PhD DEFENSE BY TIJAN MEDE

This PhD project was co-supervised by Guillaume Chambon and François Nicot from IRSTEA, and Pascal Hagenmuller from the CEN, and co-financed by Tec21 and the INRAE Grenoble.
The defense will take place at 2 pm in the room Ecrins (INRAE building) on the university campus.

Abstract

Dry slab snow avalanches represent a major natural hazard that is extremely dicult to manage. Attempts to model this phenomenon are hindered by the lack of a constitutive law that would describe the mechanical behaviour of snow on a material scale. In particular, relatively little is known on the failure and post-failure response of snow at high loading-rates. The highly fragile nature of the material in this deformation regime renders experimental investigation diffcult and complicates observation at the microstructural level. As an alternative to experiments, a Discrete Element Method-based numerical model of snow is developed in this thesis. The model enables us to simulate the response of snow to mechanical loading, while accounting for actual snow microstructure by using X-ray attenuation images of snow microstructure as input. Snow is considered as a cohesive granular material and an original methodology is developed in order to model the shape of each grain. Individual grains are bound into the snow matrix by modelling cohesion between neighbouring grains. The model is then used to explore the macroscopic mechanical response of different snow samples to mixed-mode loading. Three typical modes of failure are observed in all tested snow samples, depending on the level of applied normal stress: a localized shear failure at low normal stress (mode A), a shear failure-induced volumetric collapse at intermediate levels of normal stress (mode B), and a normal failure and collapse for high values of normal stress (mode C). The observed failure modes result in closed failure envelopes and no qualitative dierence is observed between the mechanical responses of dierent snow types. The internal mechanisms that lead to volumetric collapse are further examined on the microscale. Force chain buckling is identied as a trigger of this material instability. Additionally, force chain stability appears to be controlled by the contacts between the force chain members and the surrounding grains. The failure in these contacts, which is evidenced in modes B and C, allows force chain buckling to develop and results in subsequent volumetric collapse.