Collaborative Research: Multiscale Investigation of Thixotropy in Soft Clays

触变性是一种描述物质如何随时间“变厚”的材料特性。家里可用的触变材料的简单例子是厨房（番茄酱），浴室（牙膏），艺术工作室（油漆）和玩具盒（橡皮泥）。这些材料如果不动会变得更硬，但当材料被加工（或使用）时，它会更容易移动。这个概念也被用来描述一些自然形成的粘土的行为。科学家和工程师长期以来一直观察到粘土中的这种现象，但迄今为止，只有一种理论或假设的解释被提出来解释潜在的机制。本研究项目旨在考察和模拟不同环境条件（时间、水化学和温度）和不同粒径下粘土触变性的颗粒尺度发展。这种理解触变性机制的创新多尺度方法有助于推进NSF的使命，即通过利用理论、实验和计算方面的最新进展填补信息空白，促进科学进步。在这项工作中获得的新知识将有助于设计和建造涉及软粘土的工程系统，如深桩基础、海上管道、风电场基础、疏浚材料的处理、钻井泥浆的稳定性和海底粘土的声学特性等。该项目还包括一个重要的外展计划，通过出版物和K-12学校活动和示范，帮助吸引代表性不足的少数民族学生学习STEM学科。触变性是一种基本的土壤行为机制，它控制着软粘土的多种随时间变化的工程特性（例如，刚度、强度和敏感性随时间的演变）。虽然自20世纪20年代初触变性领域开始以来，对胶体体系的触变性已经取得了重要的认识，但目前关于土壤触变性的知识仍然主要基于20世纪60年代及其之前进行的一些开创性工作，从那时起，新的发展就很少，而且支离破碎。这种新发现的缺乏以及胶体科学与土力学在触变性研究和进展方面的差距为这一研究提供了动力。因此，这个结合多尺度实验和计算的合作项目是研究软粘土的触变性。该项目的总体目标是建立软粘土触变的宏观力学和微观结构机制方面的知识，从而在岩土工程知识库中增加一些新的随时间变化的土壤行为。为了实现这一目标，麻省大学阿默斯特分校和德雷塞尔大学的三位研究人员在宏观机械测试、微观织物成像、颗粒取向的定量表征和粗粒度分子动力学模拟方面进行了协同合作，设计了一个由三个主要方向组成的一致而全面的研究项目。项目的智力价值体现在三个方面：(1)拓宽了岩土工程对土壤触变性的知识基础，特别是对温度、孔隙水化学等理化因素的影响有了新的认识；(2)通过多尺度试验和计算研究，揭示软黏土触变硬化的宏观力学和微观结构机制；(3)建立黏土织物定量时变演化与宏观触变过程之间的联系。由于土壤触变性在许多工程问题中起着重要的作用，因此该项目也会对岩土工程，特别是涉及软粘土的工程系统的设计和施工产生重大的实际影响。例子包括桩和吸力沉箱设置的评估、风电场基础的设计、疏浚材料的处理等。此外，通过本项目开发的多尺度调查方法可以推广到其他更复杂的土壤研究课题，也可以作为其他基础研究问题的通用方法。

Thixotropy is a material property that describes how a substance can "thicken" over time. Simple examples of thixotropic materials available in the house are in the kitchen (ketchup), the bathroom (toothpaste), the art studio (paints), and the toy box (silly putty). Each of these materials if left untouched will become stiffer, but when the material is worked (or used) it will move more easily. This concept also is used to describe the behavior of some naturally occurring clay soils. Scientists and engineers have long observed this phenomenon in clays, but only a theorized or hypothesized explanation has been presented to date to explain the underlying mechanisms. The work in this research project aims to examine and simulate the clay particle-scale development of thixotropy under various environmental conditions (time, water chemistry, and temperature) and at different size scales. This innovative multiscale approach to understanding the mechanisms of thixotropy serves to advance the NSF mission of promoting the progress of science by filling a void of information utilizing recent advances in theory, experimentation, and computing. The new knowledge gained in this work will aid in the design and construction of engineering systems involving soft clays, such as deep pile foundations, offshore pipelines, wind farm foundations, disposal of dredged materials, drilling mud stability, and seabed clay acoustic properties, among others. The project also includes a significant outreach program to help attract under-represented minority students to STEM disciplines through publications and K-12 school activities and demonstrations.Thixotropy is a fundamental soil behavior mechanism that governs multiple time-dependent engineering properties of soft clays (e.g., the evolution of stiffness, strength, and sensitivity over time). While significant understanding of thixotropy of colloid systems has been achieved since the initiation of the field of thixotropy in the early 1920s, current knowledge on soil thixotropy is still based primarily on some pioneering work performed in and prior to the 1960s and, since then, new developments have been scarce and fragmental. Such a paucity of new findings and the disparity in thixotropy research and advancement between colloid science and soil mechanics provide an impetus to this research. Therefore, this collaborative project that integrates multiscale experimental and computational efforts is to study soft clay thixotropy. The overall goal of the project is to create the enabling knowledge on the macroscale mechanical and microscale structural mechanisms of soft clay thixotropy and hence to append some new time-dependent soil behavior to the geotechnical knowledge base. To achieve this goal, a congruent and comprehensive research program consisting of three primary thrusts is designed with synergistic collaboration among the three investigators from UMass Amherst and Drexel University with complementary expertise in macroscale mechanical testing, microscale fabric imaging, quantitative characterization of particle orientations, and coarse-grained molecular dynamics simulations. The intellectual merit of the project stems from three aspects: (1) the geotechnical knowledge base on soil thixotropy will be expanded with new understanding, particularly the effects of physico-chemical factors such as temperature and porewater chemistry; (2) both the macroscale mechanical and microscale structural mechanisms of thixotropic hardening of soft clays will be uncovered via multiscale experimental and computational research; and (3) the linkage between quantitative time-dependent clay fabric evolution and macroscale thixotropic processes will be developed. Because soil thixotropy plays an important role in many engineering problems, the project also can generate significant practical impacts to geotechnical engineering, particularly the design and construction of engineering systems involving soft clays. Examples include evaluation of pile and suction caisson setup, design of wind farm foundations, and disposal of dredged materials, among others. Moreover, the multiscale investigation methodology developed through this project can be generalized to other more complex soil research topics and can also serve as a generic approach for other basic research queries.