Digital Sand: On the investigation of the mechanical behaviour of granular soils by linking in situ X-ray CT experiments with numerical simulations

• Applicant: Professor Dr.-Ing. Marius Milatz
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2026
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 576182042

The research project aims at laying the foundation for a “digital soil laboratory” for granular soils, that allows for the numerical simulation of the (micro-)mechanics of sand based on digital twins obtained from computed tomography (CT) scans. This procedure is supposed to yield new insights into the granular material behaviour, classically described by continuum mechanical constitutive models, and to improve our modelling frameworks by means of a focus on the interface between micro- and macromechanics. As a starting point, we create an open access sand particle database for single grains and particle packings of sands with different grain sizes and grain shapes, which can be extended in the future, allowing to extract numerical models and statistical studies on granulometry. The chosen numerical method is the Finite Discrete Element Method (FDEM), a coupling of FEM and DEM in which the geometry of each sand grain is captured by an individual FE mesh. Besides the particle database, a software for model generation is developed in order to import the digital sand grains and particle packings into FDEM simulations. Subsequently, we investigate the influence of mesh resolution of digital sand grains, of the FE modelling approach as well as of the micromechanical material parameters in the framework of systematic FDEM studies. This working package aims at an optimisation of the modelling technique, enabling it to realistically model the physics of thousands of arbitrarily shaped sand grains in contact in an appropriate simulation time. Within the scope of experiments, we further develop miniaturised soil mechanical experimental apparatuses. These are applied in “in situ CT experiments” for particle scale testing of the sands with their different granulometries by means of sequential CT scans during oedometric compression and shear loading in a direct shear test and simple shear test. The CT scans capture all sand grains contained in a specimen (several thousands) as well as the mechanical boundary conditions with sufficiently high spatial resolution. In combination with the measurement of forces and displacements at the boundaries, this approach also allows for the quantification of the macroscopic specimen response. The interactions of sand grains, trajectories, rotations, and contact changes, are measured from the temporal 3D CT data. After the optimisation of the FDEM modelling approach, we use the CT data from the in situ CT experiments to create digital twins for direct back calculation of the experiments based on the real grain skeletons. Thereby, the micro- and macromechanical behaviour as modelled by FDEM is supposed to be validated with regard to the experimental data. We aim at the investigation of granulometric influences on the particle scale and macroscopic behaviour of the sand grains and of the overall specimen in the selected classic soil mechanical lab experiments.

• DFG Programme: Research Grants