Capacity degradation of cyclic axially loaded piles in sand

根据循环次数的不同，承受循环轴向荷载的桩通常会经历桩变形的增加和承载力的降低。其主要原因被认为是作用在桩周围区域的土壤中的水平应力的松弛，这是循环剪切引起的桑迪中的颗粒重排的结果。在文献中可以找到从试验结果中得出的各种“相互作用图”，对于给定的平均荷载和循环荷载幅值，可以从中得出导致桩破坏的荷载循环次数。然而，由于缺乏数据，无法考虑桩类型和具体桩尺寸的差异。用相互作用图根本不能预测桩塑性变形的增加。然而，塑性隆起或沉降与设计相关，因为它导致基础的永久倾斜，这必须受到限制，特别是对于海上风能发电厂。在该项目的框架内，将进一步开发一种方法，该方法将桩承载行为的数值模拟与循环简单剪切试验联系起来，因此，原则上，可以计算给定桩几何形状和底土条件的相互作用图，以及桩变形和循环荷载导致的承载能力变化。这种方法，被称为“能力退化法”（CDM），因此承诺的基本见解桩的行为受到循环轴向载荷以及实践相关的调查结果，关于承载能力的减少和永久变形。在本项目范围内，将对CDM的数值模型进行改进，并将进行循环简单剪切试验，以改善循环剪切下土壤行为的描述。目的是，一方面，更好地了解决定循环承载性能的参数，另一方面，发展一个实用的设计程序，循环轴向加载桩在桑迪土。

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.