Multi-scale behaviour of aggregated soils:Experimental characterization and Constitutive modelling

Natural and other aggregated or structured soils present peculiar mechanical behaviour which differs, in some parts, from the behaviour of soils reconstituted in laboratory. This difference can be explained by a specific structure which gathers the particles arrangement and bonding. However, numerous constitutive models were originally based on the behaviour of reconstituted soils. Therefore, classical models should be extended in order to reproduce more complex features of behaviour linked to the soil structure. This work provides a general framework to describe and predict the behaviour of aggregated and reconstituted soils. A multi-scale study was performed to understand the effect of the structure and its evolution upon loading. From an experimental point of view, a preliminary review of literature has been completed by an experimental program carried out on a silty soil in which different structures have been generated by various compaction conditions. At the macro-scale, conventional mechanical tests evidence the overconsolidation effect induced by structure and the important compressibility during its degradation. At the micro-scale, the fabric evolution during the destructuration process (induced by saturation and loading) has been mainly quantified by the evolution of the pore size distribution. This characterisation has been done by Mercury Intrusion Porosimetry. The samples compacted on the dry side of optimum exhibit a double porosity characterized by a bimodal pore size distribution, by opposition to the samples compacted on wet side of optimum that show a single class of pores. The deformation induced by mechanical loading is related to the closure of the biggest pores and the pore size distribution of aggregated soils tends towards unimodal shape upon the destructuration process. A constitutive critical state model (ACMEG) has been extended, based on experimental observations, in order to reproduce the behaviours of aggregated and reconstituted soils. In this purpose, a structure variable has been introduced in the yield function and its evolution has been integrated in the hardening law. Explicit and implicit methods of integration have been discussed and implemented to reproduce stress-strain responses on geomechanical tests (oedometric, isotropic and triaxial compression tests). Finally, the model has been validated by comparison of model predictions with the experimental results.Doctorat en Sciences de l'ingénieur et technologie