Investigation of the Grain-scale Mechanical Behavior of Granular Soils under Shear Using X-ray Micro-tomography

基於X射線顯微斷層掃描的顆粒土在剪切作用下的顆粒尺度力學行為研究

Student thesis: Doctoral Thesis

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Award date11 Jul 2018

Abstract

Grain-scale mechanical behavior (e.g., particle kinematics, inter-particle contact interaction and particle crushing) plays a very important role in the macro-scale mechanical response of granular soils. Over the past few decades, most of our understanding of it and its relation to the macro-scale mechanical response of granular soils has come from discrete element method (DEM) modeling, and relatively few attempts have been made to quantitatively study the grain-scale mechanical behavior using non-destructive experimental methods. The micromechanics that govern the grain-scale mechanical behavior have not been well understood. The aim of this research is to experimentally examine the grain-scale mechanical behavior of granular soils and quantitatively investigate its evolution during shear using X-ray micro-tomography.

The test materials are Leighton Buzzard sand (LBS) and glass beads (GB), which are compressed triaxially within a synchrotron radiation scanner. A miniature triaxial apparatus is developed for the in-situ testing in combination with the X-ray micro-tomography facility. Both materials are made into dry cylindrical samples with a size of 8×16 mm (diameter×height) and scanned with a spatial resolution of 6.5 μm. A series of CT images are acquired, and image processing and analysis skills are utilized to extract individual grains from the CT images and acquire their grain quantities (e.g., particle volume, particle surface area and inter-particle contact). To obtain the particle kinematics (i.e., particle translation and particle rotation) evolution of the samples throughout the shear, a particle-tracking approach is developed to track the individual particles of samples between any two consecutive scans. A framework is also presented to investigate the inter-particle contact evolution (i.e., the contact loss, contact gain and contact movement) and its effects on the fabric evolution of the samples. In addition, a meshfree method is presented to quantify the strain evolution of the samples, based on the particle translations and particle rotations obtained from the X-ray imaging and particle-tracking techniques.

It is found that all tested samples fail through well-defined single shear bands with localized particle kinematics and strains. The samples gradually exhibit a directional preference of branch vectors towards the loading direction as the shear progresses. The contact gain and loss, which contribute to the directional preference, and the contact movement, which leads to attenuation of this preference, are shown to be the two competing factors determining the evolution of fabric anisotropy within the samples. The higher degree of fabric anisotropy within the shear bands is to be attributed to the higher percentages of contact gain and contact loss when compared to entire samples.

In parallel with the above experimental work, a theoretical model for the quantification of particle crushing and grading evolution of granular soils subject to static biaxial shearing is presented, based on the statistical analysis of inter- and intra-particle stresses derived from DEM modeling. The model-calculated results show good agreement with those from published literature.

    Research areas

  • In-situ triaxial test, Granular materials, particle translation and particle rotation, contact gain and contact loss, strain localization, theoretical model, particle crushing and grading evolution