Compaction of granular material inside confined geometries

TL;DR

This study investigates how granular materials compact within confined, quasi-two-dimensional spaces, revealing exponential stress growth and extending analytic models to better predict stress distribution under various conditions.

Contribution

It extends existing models to accurately describe stress in confined granular packings and explores effects of system size, piston rate, and initial packing fraction.

Findings

Stress increases exponentially with accumulated grain length.

Extended analytic models fit measured stress data.

Identified limitations of current models for small systems.

Abstract

In both nature and engineering, loosely packed granular materials are often compacted inside confined geometries. Here, we explore such behaviour in a quasi-two dimensional geometry, where parallel rigid walls provide the confinement. We use the discrete element method to investigate the stress distribution developed within the granular packing as a result of compaction due to the displacement of a rigid piston. We observe that the stress within the packing increases exponentially with the length of accumulated grains, and show an extension to current analytic models which fits the measured stress. The micromechanical behaviour is studied for a range of system parameters, and the limitations of existing analytic models are described. In particular, we show the smallest sized systems which can be treated using existing models. Additionally, the effects of increasing piston rate, and variations of the initial packing fraction, are described.