Experimental and modelling issues in unsaturated soils mechanics: Role of microstructure

This paper brings some insights into the role played by the microstructure of soils on their hydromechanical behaviour. First, a discussion on the definition of an effective stress for unsaturated soils is presented. Based on an energetic approach, the role of microstructure in the definition of Bishop's parameter is investigated. Based on experimental data, it is then shown that the usual assumption identifying this parameter to the degree of saturation is not verified for fine soils. Second, the wetting induced collapse of an unsaturated sand is studied using X-ray computed tomography. Quantitative analyses are performed using DIC and show that the size of the REV depends on the quantity of interest and evolves with the water content. Heterogenous water content and strain distributions are observed at the local scale. Interestingly, the local behaviour is compatible with the average behaviour at the scale of the specimen.