Date of Graduation

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Engineering (PhD)

Degree Level

Graduate

Department

Civil Engineering

Advisor/Mentor

Michelle L. Bernhardt-Barry

Committee Member

Richard A. Coffman

Second Committee Member

Clinton M. Wood

Third Committee Member

Paul Millett

Keywords

Analogue Soil, Direct Shear Testing, Discrete Element Method, Geotechnical, Idealized Granular Material, Sand

Abstract

Discrete element method (DEM) modeling, is becoming widely used for understanding the micro-mechanical behavior of granular materials at the particle level. This dissertation presents important considerations regarding the experimental boundary conditions and idealized granular materials used in DEM validations.

A common issue in DEM validation studies for direct shear and direct simple shear is selecting top and bottom cap boundary conditions that allow for shear transmission while also being efficiently modeled in the DEM simulations. While the traditional grid boundary types used in the laboratory can be replicated in the numerical models, researchers have also used fixed-particle boundaries, sawtooth boundaries, and high friction boundaries with no projections.

To examine any effects from these boundaries at the macro-scale, direct shear tests on physical specimens of dense and loose sand, and dense and loose steel ball bearings were conducted. The influence of friction on the boundary was further examined at the particle scale using two simple DEM simulations replicating the physical steel ball bearing specimens with a high and a low friction coefficient on the top and bottom boundaries. The DEM stress-displacement responses are compared to the laboratory results for tests using the boundary plates with no projections and then the particle-scale results of the validated simulations are analyzed.

In addition to boundary effects, the idealized materials used in validation studies were also examined. Commonly used steel ball bearings or glass ballotini restrict the direct comparison of DEM simulations to spherical particles, thus limiting understanding of real materials with more complex shapes. This study shows that additive manufacturing (AM) can be used to create analogue soils with a variety of shapes which can be used for DEM validations. The AM particles were characterized to ensure their suitability for laboratory testing and to determine the material properties to input into future DEM models. The second part of the dissertation describes the determination of material properties and surface characteristics of two AM materials. Because a Hertzian contact law is typically used in DEM models, a comparison of the materials’ response under uniaxial compression and the theoretical response for Hertzian behavior was carried out.

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