It is well known that the bearing capacity and stiffness of displacement piles depends on short term effects during and shortly after pile installation as well as long term effects, a phenomenon usually referred to as set-up. During pile installation the surrounding soil is influenced by the installation process. The soil state e.g. the stress state, pore pressure or void ratio are changing due to complex mechanical processes in the soil. The change of the soil state has a great influence on the pile capacity and the behavior of the pile under vertical and horizontal loading. During the installation process phenomena like a temporary and locally limited liquefaction of the soil can occur. After the pile installation the bearing capacity may increase significantly with time due to set-up effects. In cohesive soils the set-up is commonly explained by consolidation processes in which the dissipation of the excess pore pressures around the pile leads to an increase in effective radial stresses. Case histories show that set-up may also occur in sand over a period which exceeds by far the consolidation process. Therefore, other effects apart from the dissipation of excess pore pressures must contribute to this long-term set-up effect. An understanding and a correct estimation of these effects is of great importance for a economical design of pile-founded structures e.g. offshore wind turbines. For the investigation of these phenomena different methods like numerical simulations, model scale or full scale tests are applicable. In this paper two methods are used: The installation process is investigated with numerical simulations and the set-up effects are investigated by long term in-situ measurements.

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