Abstract The performance of monopiles in cohesive soils is of great interest for future offshore wind farm developments, particularly under the cyclic loads experienced in the ocean environment. Clay behaviour during undrained cyclic loading is complex and involves the accumulation of plastic strains, generation of excess pore-water pressures and degradation of initial stiffness. In this paper, the cyclic performance of a laterally-loaded monopile in spatially variable clay is investigated for the first time. A kinematic hardening constitutive model is used in a 3D finite element analysis to capture the hysteretic stress-strain behaviour of the clay. The monopile is installed in overconsolidated London Clay, which is present at several offshore wind farms in the Thames Estuary. The finite element model is coupled with random field representations of initial stiffness and clay structure. The statistical characterisation of the random fields was undertaken considering parameter ranges observed in laboratory tests. Under one-way cyclic loading, the monopile showed ratcheting behaviour, where pile rotation accumulates with increasing numbers of load cycles. The cyclic secant stiffness also increased due to the generation of negative excess pore-pressures in the clay. This behaviour occurred in both homogeneous and spatially variable clay. The monopile was also subjected to an extreme dynamic event and the soil response around the monopile showed increasing variability in stress-strain response and generation of excess pore-water pressure over time as plastic strain accumulated. However, the overall behaviour of the foundation was governed by a spatial average of the mobilised clay. The range in monopile response demonstrates how the natural spatial variability of clay can have a strong influence on monopile performance.

中文翻译：

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使用高级本构模型在空间可变粘土中单桩的循环性能

摘要 粘性土壤中单桩的性能对未来海上风电场的发展具有重要意义，特别是在海洋环境中经历的循环载荷下。不排水循环荷载期间的粘土行为是复杂的，涉及塑性应变的积累、超孔隙水压力的产生和初始刚度的降低。在本文中，首次研究了空间可变粘土中横向加载单桩的循环性能。在 3D 有限元分析中使用运动硬化本构模型来捕捉粘土的滞后应力应变行为。单桩安装在过度固结的伦敦粘土中，该粘土存在于泰晤士河口的几个海上风电场中。有限元模型与初始刚度和粘土结构的随机场表示相结合。随机场的统计特征是考虑到在实验室测试中观察到的参数范围而进行的。在单向循环载荷下，单桩表现出棘轮行为，随着载荷循环次数的增加，桩的旋转会累积。由于粘土中产生负超孔隙压力，循环割线刚度也增加。这种行为发生在均质粘土和空间可变粘土中。单桩还受到极端动力事件的影响，随着塑性应变的积累，单桩周围的土壤响应显示出随着时间的推移，应力-应变响应和超孔隙水压力的变化越来越大。然而，地基的整体行为受流动粘土的空间平均值控制。单桩响应的范围表明粘土的自然空间可变性如何对单桩性能产生重大影响。