeper waters, there is a need to continuously improve engineering design approaches to ensure costs remain viable
This was justified by an assessment of the cyclic loading that would be experienced by a pile over a 200-day period prior to installation of the wind turbine, which was deemed not to have a detrimental effect on pile capacity or prevent pile ageing
It is clear that a greater understanding of cyclic loading effects on aged piles in sand is required if pile ageing factors are to be widely adopted for jacket pile design

Summary

Introduction

As the offshore wind industry expands and moves into less favorable sites with deeper waters, there is a need to continuously improve engineering design approaches to ensure costs remain viable. Despite the widely accepted evidence of the beneficial effects of pile ageing from numerous sources, there has been a reluctance in the industry to include pile ageing in design or to incorporate pile ageing factors in design standards This is partly due to conservatism within the industry and the slow adoption of new design approaches but mainly due to a lack of full-scale data to verify these aged capacities and a poor understanding of the mechanisms controlling the ageing process. Some limited allowances for pile ageing effects were used as part of a mitigation strategy in case of pile driving refusal to offset some of the shaft capacity degradation if drilling was required This was justified by an assessment of the cyclic loading that would be experienced by a pile over a 200-day period prior to installation of the wind turbine, which was deemed not to have a detrimental effect on pile capacity or prevent pile ageing. It is clear that a greater understanding of cyclic loading effects on aged piles in sand is required if pile ageing factors are to be widely adopted for jacket pile design

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