Accumulation of lateral pile displacements under general one- and two-way cyclic loading - Part 2

对于直径较大的桩，特别是近海桩，往往要承受循环次数较多的横向循环荷载。这导致不可逆位移的积累和桩头的永久旋转。这种永久旋转的预测在海上风能变流器的单桩基础设计中尤为重要。对于单向载荷，可以进行相当准确的预测，而对于一般的单向和双向载荷，则无法进行可靠的预测。从试验研究可知，累积速率与加载类型（由一个周期内最大和最小水平荷载的比值表示）的依赖关系可以用“加载类型函数”来描述。通过一系列的模型试验，阐明系统和荷载参数对荷载类型函数的影响。此外，还计划进行小规模模型试验，利用粒子图像测速仪（PIV）观察靠近桩的砂土中的过程。该项目的第一部分，包括在中密细砂到中砂中对刚性和半柔性桩的试验，结果出乎意料地发现，所有研究系统的加载类型函数对于非对称双向加载的最大值相对较小，当荷载的杠杆臂改变时，该函数也没有显着变化。在第2部分中，应澄清砂的相对密度的变化如何影响结果，以及该函数是否取决于砂的类型。此外，还应研究一个非常灵活的桩-土系统。工程成果将用于开发任意单向和双向荷载下位移累积的计算方法。

Piles of relatively large diameter and in particular offshore piles are often subject to cyclic lateral loads with a great number of cycles. This leads to an accumulation of irreversible displacements and a permanent rotation of the pile head. The prediction of such permanent rotations is crucial in particular in the design of monopile foundations of offshore wind energy converters. A fairly accurate prediction is possible for one-way loading, whereas for general one- and two-way loads with varying magnitude no reliable prediction is possible. It is known from experimental investigations that the dependence of the accumulation rate on the type of loading (represented by the ratio of maximum and minimum horizontal load in a cycle) can be described by a “loading type function”. By a series of model tests, the project shall clarify how the loading type function is affected by system and load parameters. Supplementary, small-scale model tests are planned, in which the processes in the sand close to the pile are observed by means of Particle Image Velocimetry (PIV). Result of Part 1 of the project, which comprised tests on rigid and semi-flexible piles in medium dense fine to medium sand, was unexpectedly that the loading type function for all investigated systems had a relatively small maximum value for asymmetric two-way-loading, which also did not vary significantly when the lever arm of the load was changed. In Part 2, it shall be clarified how a variation of the sand’s relative density affects the results and whether the function depends on the type of the sand. Additionally, a very flexible pile-soil system shall also be investigated. The project results shall be utilized for the development of a calculation approach for displacement accumulation under arbitrary one- and two-way loading.