Numerical modeling of vibro-injection pile installation

Zusammenfassung（englisch）该项目涉及水饱和砂中振动灌注桩安装的数值模拟，并提供基于有限元法（FEM）的适当计算程序。城市深基坑采用振冲灌注桩作为基坑支护的基础。其安装过程的特点是多物质流与大的材料变形，由时间相关的材料界面，以及砂和孔隙水的复杂耦合行为。经典的拉格朗日和欧拉有限元法无法对此类问题进行数值模拟。因此，一个所谓的多材料任意拉格朗日欧拉（MMALE）有限元方法将开发，实施和验证模型试验的基础上，在研究项目。MMALE方法描述了独立于材料运动的单元网格的运动。材质界面可以在网格中移动，因此元素可能包含两种或多种材质。多孔介质的两方程模型（u-p公式）和由气泡函数稳定的线性混合三角形单元提供了发展的基础。每一个全局计算步骤被细分为拉格朗日步骤和欧拉步骤。在拉格朗日步骤中，网格遵循材料变形，特定材料的演化方程及其在单元内的体积分数在时间步长上被积分。采用先进的本构方程描述砂土的力学行为。在欧拉步骤中，网格被平滑，并且通过使用流体动力学算法将拉格朗日步骤之后获得的解变量传输通过平滑的网格。此外，为了准确计算待输运的物料体积，并便于在单元网格内显示界面，还对物料界面进行了重构。为了验证MMALE方法，除了中心项目外，还在带有玻璃面板的试验室中进行了关于振动灌注桩安装的小规模模型试验。安装过程通过视频记录进行记录，这些视频记录由图像相关软件处理，涉及土壤变形和多材料流的特性（前进前沿，速度和应变场）。通过使用该项目中开发的经过验证的计算程序，可以1）预测桩附近的土壤应力和密度变化及其对相邻结构的影响，2）实际确定成品桩的结构行为。

Zusammenfassung (englisch)The project deals with the numerical modeling of vibro-injection pile installation in water-saturated sand and the provision of an adequate calculation procedure based on the finite element method (FEM). Vibro-injection piles are used to tie back the base slab of deep excavations in urban area. Their installation process is characterized by multimaterial flow with large material deformations, by time-dependent material interfaces, as well as by the complex coupled behavior of sand and pore water. The numerical simulation of such problems can not be accomplished by using the classical Lagrange and Euler formulations of FEM. Therefore, a so-called multi-material arbitrary Lagrangian-Eulerian (MMALE) method for finite elements will be developed, implemented, and validated based on model tests during the research project. The MMALE method describes the motion of the element mesh independent of the material motion. Material interfaces can move through the mesh, so that elements potentially contain two or more materials. The two-equation model for porous media (u-p formulation) and the linear mixed triangle element stabilized by a bubble function provide a basis for the developments. Every global calculation step is subdivided into a Lagrangian step and a Eulerian step. During the Lagrangian step, the mesh follows the material deformations and the evolution equa-tions of the particular materials and their volume fractions within the elements are integrated over the time step. Advanced constitutive equations are incorporated which describe the mechanical behavior of sand. During the Euler step, the mesh is smoothed and the solution variables obtained after the Lagrange step are transported through the smoothed mesh by using fluid dynamical algorithms. Moreover, the material interfaces are reconstructed in order to accurately calculate the material vol-umes to be transported and to facilitate the display of interfaces within the element mesh. For reasons of validation of the MMALE method, small-scale model tests concerning the installation of vibro-injection piles are conducted in a testing chamber with glass panel in addition to the central project. The installation process is documented with video recordings which are processed by image correlation software with regard to soil deformations and properties of the multi-material flow (advancing fronts, velocity and strain fields). By using this verified calculation procedure developed in the project, one is able to 1) predict the soil stress and density changes in the vicinity of the pile and their impact on adjacent structures and 2) realistically determine the structural behaviour of the finished pile.