Homogeneous elements within coarse-grained soils during strain localisation

• Applicant: Professor Dr.-Ing. Ivo Herle
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 508562211

Triaxial tests are commonly interpreted as element tests where the specimen is considered one homogeneous element and thus a material point. However, this assumption is violated as soon as one or multiple shear bands develop inside the specimen. It has been revealed that specimens are heterogeneous even from the very beginning of the tests which leads us to strongly question the interpretation of triaxial tests as element tests. Considering this obvious heterogeneity of the soil specimen leads to a new concept with implications for the constitutive modelling of the soil behaviour. In our preliminary works we have shown that the soil may behave much simpler when observed on a smaller scale. For this purpose, the soil behaviour on the mesoscale can be extracted from the evolving shear band using x-ray CT. It has been observed that much simpler constitutive models can be used to reproduce the highly complex soil behaviour in triaxial tests when combined with a realistic heterogeneous initial state. It is obvious that the soil behaviour inside and outside the localised zone is vastly different. The findings suggest that the high complexity of the macroscopic soil behaviour is a result of the homogenisation of a highly heterogeneous specimen with zones of very different behaviour. This heterogeneity can be characterised in several ways. In any case, the size of a representative elementary volume (REV) needs to be determined which is then placed either in a regular grid throughout the specimen or in a selected region. The identification of a suitable REV size is crucial as it should reveal the variation of the soil variables without being too sensitive to micro-scale fluctuations. Two approaches for the REV size determination from x-ray CT images were proposed in our preliminary work. These methods will be further enhanced to achieve a robust, objective and preferably automated procedure for a multitude of different soil specimens. In this way, the influence of the granulometric properties as well as preparation methods or test conditions on the REV size will be analysed. The heterogeneity of the soil specimen and the mesoscale soil behaviour will be extracted from x-ray CT images with respect to the local strain, the void ratio and the contact fabric. As the evolution of the contact fabric anisotropy is closely connected to the stress evolution, this gives a good approximation of the mesoscale stress-strain relationship. This assumption will be further investigated with simulations of the triaxial tests using the discrete element method (DEM). The DEM will provide an additional valuable insight into the mesoscale, especially regarding the contact forces. The expected results of this project will form the basis for the incorporation of the soil heterogeneity into constitutive modelling and the development and calibration of constitutive models based on the mesoscale soil behaviour.

• DFG Programme: Research Grants
• International Connection: Australia, France
• Cooperation Partners: Professor Dr. Itai Einav; Professor Dr. Gioacchino Viggiani; Dr.-Ing. Max Wiebicke