Representative Elementary Volumes of coarse-grained soils

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

Standard laboratory tests, such as the triaxial test, are commonly used to calibrate the constitutive models aiming to correctly describe the soil behaviour. In order to obtain true material behaviour, those tests are designed and assumed to be element tests. The specimen is considered to be a representative elementary volume (REV), i.e. an element with homogeneous properties and homogeneous behaviour, representing the soil behaviour and is therefore interpreted as a material point. This is obviously not true for the triaxial test where shear banding is usually visible to the naked eye or complex patterns of shear bands form inside the specimen leading to a highly heterogeneous response from very early on.These obvious discrepancies between the phenomena observed in laboratory tests and the assumption of homogeneity forming the basis for the experimental evaluations, the constitutive model calibration and the description of the soil behaviour have to be resolved. The specimen is, in fact, not a single REV but rather consists of multiple REVs, each representing a certain region inside the specimen. The size of such a REV depends on the internal length scales, e.g. the particle size or the shear band thickness, governing the global behaviour. The REV size was also found to depend on the analysed variable, e.g. the void ratio or the contact fabric.Based on the analysis of x-ray computed tomographies (CTs) acquired during the loading of a specimen, we have developed an approach for the determination of the REV size considering three different criteria. This approach will now be used to characterize the different influences on the REV size and to identify the heterogeneous state of the specimen throughout loading. Using x-ray CT for this study, one is limited to the fabric and kinematics of the soil. Ultimately, we want to connect the REV behaviour to the globally observed one characterised by a stress-strain-relationship measured at the boundary of the specimen.At the micro-scale, we also need to include the stresses, which cannot be obtained with x-ray CT. For such a complete description, the discrete element method (DEM) can be introduced. Combining the experimentally and the numerically obtained evolution of the REVs will offer the fundamentals for a comprehensive comparison between the global and the local behaviour. The study of the soil heterogeneity also allows for its incorporation in finite element models and an enhancement of the constitutive model calibration. The consideration of a realistic initial state has already been shown to improve the model calibration. Constitutive models could even be calibrated for chosen REVs, e.g. those inside the shear band, which have been assumed to be the only ones reaching a true critical state. The results of these investigations will form the basis for a link between conventional assumptions and evaluations on the one hand and micro-scale observations and phenomena on the other hand.

• DFG Programme: Research Grants