The shape of sand particles: Assessments of three-dimensional form and angularity

Particle shape affects the mechanical behaviour of soil and is thus a parameter of interest in geotechnical engineering. Shape is commonly described by form, angularity and roughness. Form describes the overall aspect ratio, angularity the sharpness of the edges and corners, and roughness the small surface irregularities. This work explores the characterisation of form and angularity of sand particles. Our results show that focus variation microscopy principles can be implemented in a conventional compound microscope to measure particle heights as small as 60 μm without having to observe a lateral view of the particle. The robustness of the procedure is demonstrated by implementing it on sand-sized particles from six different sources. Importantly, the compound microscope employed by the procedure is likely to be accessible to many soil laboratories. Heights measured using focus variation were used to assess particle form. Contrary to assumptions in previous works, form varied significantly within a given soiltype and a narrow particle size range. Regarding angularity, there is a systematic correlation between particle form and the angularity metric known as 'ellipseness'. Furthermore, while ellipseness is adequate to distinguish between angular and rounded particles, it cannot distinguish between sub-rounded and well-rounded particles.