EAGER: Analysis of the Depositional Fabric and Deformation Characteristics of Sands Using 3-D Computed X-Ray Tomography and DEM Simulation

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Understanding the behavior of sands under different loading conditions is an important aspect of being able to design foundations for all types of structures. One of the great challenges is obtaining an accurate understanding of how the way the sands were deposited affects their properties. Thus, the objective of this EArly-concept Grant for Exploratory Research (EAGER) project is an international cooperative effort to evaluate the use of computed X-ray tomography to produce 3-D images of sands and silts deposited in different environments. Computed X-ray tomography involves the use of high energy beam to obtain high resolution 3-D images of microscopic objects. The technique has been demonstrated using samples of granular materials and uniform sands prepared in the laboratory. However, to-date the same has not been attempted with undisturbed samples collected from natural deposits. Thus, the practical aspect of the project is the development of the necessary laboratory techniques for imaging undisturbed samples using computed X-ray tomography in order to determine the actual depositional geometry of the grain assemblage. These data will then allow analysis of the influence of the mode of deposition on the mechanical properties of these materials using advanced particle tracking numerical models. The ultimate goal is the development of more nuanced approach to geotechnical analysis and design tailored to a specific material and its mode of deposition. Overall, this effort represents an early exploration of advanced techniques, taking advantage of the latest developments in visualization and simulation of granular media. This award addresses the NSF mission "to promote the progress of science." The principal objective of this research is to explore the use of computed X-Ray tomography to demonstrate that different depositional environments and sources of sediments produce different types of fabric that are characteristic of that environment. This will allow the development of a data base of types of fabric characteristic of different source materials and depositional environments. This data base will then provide baseline attributes of expected material behavior that are specific to the local and regional geologic setting. Assuming that the scanning methodology can be sufficiently standardized, this goal should become quite achievable with the extensive data bases of the in-situ properties of recent sediments being developed in New Zealand, Japan, Chile, and California, in particular. Advanced visualization and DEM simulation tools will then be explored to develop a realistic mathematical description of the micro-mechanical behavior of naturally deposited sands and silts. The challenge is in being able to accurately represent the grain geometry and then numerically track the movement of grains and their interaction at points of contact. At present this type of detailed numerical simulation has been demonstrated only on an assemblage of sand grains with relatively open fabric. Thus, the objective is to extend the state of the art to a more compact grain packing and more complex geometry in order to explore the influence of fabric on the deformation characteristics of natural deposits. This should allow for future advances in the state of the art of modeling the behavior of granular sediment using multi-scale constitutive models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.