Evolution of shear zones in granular packings under pressure

TL;DR

This study explores how shear zones in granular materials evolve under pressure and boundary slip, revealing their shifting position, widening, and transition from open to closed zones, with minimal influence from grain size.

Contribution

It provides new insights into the effects of confining pressure, boundary slip, and material properties on shear zone characteristics in granular media.

Findings

Shear zone center shifts toward the cylinder's center with increased pressure.

Shear zone width increases as confining pressure rises.

Boundary slip induces transition from open to closed shear zones.

Abstract

Stress transmission in realistic granular media often occurs under external load and in the presence of boundary slip. We investigate shear localization in a split-bottom Couette cell with smooth walls subject to a confining pressure experimentally and by means of numerical simulations. We demonstrate how the characteristics of the shear zone, such as its center position and width, evolve as the confining pressure and wall slip modify the local effective friction coefficient of the material. For increasing applied pressure, the shear zone evolves toward the center of the cylinder and grows wider and the angular velocity reduces compared to the driving rate of the bottom disk. Moreover, the presence of slip promotes the transition from open shear zones at the top surface to closed shear zones inside the bulk. We also systematically vary the ratio of the effective friction near the bottom plate and in the bulk in simulations and observe the resulting impact on the surface flow profile. Besides the boundary conditions and external load, material properties such as grain size are also known to influence the effective friction coefficient. However, our numerical results reveal that the center position and width of the shear zone are insignificantly affected by the choice of the grain size as far as it remains small compared to the radius of the rotating bottom disk.