Collaborative Research: Imaging and Modeling the Microstructure of Unsaturated Soils for Improved Prediction of Macroscale Response

非饱和土在各种自然土过程和工程土系统中起着重要作用。非饱和土壤系统中的孔隙水形成了一个复杂的结构，由负压下的饱和水袋和颗粒接触点附近形成的液桥网络组成。水通过饱和孔隙中的负压改变颗粒间应力，并通过液体桥提供颗粒间结合力，从而影响土体的行为。然而，每种机制的大小和相关性都高度依赖于孔隙水结构，孔隙水结构很容易随着吸力、饱和度、润湿方向、外部应力以及整体或局部变形的变化而改变。很明显，在机械或水力荷载作用下非饱和土壤微观结构的变化会影响土壤的宏观行为，但其表征的困难限制了基于微观结构的预测宏观土壤反应框架的发展。该合作项目旨在通过利用无损成像技术的最新进展来观察和量化非饱和土壤的多相结构。微聚焦x射线计算机断层扫描将与一系列特殊加载阶段相结合，这些加载阶段设计用于在受控的吸力和应力条件下以及在大范围的饱和和应变下对非饱和砂试件的微观结构进行成像。将对图像进行分析，以表征多相织物的显著特征，包括3D晶粒取向、颗粒接触法线、液体桥构型以及液体和气体饱和空隙的分布。描述这些特征的张量将被量化，并随着试样在吸力、润湿方向、压缩和剪切方面的受控变化而跟踪它们的演变。晶粒尺寸、密度、各向异性、吸力、围应力和应变率将被视为实验变量。微观结构观察将被整合到非饱和土壤行为的一个新的本构框架中，该框架明确地说明了固体、液体和气体结构的元素。该研究将致力于解决非饱和土微观结构与宏观尺度响应之间的联系，并将通过一个新的本构平台来预测工程行为。观察织物在水力和机械载荷下的演变将为解决瓶颈问题提供直接证据，这些瓶颈问题目前限制了我们对非饱和土行为的预测性理解，包括干湿滞后、吸力、饱和和变形之间的耦合、液桥破裂、膨胀和速率效应。了解填充颗粒与润湿流体的多相相互作用对于处理过滤、干燥、制药和陶瓷团聚以及采油等物理现象的其他科学领域也至关重要。通过研究生和本科生的参与和教育模块的开发，包括密苏里大学和华盛顿州立大学专门针对妇女和少数民族的活动，教学和多样性将得到加强。

Unsaturated soils play an essential role in a variety of natural earth processes and engineered earthen systems. The pore water in an unsaturated soil system forms a complex fabric consisting of saturated pockets of water under negative pressure and a network of liquid bridges formed near the particle contact points. Water influences bulk soil behavior by modifying intergranular stress through negative pressure in the saturated pores, and by providing an intergranular bonding force through the liquid bridges. The magnitude and relevance of each mechanism, however, is highly dependent on the pore water fabric, which is readily altered with changes in suction, saturation, wetting direction, external stress, and global or localized deformation. It is evident that changes to unsaturated soil microstructure under mechanical or hydraulic loading will influence macroscopic soil behavior, but the difficulties associated with its characterization have limited development of microstructure-based frameworks for predicting macro soil response.This collaborative project seeks to observe and quantify the multiphase fabric of unsaturated soils by making use of recent advances in non-destructive imaging techniques. Microfocus X-ray computed tomography will be integrated with a series of special loading stages designed to image the microstructure of unsaturated sand specimens under controlled suction and stress conditions and over a wide range of saturation and strain. Images will be analyzed to characterize salient features of the multiphase fabric, including 3D grain orientation, particle contact normals, liquid bridge configurations, and the distribution of liquid- and gas-saturated voids. Tensors describing these features will be quantified and their evolution tracked as specimens are subject to controlled changes in suction, wetting direction, compression, and shear. Grain size, density, anisotropy, suction, confining stress, and strain rate will be treated as experimental variables. Microstructural observations will be integrated into a new constitutive framework for unsaturated soil behavior that explicitly accounts for elements of the solid, liquid, and gas fabric.The research will work to resolve the links between unsaturated soil microstructure and macroscale response, and will implement them through a new constitutive platform for predicting engineering behavior. Observations of fabric evolution with hydraulic and mechanical loading will provide direct evidence to address the bottleneck issues that currently limit our predictive understanding of unsaturated soil behavior, including wetting-drying hysteresis, coupling between suction, saturation and deformation, liquid bridge rupture, dilation, and rate effects. Understanding multiphase interactions in packed particles with wetting fluids is also critical to other scientific fields that deal with physical phenomena such as filtration, drying, pharmaceutical and ceramic agglomeration, and oil recovery. Teaching and diversity will be enhanced through graduate and undergraduate student involvement and educational module development, including activities targeted specifically for women and minorities at the University of Missouri and Washington State University.