Doctoral Dissertations

Date of Award

5-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Civil Engineering

Major Professor

Khalid Alshibli

Committee Members

Dayakar Penumadu, Eric Drumm, Angel Palomino

Abstract

The macro-scale behavior of uncemented granular materials is governed by the particle-to-particle interactions. Therefore, accurate assessment of the micro-scale mechanics is essential for better understanding of the fundamental behavior of granular materials. Particle fracture phenomenon and force transmission mechanisms in natural granular assemblies such as sands have not been fully understood due to the lack of micro-scale experimental measurements. The objective of this dissertation is to provide key quantitative measurements about these issues using powerful non-destructive experimental 3D x-ray diffraction (3DXRD) and synchrotron micro-tomography (SMT) techniques, as well as distinct element method (DEM). 3DXRD was employed to measure the volume-averaged lattice strain of individual silica sand particles within a sand assembly under 1D compression loading condition. The evolution and distribution of particle fracture, particle fracture mechanism and deformation characteristics of sand particles subjected to 1D compression across the scales were also investigated and quantified using SMT and DEM methods. Sand particles were modeled in DEM as crushable agglomerates composed of many spherical sub-particles that were linked by parallel bonds. DEM simulations were first calibrated and validated using laboratory experiments, and then were used to quantify micro-scale processes including the contact force network, and the fracture mechanics of crushable agglomerates. In addition, DEM was adopted to examine particle kinematic behavior and the influence of boundary conditions in triaxial testing.

In first set of lattice strain measurements performed on a column composed of three sand particles, the normal strain along the loading direction increased in a linear fashion as the compression proceeded until one of the sand particle fractured. However, significant variation and fluctuations were observed in the measured lattice strain tensor components of sand particles for relatively larger specimen due to complex deformation behavior and a non-homogenous contact force network. The SMT images and DEM model revealed that particle fracture concentrates at certain locations close to the loading platen, and the onset of particle fracture and specimen yielding occur at the same strain level in a 1D compression test. Finally, triaxial test DEM simulations showed that a flexible membrane better replicates the uniformly applied confining stress compared to a rigid boundary.

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