Mechanisms Controlling Swelling of Clays

虽然粘土的膨胀(以及相应的收缩)在美国的许多地区(中西部)和世界上许多其他国家(如斯堪的纳维亚国家、加拿大和南非)都是一个问题，但目前还没有可靠的方法来对岩土设计所需的定量评估。基于塑性极限和水分含量的经验方法在许多情况下都失败了。膨胀土通常含有丰富的粘土矿物，如蒙脱石。当足够多的水填满颗粒间和颗粒内空间时，就会发生膨胀。虽然对膨胀科学的探索和现代土力学一样古老，但过去的研究导致了更多的问题而不是答案。例如，在最近的一些研究之后，双层斥力和粘土矿物表面水化所产生的力(结构力)对整体膨胀的相对贡献已经变得有争议。如果不解决这一根本问题，就无法建立在实践中使用的合理关联。随着计算机技术、分子动力学(MD)和离散元分析方法(DEM)的发展，可以明确地解决这一问题，并建立基于实际物理的粘土膨胀预测模型。由Alder和Wainwright在20世纪50年代提出的MD背后的原理几乎保持不变，但多年来解释原子之间相互作用的方法已经变得非常复杂。MD已被用于研究液体、气体、电解质、胶束、胶体、晶体结构、在多孔介质中的吸附、金属和蛋白质的行为，仅举几例。在这个项目中，MD将被用来定量计算双层斥力和结构力对整体膨胀力的贡献，并建立作为系统变量的函数来量化它们的方程。然后，将使用DEM在微观和宏观尺度之间架起桥梁。在DEM中，包括一组粘土颗粒的数值样品被组装和加载，就像在实验室测试装置中加载真实样品一样。输出结果的准确性取决于颗粒间作用力的量化精度。在过去的二十年里，PI和他的学生们一直在追求这一技术，并开发了量化机械力、双层排斥力和范德华力等力的方法。根据拟议的MD研究，将建立一种量化结构力的方法，并用于DEM研究中的结构力计算。为了验证目的，计划进行一系列的实验室膨胀实验。在这项研究的基础上，将根据土壤的组成、阳离子交换量和比表面积等可测量的基本特性，开发出简单、科学的方法，用于岩土工程实践。为了更广泛的影响，计划了几个项目。其中最重要的是计划开发一个基于蒙脱石膨胀和计算机模拟的演示模块，并将该模块用于约翰·霍普金斯大学、巴尔的摩选定的高中和马里兰科学中心的课程，以教育膨胀粘土的不利影响以及如何利用高级研究结果来缓解这些不利影响。研究团队将由一名新近毕业的博士、一名研究生、几名本科生、几名高中生和PI组成。

While swelling of clays (as well as its counterpart, shrinkage) is a problem in many parts of the U.S. (middle and western parts) and in many other countries around the world (e.g., Scandinavian countries, Canada and South Africa), there is currently no reliable method for the quantitative evaluation needed in geotechnical design. The empirical methods based on plastic limits and water contents have failed in a large number of cases. Expansive soils are normally rich in clay minerals such as montmorillonite. Swelling occurs when sufficient amount of water fills the inter-particle and intra-particle spaces. While the quest for the science of swelling is as old as modern soil mechanics, the past research has led to more questions than answers. For example, the relative contributions of double-layer repulsion and forces due to hydration of clay mineral surface (the structural forces) to the overall swelling have become controversial after some of the recent studies. Without resolving this fundamental issue, a rational correlation for use in practice cannot be established. With the recent advances in computer technology, the molecular dynamics (MD) and the discrete element analysis methods (DEM), it is now possible to unequivocally resolve this issue, and to develop a predictive model for clay swelling based on the actual physics. Introduced by Alder and Wainwright in the 1950s, the principle behind MD remains almost the same, but the methods of accounting for the interactions between atoms have become very sophisticated over the years. MD has been used to study the behavior of liquids, gases, electrolytes, micelles, colloids, crystal structures, sorption in porous media, metals and proteins, just to name a few. In this project, MD will be used to quantitatively calculate the contributions of double-layer repulsion and structural forces to the overall swelling force, and develop equations for quantifying them as a function of system variables. DEM will then be used to bridge the micro and macro scales. In DEM, a numerical specimen comprising a collection of clay particles is assembled and loaded just as a real specimen is loaded in a laboratory testing setup. The accuracy of the output results depends on the accuracy with which the interparticle forces are quantified. The PI and his students have pursued this technique for the past two decades and developed methodologies for quantifying such forces as the mechanical forces, double-layer repulsive forces and van der Waals attractive forces. Based on the proposed MD study, a methodology will be established for quantifying the structural force and used to calculate the structural force in the DEM study. A series of laboratory swelling experiments are planned for verification purposes. Based on the proposed study, simple, scientific methods based on measurable basic characteristics of soil such as the composition, cation exchange capacity and specific surface will be developed for use in geotechnical engineering practice. Several items are planned for the broader impact purposes. The most important among these is the plan to develop a demonstration module based on expansion of montmorillonite and computer simulations, and to use this module in courses at Johns Hopkins University, selected high schools in Baltimore, and at the Maryland Science Center to educate on the adverse effects of expansive clays and how the results from advanced research are used to mitigate them. The research team will consist of a recent doctoral degree graduate, one graduate student, several undergraduate students, a few high school students and the PI.