Strain localisation and grain breakage in sand under shearing at high mean stress: insights from in situ X-ray tomography

Abstract

This work presents results from a series of triaxial compression tests on two quartz sands (differing principally in grain shape), at confining pressures high enough to cause grain breakage during shearing. Tests are performed inside an x-ray scanner, which allows specimens to be imaged non-destructively as they deform. Observation of the acquired images clearly shows different mechanisms of deformation, including shearing, dilation, compaction and grain breakage. These mechanisms are investigated quantitatively through 3D measurements of local porosity, as well as strain (obtained by 3D Digital Image Correlation), which is analysed in terms of volumetric and shear components. These tools allow the transition between macroscopically dilative (typically of a dense sand at low mean stress) and compactive behaviour to be investigated. The analysis reveals that at the high end of the confining pressure range studied (100 to 7000 kPa) the more rounded sand deforms with highly localised shear and volumetric strain-the porosity fields show a dilative band within which a compactive region (due to grain crushing) grows. The more angular material shows shear strain localisation, however its faster transition to compactive behaviour (due to a higher propensity for individual grains to crush) translates to much more distributed compactive volumetric strain.

Micro-texture and petrophysical properties of dilation and compaction shear bands in sand

“…In addition, several tests on Ottawa and Hostun Sands, at low confining pressure, 100 kPa, 22 and two tests, on Ottawa Sand, at high confining pressure, 7000 kPa, 21 was reached, and then the deviatoric loading, with constant lateral stress, was applied to reach the desired strain.…”

“…We further analyze some of the experimental data reported by Alikarami et al 21 performed on rounded Ottawa and angular Hostun sands in order to discover the evolution of dilatancy factor and porosity under different confining pressure in these two sands. Alikarami et al 21 address the effect of angularity on grain breakage in sands under different confining pressure.…”

“…Alikarami et al 21 address the effect of angularity on grain breakage in sands under different confining pressure. In this paper we present how a change in deformation mechanism causes different types of bands to form as a result of micro-textural and porosity evolution in the tested samples.…”

Evolution of deformation and breakage in sand studied using X-ray tomography

“…To allow the acquisition of images with XCT, the loading apparatus has to be transparent to X-rays and both the apparatus and the specimen must have small dimensions to allow the X-ray's cone beam to obtain a sufficient resolution. The triaxial cell and the specimen preparation method used here are described in Alikarami et al (2015). Specimens are pluviated into a neoprene membrane, stretched in a mould; the membrane's contribution to the lateral stress has been accounted for, as described in Bishop & Henkel (1962).…”

“…It is naturally advantageous to be able to study the evolution of fabric in one specimen. Gkiousas-Kapnisis et al (2015) used XCT images of well-graded Hostun sand under triaxial loading (described in Alikarami et al (2015)), to study the evolution of grading. The angularity of the grains and the grading of the sand created important challenges during image processing, especially regarding the segmentation of the particles and the definition of a size descriptor to obtain the mean diameter of the particles.…”