Experimentelle Untersuchungen zur Verformungsakkumulation und Tragfähigkeitsreduktion von zyklisch belasteten Pfählen
Confronted with the effects of wind and waves, the foundations for offshore wind turbines are exposed to cyclic and dynamic loads. In this respect, different types of driven pile foundations, either the monopile or the multi-pile foundation, are frequently envisaged as foundation systems. Depending on the chosen variant the piles transfer the loading in different ways into the subsoil. The monopile bears the lateral loads from wind and waves through its lateral bedding and the consequent subgrade reaction. On the other hand, the lateral actions generate pairs of tension and compression axial forces in the opposite piles of a multi-pile foundation. These axial forces will be carried into the subsoil by means of the shaft friction and, whenever possible, by the pile’s tip resistance. In order to design the piles for both the ultimate and the serviceability limit states, the transient loads resulting from wind and waves are often converted into an intended “equivalent”, usually sinusoidal, cyclic loading with constant amplitude and a lower number of cycles. However, this approach neglects any influence from the order of appearance of the original loads. The implications of this fact for the discrepancy between the design predictions and the actually occurring deformations or load capacity reductions have not yet been clarified. A significant point investigated in this work is therefore to assess the independence of the deformations and load carrying capacity changes due to cyclic loading of the order of appearance of different loads in drained conditions. For this purpose, a program of experimental tests has been carried out, which for the case of the laterally loaded single pile foundation was based on physical model tests in two different scales. The validity of the hypothesis of independence of the deformations of the order of different loads for a wide range of applications has been here investigated with a variation of pertinent soil and system parameters. It is also essential to assess the quality and accuracy of the approach used for the prediction of accumulated deformations for the laterally loaded single pile foundations. This has to reflect as closely as possible the deformations that will arise in the relevant range of the numbers of load cycles. This thesis therefore also examines the quality of different empirical methods for predicting the accumulation of deformations as correlated with the results of physical model tests of a laterally loaded single pile. Thereby, the variation of the different prediction parameters with the properties of the soil system and load characteristics has been assessed. Furthermore, the validity of the prediction method across the scales is confirmed by the results of the investigations at different geometrical scales. By using different sands in the model tests, the transferability of the findings to other soil conditions is also confirmed. Concerning the axially loaded piles of a multi-pile foundation, the essential phenomena contributing to their structural behavior cannot be scaled up from a small physical model of the pile. Therefore, the experimental investigations for these piles are performed by means of element tests of the pile-soil interface at the original scale. For this purpose, the physical testing with a ring shear device has been found suitable. The results of these tests confirm the general independence of the shear strength development from the sequence of different loads in drained conditions. However, the existing empirical prediction models to estimate the change in radial stress on pile shaft due to cyclic loading, assumed to be a reason for changes in axial capacity, do not always reflect correctly the results of element tests in the shear device. For this reason, an existing empirical approach has been extended in this work in order to determine the relevant influences of the load characteristics and include them in the prediction of capacity degradation. With this extended empirical approach, the influence of the mean shear stress is taken into account on the degradation of the shear strength in addition to the influence of the shear stress amplitude. Thereby, the influence of different soil, load and system parameters were considered. Finally, the applicability of the empirical approach for the determination of the expected reduction in bearing capacity due to cyclic axial loading of pile foundations is demonstrated with a simple practical example.
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