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

加拿大北部地区的永久冻土层对气候变化的影响最为敏感，因为冻土的融化会导致强度的退化和相当大的不稳定性。永久冻土滑坡对基础设施和环境的地质环境影响可能是灾难性的，因为机械不稳定导致这些区域内的主要管道发生重大偏转，从而有可能向环境中释放有害物质。更直接的环境影响，如植被覆盖率的丧失，以及永久冻土层中甲烷的释放，也是寒冷地区山体滑坡的后果之一。尽管多年冻土区普遍存在滑坡不稳定性，但这一领域的研究现状严重依赖于经验观察和现象学方法。缺乏连贯的分析研究主要是因为将相变现象(解冻/冻结)纳入应力和应变的多尺度计算以及修改强度参数所带来的挑战。本研究的目的是建立一个物理上合理的土壤-水-冰-空气多相系统的理论框架，在热-水-力学(THM)框架内，根据颗粒尺度上的更简单的相互作用来解释冻土的整体变形和强度行为。这项研究建立在我之前对多相土壤的分析基础上，在多相土壤中，土/水/空气界面的关键但经常被忽视的影响被一致地纳入应力和应变的公式中。在继承了我以前对非饱和土力学分析的精妙之处的同时，本研究将通过结合孔间水的热效应和相变、运移方面以及随后的变形特征来进一步发展冻土的力学。融化冻土强度参数的整体退化在微观上自然是由于粘性冰键的消失，以及活跃层温度波动引起的连续融化/冻结循环时的不可逆变形。短期内，这项研究将有助于更好地了解永久冻土层地质力学不稳定性的物理基础，提高我们实际规模建模和预测的准确性，并提高北方当前和未来基础设施设计的安全性和可靠性。此外，从长远来看，这种对导致土壤行为的基本成分的基本见解最终将被用于制定有效的战略，以遏制和防止寒冷地区的大量山体滑坡。

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.