Effects of grain shape on packing and dilatancy of sheared granular materials

TL;DR

This study investigates how grain shape influences packing density, particle orientation, and dilatancy in sheared granular materials, revealing complex interactions between alignment and dilation effects for different particle geometries.

Contribution

It provides detailed insights into the effects of grain shape on packing and dilatancy, using X-ray tomography to compare anisometric and spherical particles under shear.

Findings

Anisometric particles reorient and partially compensate for dilatancy.

Crystallization occurs in monodisperse spheres but is inhibited by shape deviations.

Surface deformation correlates with particle alignment and dilation effects.

Abstract

Granular material exposed to shear shows a variety of unique phenomena: Reynolds dilatancy, positional order and orientational order effects may compete in the shear zone. We study granular packings consisting of macroscopic prolate, oblate and spherical grains and compare their behaviour. X-ray tomography is used to determine the particle positions and orientations in a cylindrical split bottom shear cell. Packing densities and the arrangements of individual particles in the shear zone are evaluated. For anisometric particles, we observe the competition of two opposite effects. One the one hand, the sheared granulate is dilated, but on the other hand the particles reorient and align with respect to the streamlines. Even though aligned cylinders in principle may achieve higher packing densities, this alignment compensates for the effect of dilatancy only partially. The complex rearrangements lead to a depression of the surface above the well oriented region while neigbouring parts still show the effect of dilation in the form of heaps. For grains with isotropic shapes, the surface remains rather flat. Perfect monodisperse spheres crystallize in the shear zone, whereby positional order partially overcompensates dilatancy effects. However, already slight deviations from the ideal monodisperse sphere shape inhibit crystallization.