Numerical modeling of vibro-injection pile installation

Zusammenfassung(Englisch)，该项目涉及饱和水砂土中的振动喷射桩安装的数值模拟，并提供基于有限元方法的适当的计算程序。在城市深基坑工程中，采用振冲注浆桩对底板进行锚固。它们的安装过程具有多材料流动和大材料变形、随时间变化的材料界面以及砂土和孔隙水复杂的耦合行为等特点。这类问题的数值模拟不能用经典的有限元拉格朗日公式和欧拉公式来完成。因此，在研究过程中，将基于模型试验，开发、实现和验证所谓的多材料任意拉格朗日-欧拉方法(MMALE)。MMALE方法描述了与材料运动无关的单元网格运动。材质界面可以在网格中移动，因此元素可能包含两种或更多材质。多孔介质的两方程模型(u-p列式)和气泡函数稳定的线性混合三角形单元为发展提供了基础。每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，特定材料及其在单元中的体积分数的演化方程在时间步长上积分。采用先进的本构方程描述砂土的力学行为。在欧拉步骤中，对网格进行光顺，并利用流体动力学算法将拉格朗日步骤后得到的解变量传输到光顺的网格中。此外，还对材料界面进行了重建，以便准确计算要输送的材料体积，便于显示单元网格内的界面。为了验证MMALE方法的有效性，除中心工程外，还在带玻璃面板的试验室中进行了关于振冲桩安装的小比例模型试验。安装过程用录像记录下来，这些录像由图像相关软件处理，涉及土壤变形和多材料流动的特性(推进前沿、速度和应变场)。利用本工程开发的验证计算程序，能够1)预测桩附近土体应力和密度的变化及其对邻近结构的影响；2)真实地确定成桩的结构性状。

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.