Mechanisms Controlling Swelling of Clays

虽然粘土的膨胀（以及其对应物，收缩）在美国的许多地方（中部和西部）和世界上许多其他国家（例如，斯堪的纳维亚国家、加拿大和南非），目前还没有可靠的方法来进行土工设计所需的定量评估。基于塑性极限和含水量的经验方法在许多情况下都失败了。膨胀土通常富含粘土矿物，如蒙脱石。当足够量的水填充颗粒间和颗粒内空间时发生溶胀。虽然对膨胀科学的探索与现代土壤力学一样古老，但过去的研究带来的问题多于答案。例如，在最近的一些研究之后，由于粘土矿物表面的水化（结构力）而引起的双层排斥力和力对整体膨胀的相对贡献已经变得有争议。如果不解决这个根本问题，就无法建立一个合理的相互关系，供实际使用。随着计算机技术、分子动力学（MD）和离散元分析方法（DEM）的最新进展，现在可以明确地解决这个问题，并开发基于实际物理的粘土膨胀预测模型。由桤木和温赖特在20世纪50年代提出的分子动力学，其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。分子动力学已被用于研究液体、气体、电解质、胶束、胶体、晶体结构、多孔介质中的吸附、金属和蛋白质等的行为。在这个项目中，MD将用于定量计算双层排斥力和结构力对整体膨胀力的贡献，并开发用于量化它们作为系统变量的函数的方程。然后，数字高程模型将被用来连接微观和宏观尺度。在DEM中，包括粘土颗粒集合的数值试样被组装和加载，就像在实验室测试设置中加载真实的试样一样。输出结果的准确性取决于颗粒间力量化的准确性。PI和他的学生们在过去的二十年里一直在追求这种技术，并开发了量化机械力、双层排斥力和货车范德华吸引力等力的方法。根据拟议的MD研究，将建立一种方法来量化结构力，并用于计算DEM研究中的结构力。计划进行一系列实验室膨胀实验以进行验证。在此基础上，提出了基于土的组成、阳离子交换量和比表面等可测基本特性的简单、科学的岩土工程方法。为产生更广泛的影响，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，并在约翰霍普金斯大学、巴尔的摩选定的高中和马里兰州科学中心的课程中使用该模块，以教育膨胀性粘土的不利影响以及如何使用先进研究的结果来减轻这些影响。研究小组将由一名博士毕业生、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.