Numerical modeling of vibro-injection pile installation

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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.