Determination of the heterogeneity of a sand specimen and its evolution in triaxial compression

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4. Determination of the heterogeneity of a sand specimen and its evolution in triaxial compression

Standard laboratory tests, such as the triaxial compression test, have been extensively studied in the past decades with the aim to gain a better understanding of the micro-mechanical mechanisms that drive the overall behaviour of the specimen. For this purpose, CT images of specimens in different loading states have been acquired and analysed regarding e.g. the soil fabric or grain kinematics. It has been found that, although the specimen seems to be homogeneous before reaching the peak stress state when observed from the outside, it is in fact strongly heterogeneous even at the initial state.

These observations underline the necessity to change the way constitutive models - aiming to correctly describe the soil behaviour - are developed and calibrated. It becomes obvious that common assumptions, such as the homogeneity of the soil specimen, are wrong and that the entire soil specimen cannot be treated as one single representative elementary volume (REV). Instead, the specimen consists of multiple smaller homogeneous regions. With the help of the REV, the specimen heterogeneity and its evolution can be characterised so that it can be considered in the constitutive modelling of the soil.

In this study, the size of the REV is determined for a Hostun sand specimen in triaxial compression regarding not only the void ratio but also two contact fabric descriptors - the coordination number and the scalar anisotropy factor. The REVs are then used to study the heterogeneity of the soil specimen in different loading states. All of the variables localise during the triaxial test but only the contact fabric shows early signs of localisation. Furthermore, the influence of the grid size on the distribution of the variables is analysed as well as the evolution of specific specimen regions. While there is much smaller change below the shear band, the soil above and inside the shear band, respectively, behaves oppositely.

28th European Young Geotechnical Engineers Conference (EYGEC2024)

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