Micromechanics of unsaturated porous media across the saturation regimes: Applications to stability and resilience of geostructures under climate change

点击翻译按钮获取中文摘要

In nature, unsaturated porous media are ubiquitous, e.g., footprints in sand and snowballs. How water occupies pore spaces of a soil such as in a slope is crucial-the cyclical wetting and drying of soils due to fluctuating climate can be disastrous, triggering landslides by liquefaction. Global warming is also causing permafrost thaw, creating soil moisture changes that cascade into CO2 release and landslides in the Canadian North. The central question is: How does water saturation in soils lead to distinct capillary regimes that can spontaneously modify their behaviour from solid to fluid state? How do we mathematically describe such a brutal transition that translates into a wetting collapse of geostructures? It turns out that this longstanding question has microstructural origins, being intimately linked to the loss of suction between grains. Whereas most studies take rather a phenomenological view that does not go deep into the physics of the microstructure, thus hindering progress, the proposed research investigates formidable issues such as how grain contacts, water menisci and interfaces between phases in an unsaturated soil evolve as water saturation increases (decreases) during wetting (draining). A multiscale approach is thus mandated where an element volume capturing all the local microstructural complexities of fluid and solid interactions is studied, and thereafter statistically upscaled to establish relationships between stress, strain, and capillary stresses at the continuum level for the analysis of a geostructure. A new framework is developed that underscores the collective interactions of water and air in pore spaces coupled with solid particles through discrete element modelling for solids and the Lattice Boltzmann method for water and air. This facilitates the modelling of complex formation, coalescence, and rupture of water menisci in the pore space. Unsaturated constitutive laws that account for these transitions are developed and implemented into a Material Point Method computational code. This groundbreaking modelling paradigm provides a powerful tool that can precisely analyze the complex response of geostructures against climatic fluctuations, especially flow-type failures. The research program will significantly advance the knowledge in unsaturated soil mechanics by elucidating the dynamics of wetting and drying under climatic fluctuations. The developed computational tool will offer quantitative analysis of existing geostructures combined with field monitoring and machine learning for mitigating geohazards. Building on existing successes, this goal will be achieved through the training of HQP in interlinked tasks, and in collaboration with industry and an international research network. The highly impactful nature of the research in the light of growing uncertainly around climate change today, such as the recent flooding and landslide events in Europe, makes this proposal an extremely valuable endeavour.