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

在自然界中，非饱和多孔介质是无处不在的，例如沙子和雪球中的足迹。水如何占据土壤的孔隙空间(例如在斜坡中)是至关重要的--气候波动导致土壤周期性的湿润和干燥可能是灾难性的，通过液化引发山体滑坡。全球变暖还导致永久冻土融化，造成土壤水分变化，进而导致加拿大北部的二氧化碳释放和山体滑坡。核心问题是：土壤中的水分饱和度如何导致不同的毛细制度，从而自发地将它们的行为从固体状态改变为液体状态？我们如何在数学上描述这样一种残酷的转变，转化为地质结构的湿润坍塌？事实证明，这个长期存在的问题源于微观结构，与颗粒之间吸力的丧失密切相关。虽然大多数研究采取的是现象学的观点，不深入微观结构的物理学，从而阻碍了进展，但拟议的研究调查了一些令人敬畏的问题，如非饱和土壤中的颗粒接触、水弯月面和相间界面如何随着水分饱和度的增加(减少)而在湿润(排水)过程中演变。因此，要求采用多尺度方法，其中研究了捕捉流体和固体相互作用的所有局部微观结构复杂性的单元体积，并在此后进行统计放大，以建立连续介质水平上的应力、应变和毛细管应力之间的关系，用于分析地质结构。通过对固体的离散元模拟和对水和空气的格子Boltzmann方法，建立了一个新的框架，强调了在与固体颗粒耦合的孔隙空间中水和空气的集体相互作用。这有助于模拟孔隙空间中水半月板的复杂形成、合并和破裂。建立了考虑这些转变的非饱和本构关系，并将其应用到物质点方法计算程序中。这一开创性的建模范例提供了一个强大的工具，可以精确地分析地质结构对气候波动的复杂响应，特别是流型故障。该研究计划将通过阐明气候波动下的干湿动力学，极大地促进非饱和土力学知识的发展。开发的计算工具将提供对现有地质结构的量化分析，并结合现场监测和机器学习来减轻地质灾害。在现有成功的基础上，将通过在相互关联的任务中培训总部总部，并与工业界和一个国际研究网络合作，实现这一目标。鉴于当今围绕气候变化的不确定性越来越大，例如最近欧洲发生的洪水和山体滑坡事件，这项研究的高度影响力使这一提议成为一项极其有价值的努力。

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.