复合加载下自升式平台桩土耦合作用机理及其极限承载能力研究

Jack-up platform is an important marine construction equipment in the world, environmental loads including sea wave, ocean current make the platform subjected to combined action of horizontal, vertical, moment and torque loads. It is very important for the evaluation of ultimate bearing capacity and stability of platform pile foundation to study the coupling interaction between pile legs of jack-up platform and soil around the pile legs under combined loading. Some relevant scientific problems of pile foundation stability of jack-up platform under combined loading are researched in this project in detail, which include the soil deformation law and failure mechanism during the process of driving and pulling piles of jack-up platform under combined loading; the effect of pile group to the punch-through failure mechanism of layered seabed sediment during driving piles and the ultimate bearing capacity of platform pile foundation; and the effection of the weaken mechanism of soil shear strength on the ultimate bearing capacity of pile foundation of platform under the cyclic loads of sea wave. The scouring effect of soil on the stability of platform pile foundation. On the basis of numerical analysis, theoretical derivation and experimental study, a nonlinear mathematical model coupling combined loads, viscoelastic plastic seabed and spudcan legs is established, and it’s relevant numerical analysis method including static calculation and dynamic analysis are given. The nonlinear coupling action between jack-up platform pile legs and seabed soil under combined loading would be solved, and some key scientific problems about soil deformation law under six degrees of freedom would be solved in this project too. As results, some computational formulas of ultimate bearing capacity of pile foundation with six degrees of freedom is achieved, and the results of this project can provide scientific basis for intensive study on stability of platform pile foundation.

自升式平台是国内外海上施工的重要设备，波浪、海流等环境荷载使得平台承受水平、竖向、力矩与扭矩的复合加载作用，深入研究复合加载下桩腿与海床土体间的耦合作用机理，在准确计算海床地基的承载能力与评估桩基稳定性上具有举足轻重的地位。项目主要研究复合加载下自升式平台桩基稳定性的有关科学问题，包括：6自由度复合加载下自升式平台插拔桩过程桩周土体的变形规律与失稳机理；群桩效应对土层穿刺机理与极限承载能力的影响；波浪循环荷载下海床土体强度弱化对桩基承载能力的影响；桩周土体冲刷效应对桩基稳定性的影响。通过上述分析，建立复合加载下粘弹塑性海床地基与平台桩腿非线性耦合作用的数学模型和静、动态分析计算方法；拟解决复合加载下平台桩腿与海床土体非线性耦合作用，以及6自由度下桩基土体失稳机理等关键科学问题。给出较为准确的6自由度桩基极限承载力计算公式，为深入研究平台桩基稳定性提供科学依据。

21世纪是全面发展海洋资源的时代，人们正逐渐由沿海走向浅海、迈向深远海，需要面临与解决复杂的海洋环境荷载带来的种种工程困难，其中海洋结构物与海床土体的非线性耦合作用显得非常重要，成为直接影响结构物安全作业的关键问题之一。. 本项目研究内容，包括：①从理论上推导了环境荷载下地基的整体稳定性，对平台桩腿插桩入泥、拔桩阻力、层状穿刺以及桩周土体孔隙水压力场的产生、发展、累积与消散规律进行了数值仿真；②分析了波浪循环荷载下海床土体抗剪强度弱化、刚度软化以及土体冲刷规律，推导出了考虑中主应力旋转的土体动力本构模型；③建立了循环荷载下桩基土体的变形规律与荷载分量间的对应关系，并研发出了六自由度土工加载仪、土体大变形循环环剪仪。. 通过采用理论推导、数值计算与比尺试验同时进行的方法，取得了如下成果：①推导出了海床土体破坏滑裂面方程，建立了循环荷载下桩基稳定性评估方法；②推导出了循环荷载下土体动力本构方程，分析了土体强度弱化与刚度软化的特性；③研究了平台桩靴贯入过程中的超深入泥、拔桩过程中的极限上拔力以及层状土层中的穿刺机理；④建立了循环荷载下土体渐变破坏理论，分析波流作用下人工岛护岸的整体稳定性，研究了人工岛填筑过程中初始固结与压密不均匀性造成的地基土体内部塌陷等安全问题。截止2020年共发表科研论文13篇，出版海洋土力学专著1部，获批国家发明专利2项。. 本项目具有如下科学意义：①建立了三维荷载空间内粘弹塑性海床与结构物非线性耦合作用数学模型，构建了相应的静、动态数值计算方法，为我国海洋工程领域内的科研工作者在数值仿真方面提供了一定的科学参考依据；②本项目给出的循环荷载作用下土体渐变破坏理论，在南中国海岛礁填筑工程中得到了合理的验证与应用，为进一步扩大人工岛建设提供了一定的理论参考。③本项目研发的六自由度土工试验仪和大变形循环环剪仪，对试验室从事比尺研究的科研人员提供了较好的参考依据。