Date of Award

Spring 1-1-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Second Advisor

Richard Regueiro

Third Advisor

Ronald Pak

Fourth Advisor

Jeong-Hoon Song

Fifth Advisor

Franck Vernerey

Abstract

The discrete properties and different constituents, i.e. sand grains, clay matrix, pore-air, and pore-water in natural soil make it very complicated to investigate its mechanical behavior. As granular material, the macro-behavior of natural soil depends very much on the particle-level interactions. Thus it is necessary to develop a computational modeling for natural soils at grain-scale considering sand grains, clay matrix, pore-air, and pore-water.

The Discrete Element Method (DEM) is used very often to study the behaviors of sand grains. A three-dimensional DEM code ellip3d is extended to polyEllip3d to simulate sand grains using poly-ellipsoids, which are non-symmetric and have more resistance to rolling than ellipsoids. In order to bridge DEM simulation to experiments, Colorado Mason sand grains in SMT images are approximated by poly-ellipsoids and these equivalent polyellipsoids can be used directly to conduct DEM simulations. In addition, a simple particle fracture model is proposed to study the fracture behavior of sand grains. This particle fracture model will be calibrated and compared with the experimental results.

A coupling model for DEM and Peri-Dynamics (PD) is established to study the sand-clay matrix interaction, in which cohesive soil clay matrix is modeled using PD with ability to fracture and fragment clay constituent. Both DEM-PD coupled model and Finite Element Analysis (FEA) are used to solve a rigid inclusion problem and their results are compared to semi-verify our DEM-PD model. In the case that the soil is purely cohesionless (e.g.,only sand grains) and modeled using DEM, then the pore-water is modeled using another particle method, Smoothed Particle Hydrodynamics (SPH). DEM-SPH coupled model is also developed to study the interaction between fluid and particles, which can be used in the future to conduct more simulations in sand-pore-water-system, such as drainage problem. Several test examples are conducted to verify the DEM-SPH coupling model in this research. The various C++ codes developed, such as polyEllip3d, DEM-SPH, and DEM-PD are parallelized using hybrid MPI/OpenMP.

Finally, stress and strain measures for granular material at large deformation are developed and implemented to study the macroscopic mechanical behavior of natural soils. Thus these stress and strain measures can be used in upscaling to bridge microscale (e.g., grain-scale in this research) to the macroscale (e.g., continuum scale of natural soil).

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