The evolution of morphology and fabric of a sand during shearing
File(s)
Author(s)
Fonseca, J
Type
Thesis
Abstract
Over the past 50 years, experimental studies have repeatedly demonstrated that the
mechanical behaviour of sand is sensitive to the material fabric, i.e., the arrangement
of the grains. Up until now there have been relatively few attempts to describe
quantitatively the fabric of sands. In fact, most of our understanding of the link
between the particle movements and interactions and the macro-scale response of
granular materials, including sand, comes from discrete element modelling (DEM)
and experiments on "analogue" sands with simple, idealized shapes. The aim of this
study had been to describe quantitatively the particle morphology and fabric of real
sand and their evolution under loading.
The material investigated was Reigate sand (from Southeast England), a geologically
old sand which, in its intact state, exhibits signi cant grain interlocking and no
bonding. To explore the effects of fabric on the mechanical response of the soil, intact
and reconstituted specimens both having similar densities were tested under triaxial
compression. The specimens were impregnated with an epoxy resin at three different
stages of shear deformation and small cores from each specimen were scanned using
X-ray micro-tomography. Different systems and scanning parameters were explored
in order to obtain three-dimensional high-resolution images with a voxel size of 5μm
(0:018d50) and a quality level required for the identi cation of the individual particles
and the surface defining each particle-particle contact.
The quantification of particle size and shape has shown that breakage of fractured
grains, along existing fissures, occurs both during reconstitution and shearing of
the intact soil, a phenomenon that cannot be observed using invasive techniques
such as sieve analysis. Statistical analyses of the distribution of fabric directional
data in terms of particle orientations, contact normals, branch vectors and void
orientations were carried out at each loading stage. It has been shown that the initial
particle orientation fabric that develops during the deposition of the material tends
to persist during shearing, while the contact normals seem to be reorientated along
the direction of the major principal stress in the post-peak regime. Different patterns
were observed within the shear band as both the particles and the contact normals
appeared to rotate towards the direction of the shear plane. The measurements
from the tomographic data were complemented with a qualitative description of the
morphology and fabric using SEM and optical microscope images of thin sections.
mechanical behaviour of sand is sensitive to the material fabric, i.e., the arrangement
of the grains. Up until now there have been relatively few attempts to describe
quantitatively the fabric of sands. In fact, most of our understanding of the link
between the particle movements and interactions and the macro-scale response of
granular materials, including sand, comes from discrete element modelling (DEM)
and experiments on "analogue" sands with simple, idealized shapes. The aim of this
study had been to describe quantitatively the particle morphology and fabric of real
sand and their evolution under loading.
The material investigated was Reigate sand (from Southeast England), a geologically
old sand which, in its intact state, exhibits signi cant grain interlocking and no
bonding. To explore the effects of fabric on the mechanical response of the soil, intact
and reconstituted specimens both having similar densities were tested under triaxial
compression. The specimens were impregnated with an epoxy resin at three different
stages of shear deformation and small cores from each specimen were scanned using
X-ray micro-tomography. Different systems and scanning parameters were explored
in order to obtain three-dimensional high-resolution images with a voxel size of 5μm
(0:018d50) and a quality level required for the identi cation of the individual particles
and the surface defining each particle-particle contact.
The quantification of particle size and shape has shown that breakage of fractured
grains, along existing fissures, occurs both during reconstitution and shearing of
the intact soil, a phenomenon that cannot be observed using invasive techniques
such as sieve analysis. Statistical analyses of the distribution of fabric directional
data in terms of particle orientations, contact normals, branch vectors and void
orientations were carried out at each loading stage. It has been shown that the initial
particle orientation fabric that develops during the deposition of the material tends
to persist during shearing, while the contact normals seem to be reorientated along
the direction of the major principal stress in the post-peak regime. Different patterns
were observed within the shear band as both the particles and the contact normals
appeared to rotate towards the direction of the shear plane. The measurements
from the tomographic data were complemented with a qualitative description of the
morphology and fabric using SEM and optical microscope images of thin sections.
Version
Open Access
Editor(s)
O'Sullivan, C
Coop, M
Date Issued
2011-05
Date Awarded
2011
Citation
2011
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
O’Sullivan, Dr. Catherine
Coop, Professor Matthew
Sponsor
Fundacao para a Ciencia e a Tecnologia
Publisher Department
Civil and Environmental Engineering
Publisher Institution
Imperial College London
Subjects
soil structure/fabric, granular material, sand, 3D microscopy
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)
Author Permission
Permission granted