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.