风机棘轮致密化桩周海床在波浪作用下的液化机理与灾变防控

Large-diameter monopile is the most common foundation of offshore wind turbines. The adjacent seabed soil around the monopile tends to liquefy under extreme wave loadings, which makes the monopile foundation prone to deformation failure or even collapse. The monopile foundation commonly undergoes periodic wind loads and lateral fatigue loading of waves with small amplitudes before the extreme wave loading. It will lead to the ratcheting densification of the seabed soil around the pile which will in turn affects its liquefaction characteristics under extreme wave loading. With the massive construction of the offshore wind turbines in China, it is desirable to carry out research on the liquefaction mechanism of the densified seabed near pile under extreme wave loading, as well as the disaster prevention and control methods.. The research will be proceeded from two aspects. In the first place, cyclic drained triaxial tests and program development with the subroutine of ABAQUS-USDFLD will be conducted, so as to obtain the densification behaviors of soil and the spatial distribution of the ratcheting densification of the seabed soil near the monopile. On this basis, a centrifuge inflight wave generation device will be developed, to perform centrifuge model tests on the monopile-seabed interaction under wave loading; utilising the modified super-loading and subloading constitutive model considering the complex stress paths of soil around pile, a numerical analysis method will be proposed that solves the monopile-seabed interaction problem under wave loading, and reveals the liquefaction mechanism of the ratcheting densified seabed soil near the monopile under extreme wave loading; corresponding evaluation methods and disaster control measures will be brought forward. The research to be carried out will put an end to the situation of isolated research on seabed liquefaction and lateral cyclic interaction of pile-soil, which is expected to solve the conservative design problem of wave liquefaction resistance in practical engineering, and provide important scientific and technological support for the construction of offshore wind turbines in China.

大直径单桩是海上风机最常用的基础型式。在周期风和小幅波浪水平疲劳荷载作用下，桩周海床发生棘轮致密化；进而在极端波浪作用下，桩周致密化海床液化极易导致基础变形失效，甚至倒塌破坏。亟需开展相应的桩周海床液化机理及灾变防控研究。.本项目首先通过排水条件下循环三轴试验和ABAQUS子程序开发，分别获得风、浪疲劳荷载作用下土体致密特性及桩周海床致密化空间分布规律；在此基础上，研发离心机造波模型箱开展波浪作用下单桩-海床相互作用离心模型试验，考虑桩周土复杂应力路径修正上下负荷面本构模型，提出波浪对海床循环波压力及对桩基水平循环荷载共同作用下单桩-海床相互作用的数值分析方法，揭示极端波浪作用下棘轮致密化桩周海床的液化机理，并提出相应的评估方法和灾变控制措施。拟开展研究将改变波浪作用下海床液化和桩土水平循环相互作用两者割裂研究的局面，可望解决目前工程上依靠经验进行抗波浪液化保守设计的问题。

本项目照原计划执行，针对风和波浪荷载作用下桩周海床动力响应和桩土相互作用问题，通过开展单元体试验、波浪-海床相互作用超重力模型试验、水平循环荷载下大直径单桩水平受荷特性超重力模型试验、砂土海床交变液化特性数值模拟等，取得主要进展如下：（1）研发了我国首台用于臂式离心机的超重力波浪模拟实验装置，再现了现场大时空尺度波浪-海床相互作用；（2）通过室内单元体试验与超重力试验揭示了砂土海床地基沿深度渐进液化、持续剪缩致密化导致孔压循环累积速率慢于消散速率而发生重固结的机理，获得了长期循环作用下砂土变形和棘轮致密化分布规律；（3）建立了基于砂土交变液化和主应力轴旋转效应的波浪-单桩-海床相互作用有限元分析模型，得到不同波浪工况下桩周海床孔压分布和桩土相互作用规律，提出了桩周海床孔压累积预测模型和液化p-y曲线；界定了可将波浪等效为水平循环荷载的临界水深；（4）提出了水平循环荷载作用下单桩基础与导管架群桩基础变形与承载力分析方法，建立了与循环应力比相关的桩周土弱化分析模型，采用刚度衰减模型实现了对桩基础在风和波浪荷载作用下桩周土弱化的量化描述，并开展现场试验进行了验证。. 本研究课题共发表和录用相关学术论文17篇（均标注该项目资助），包括《Journal of Geotechnical and Geoenvironmental Engineering, ASCE》、《Journal of Waterway, Port, Coastal, and Ocean Engineering, ASCE》、《Canadian Geotechnical Journal》等本领域主流SCI期刊论文15篇， EI论文1篇，授权国家实用新型专利1项。成果中风机基础循环累积变形分析方法作为重要内容的项目获浙江省科技进步一等奖1项（项目负责人排名第1）。以本项目工作和成果为重要支撑，项目负责人朱斌入选2019年度“长江学者奖励计划”特聘教授，并作为主要审查人参与国家标准《海上风力发电场设计规范》(GBT51308-2019)制定。成果应用于广东桂山、浙江普陀等大型海上风电场工程。