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.

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