Numerical modelling of partially cemented soils in the stagnation zone

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 characterisation 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.

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