Column collapse of granular rods

TL;DR

This study investigates how granular rod columns behave upon collapse, revealing critical heights and scaling laws that depend on aspect ratio and initial geometry, with findings applicable to granular material stability.

Contribution

It identifies critical heights and transition behaviors in granular rod columns, extending understanding of collapse dynamics beyond traditional granular materials.

Findings

Collapse probability increases linearly in a transition region.

Runoff radius scales as a power-law with pile height.

Scaling behavior changes at a critical aspect ratio.

Abstract

We find the collapse of columns of granular rods to show range of behaviors that depends on particle aspect ratio (length $L$ to diameter $d$) and initial pile geometry (height/radius). For all aspect ratios $L/d$ below 24 there exists a critical height at $L/4$ below which the pile acts as a solid, maintaining its initial shape, and a second critical height at $3L/4$ above which the pile always collapses like an ordinary granular material. Separating the critical heights is a transition region in which the probability of collapse increases linearly from 0 to 1. This behavior is independent of particle length, width, or aspect ratio. When the pile does collapse, the runoff radius $r_f$ scales as a power-law with dimensionless height $\tilde H$, agreeing with previous experiments on ordinary sand. For low piles the scaling is linear, with $r_f\sim \tilde H^{1.2\pm 0.1}$. Above a critical pile aspect ratio (pile height/radius) this switches to a square-root scaling, with $H^{0.6\pm0.1}$.