Multiscale Thermo-Hydro-Mechanical Analysis of Thawing/Freezing Cycles in Partially Saturated Soils: Application to Stability of Permafrost Layers and Climate Change

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The permafrost layers in the Northern Canada regions are most sensitive to climate change effects since the thawing of frozen soil leads to degradation of strength and substantial instabilities. The geo-environmental impacts of permafrost landslide can be catastrophic for infrastructure and the environment since the mechanical instabilities induce significant deflections to, for instance, major pipelines residing in these zones, with ensuing potential of hazardous materials being released into the environment. More direct environmental effects such as loss of vegetation coverage, and the release of the methane trapped in permafrost layers are also among consequences of landslides in cold regions. Notwithstanding the prevalence of landslide instabilities in the permafrost regions, the current state of research in this field relies strongly on empirical observations and phenomenological approaches. The lack of coherent analytical studies is primarily due to the challenges associated with incorporating phase change phenomenon (thawing/freezing) into the multiscale calculation of stress and strain, as well as the modification thereof strength parameters. The objective of the current research is to develop a physically sound theoretical framework of multiphasic soil-water-ice-air systems where the overall deformational and strength behavior of permafrost soil is explained within a Thermo-Hydro-Mechanical (THM) framework, in terms of simpler interactions at the particle scale. The research is built upon my previous analysis of multiphasic soils where the crucial, but often overlooked, effect of soil/water /air interfaces are coherently incorporated into the formulation of stress and strain. While inheriting the subtleties of my previous analysis of unsaturated soils, the current research will further develop the mechanics of frozen soils by incorporating the thermal effects and phase change of interpore water, transport aspects and the ensuing deformational characteristics. The overall degradation of strength parameters of thawing permafrost soil is naturally accounted for at microscopic scale by disappearance of the cohesive ice bonds, as well as by the irreversible deformations upon continual thawing/freezing cycles caused by fluctuating temperatures in the active layer. The research will, in short term, contribute towards better understanding of the physics underlying the geomechanical instabilities in the permafrost layers, enhancing the accuracy of our real-scale modelling and predictions, as well as increasing the safety and reliability of current and future designs of infrastructure in the North. Furthermore, in the long term, such fundamental insights in the basic components contributing to the soil's behavior will eventually be used to devise effective strategies to contain and prevent numerous landslides in the cold regions.