Non-invasive characterization of particle morphology of natural sands

Particle morphologies, i.e. particle sizes and shapes, have a marked influence on the mechanical response of granular materials including soils. Until now most investigations of particle shape have been two-dimensional and size has been most often assessed using sieving. This paper makes use of recent developments in three-dimensional imaging technologies to characterize the internal features of a soil in 3D including quantification of particle morphology. The soil investigated was Reigate sand (from Southeast England), a geologically old sand, which in its intact state exhibits significant interlocking amongst the constituent grains. Intact and reconstituted specimens 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 3D, high-resolution, images with a voxel size of 5 μm (0.018×d50). The morphology measurements were compared with sieve data and measurements obtained using a 2D, image based, laser system. The sieve size is shown to correlate well with the intermediate principal axis length. Clear differences are noted between the 2D and 3D shape measurements. 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.