Capacity degradation of cyclic axially loaded piles in sand

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Piles subjected to cyclic axial loading generally experience an increase in pile deformation and a reduction in bearing capacity depending on the number of cycles. The main reason for this is considered to be the relaxation of the horizontal stress in the soil acting in the area around the pile, which is a consequence of the grain rearrangements in the sandy soil induced by cyclic shear. Various "interaction diagrams" derived from test results can be found in the literature, from which the number of load cycles leading to pile failure can be tapped for a given mean load and cyclic load amplitude. However, differences with respect to the pile type and with respect to the specific pile dimensions cannot be taken into account due to lack of data. An increase in plastic pile deformations cannot be predicted at all with an interaction diagram. However, plastic heave or settlement is relevant for the design, since it leads to a permanent tilting of the foundation, which has to be limited especially for offshore wind energy plants. Within the framework of the project, a methodology is to be further developed which links a numerical simulation of the pile load-bearing behavior with cyclic simple shear tests and thus, in principle, makes it possible to calculate both interaction diagrams for given pile geometries and subsoil conditions as well as pile deformations and changes in load-bearing capacity as a result of cyclic loads. This approach, known as the "Capacity Degradation Method" (CDM), thus promises fundamental insights into the behavior of piles subjected to cyclic axial loads as well as practice-relevant findings with regard to load-bearing capacity reduction and permanent deformations. Within the scope of the project, both the numerical model of the CDM will be refined and cyclic simple shear tests will be carried out in order to improve the description of the soil behavior under cyclic shear. The aim is, on the one hand, a better understanding of the parameters determining the cyclic load-bearing behavior and, on the other hand, the development of a practical design procedure for cyclically axially loaded piles in sandy soils.