Principles of the axial pile setup

Abstract

Piles, as widely used foundations onshore and offshore for advanced structures or wind turbines are known for their frequently observed, long-lasting, capacity increases – known as the pile setup. Not seldom, this results in a doubling of the pile capacity within the first 100 days after their installation. Many sub-mechanisms are suspected to contribute to this increase, such as changes in the pile surrounding stress field, an aging of the soil, or modifications at pile shaft surface due to physiochemical processes. However, many of the existing studies focus too much on a single mechanism as a possible cause of the setup. As a result, attempts to transfer these findings to alternative sites and/or piles frequently did not produce satisfying results. The possibility, that this process might be multi-factorial is still often rejected or just ignored. As a result, there is still no conclusive theory to explain the setup in all its facets, even though it was first mentioned now 120 years ago. An improved understanding of the mechanisms and a consequent integration of the setup into daily engineering practice has the potential of considerable cost savings. In particular if smaller, thinner or shorter piles could be used to carry the same loads as before. When piles are used as foundations for wind turbines, eliminating such inefficiencies will be of utmost importance in a world of dwindling resources and for a humanity fighting the climate crisis. The present study aims at improving our understanding of this process in three, well defined sub-studies, which concern one constant test environment. An extensive, small-diameter field study (Chapter 4) reveals that the capacity of a pile and its setup is dependent on the respective installation method as well as on the corrosion vulnerability of the pile material. Piles installed by a rather soil disturbing installation method – i.e. by pile vibration instead of impact driving or pile jacking – potentially tend to a reduced initial capacity, but simultaneously, to a more pronounced setup. It is shown, that vibrated piles can reach capacities of identical impact driven piles after not more than 100 days of aging. Indications are provided, that for longer time ranges the vibrated piles might even outperform these impact driven piles. The second study (Chapter 5) evaluates three-years of capacity monitoring of six, offshore-size piles installed by pile vibration and by impact driving. It demonstrates that vibrated, large-diameter piles provide just a third of the capacity of impact driven equivalents even after an aging period of three years. The small-pile experiment hypothesis is consequently rejected, and a size dependence of the pile setup is assumed. A final laboratory and modeling study (Chapter 6) evaluates the impacts of physiochemical effects on the overall setup. Direct shear testing of naturally-aged surfaces and subsequent capacity modeling indicate that physiochemical effects and the formation of a pile adhering sand-crust have a high potential to increase pile capacity. With these three sub-studies, this doctoral thesis provides a significant contribution to the understanding of the setup as a multi-factorial and multidimensional process.