Numerical modelling of partially cemented soils in the stagnation zone

计划中的研究项目的主要目标是更好地了解多相多孔复合材料的微观力学变形行为。为此，将详细研究一种在土木工程特别是岩土工程中经常使用的多相复合材料：水泥砂。例如，自硬化悬浮液可用作地下连续墙施工中的稳定流体。由于制造工艺的原因，在几乎完全填充水泥的颗粒结构和纯颗粒结构之间的部件边缘存在过渡区。在这个区域，称为停滞区，载荷传递取决于单个颗粒的位置和接触点。在这种情况下，所施加的载荷一方面由水泥基质承载，另一方面由嵌入其中的砂粒承载。荷载传递受颗粒的非均匀排列和球形度、混合度和水泥含量的强烈影响。为了改善与实际相关的宏观有限元模型，多孔复合材料的微观和宏观材料行为之间的桥梁是建立。基于成像技术，如X射线计算机断层扫描三维有限元模型将直接从扫描水泥砂样品获得的图像数据生成。使用数值方法，可以对以这种方式获得的模型进行变形分析。在岩土工程和施工管理研究所（Grabe教授），主要使用有限元法（FEM）。此外，有限单元法（FCM）在船舶结构设计与分析研究所（Düster教授）得到了进一步发展。这两种数值方法将在计划的研究项目过程中应用和优化微观结构问题。其目的是通过数值均匀化得到复杂多相微结构的宏观材料特性。这允许表征这种复合材料的宏观材料行为。宏观材料行为将通过实验室实验进行研究。对不同水泥含量的砂样进行了单轴压缩试验和三轴试验。由于在土力学实验室、CT扫描仪以及数值研究和开发中进行了广泛的计划调查，因此在第一个项目阶段将假设弹性材料行为。在可能的第二个项目阶段，非线性材料模型，除其他外，将用于调查较大的变形或模拟复合材料的损伤。

The primary goal of the planned research project is to gain a better understanding of the micromechanical deformation behaviour of multiphase porous composite materials. For this purpose, a multi-phase composite material commonly used in civil engineering and especially in geotechnics in many problems will be investigated in detail: cement-bound sand. For example, self-hardening suspension can be used as a stabilizing fluid in the construction of diaphragm walls. Due to the manufacturing process, there are transition zones at the edges of the components between almost completely cement-filled grain structure and pure grain structure. In this area, known as stagnation zone, the load transfer depends on the position and contact points of the individual grains. In this case, the loads applied are carried by the cement matrix on the one hand and by the sand grains embedded in it on the other. The load transfer is strongly influenced by the heterogeneous arrangement and the sphericity of the individual grains, the degree of mixing and the cement content. In order to improve macroscopic FE-models relevant for practice, a bridge between the microscopic and macroscopic material behaviour of porous composite materials is to be established. Based on imaging techniques such as X-ray computed tomography three-dimensional FE models will be generated directly from the image data obtained by scanning cement-bound sand samples. Using numerical methods, deformation analyses can be carried out on the models obtained in this way. At the Institute of Geotechnical Engineering and Construction Management (Prof. Grabe) the Finite Element Method (FEM) is mainly used. In addition, the Finite Cell Method (FCM) is further developed at the Institute of Ship Structural Design and Analysis (Prof. Düster). These two numerical approaches are to be applied and optimized to microstructural problems in the course of the planned research project. The aim is to derive macroscopic material characteristics of the complex multi-phase microstructures by numerical homogenization. This allows the characterization of the macroscopic material behaviour of such composite materials. The macroscopic material behaviour will be investigated by means of laboratory experiments. Uniaxial compression tests and triaxial tests on sand samples with different cement contents are planned. Due to the extensive planned investigations in the soil mechanics laboratory, at the CT scanner as well as the numerical investigations and developments, elastic material behaviour will be assumed in this first project phase. In a possible second project phase, non-linear material models, among others, are to be used for investigations of larger deformations or for modelling damages in the composite material.