可液化场地桩-土相互作用动态接触面模型及数值分析方法研究

The lateral deformation of liquefiable foundation can be divided into smaller motion deformation before liquefaction, large deformation after liquefaction and permanent lateral displacement after the earthquake. Pile-foundation interaction will vary with the three cases. At present, most of the pile-soil interaction numerical simulations use the contact elements at small foundation deformation, while ignoring the change of the dynamic characteristics at pile-soil contact zone which is caused by soil flow after liquefaction. This project will carry out the dynamic triaxial tests、dynamic cycle shear tests and shaking table tests to research the dynamic shear stress-strain characteristics at pile-soil interaction zone in the process of saturated sand liquefaction, establish the dynamic interaction constitutive model which can simulate slip, disengage, close and lateral load caused by soil flow. The Oka effective circulating elasto-plastic constitutive model is used to simulate the material nonlinearity, it is dispersed by element free Galerkin method (EFGM), the pile is still simulated by FEM, the dynamic contact model is added to establish the FEM-EFGM coupled method of pile-soil interaction in liquefiable site. The method can avoid calculating interruption when using FEM to simulate huge deformation caused by sand liquefaction, then it can ensure the simulation integrity. Drawing up shaking table test plan, verify and correct the simulation method until it reaches certain reliability.

可液化地基的侧向变形，可分为液化前较小的震动变形，液化后震动引起的大中变形及地震后的永久侧向位移。桩-地基相互作用会因这三种情形的不同而不同。目前桩-土相互作用问题数值模拟中，大多采用以往地基小变形时的接触单元，而忽略了液化后土体流动所带来的与桩之间接触带动力特性的变化。本项目首先开展动三轴试验、动态单剪试验和振动台试验研究饱和砂土液化过程中桩-土接触带动应力应变特性，建立可根据液化度调整的模拟滑移、脱开、闭合及土流动产生横向荷载的动态接触面本构关系。采用Oka有效循环弹塑性本构模型模拟饱和砂土的材料非线性，用无网格伽辽金法离散，桩仍用有限元模拟，基于动态接触面模型，建立液化场地桩-土相互作用的有限元-无网格耦合数值分析法，避免液化砂土产生巨大变形时采用有限元计算无法运行，保证液化过程模拟的完整性。制定振动台试验方案，对数值分析方法进行验证和校正，直到达到一定可靠度。

我国滨海、滨河、古河道、三角洲等区域分布有大面积的可液化土，地震液化诱发大变形对建（构）筑物桩基础的影响对危害评价工作有重要意义，本项目主要围绕桩-土液化动力响应分析方面展开研究。通过对不同密实度液化场地桥梁桩基础的动力响应研究，分析采用加密处理对群桩抗液化的影响。对于桩-土接触问题通过在界面采用位移协调条件、添加机械接触面模型和带有阻尼影响的接触面模型进行对比，推荐更符合理论意义的接触面模型。通过数值试验总结桩-土动力相互作用p-y曲线所确定的刚度与液化度之间的关系，并与由试验获得的p-y曲线折减系数进行比较，发现用本文数值方法获得的折减曲线适用，为地震液化情况下桩基础的数值模拟提供参考方法，避免试验消耗。对于液化引起的大变形，有限元法使用常因网格畸变而受到限制，因此，本项目开发了无网格法模拟。首先通过位移、孔压形函数和边界条件的处理，将无网格法应用到单桩和边坡模型的模拟中，验证其适应性和有效性；同时为了解决无网格法计算时长问题提出采用自适应程序提供计算效率。开发了场地液化大变形的耦合算法，对于液化变形大的区域采用无网格法离散，变形小的区域用有限元法离散，既解决了有限元法因网格畸变产生的计算中断问题，又节约计算成本。