How Sandcastles Fall

TL;DR

This paper investigates how capillary forces influence sandpile stability, revealing size-dependent effects on the critical angle and bulk failure, with a theoretical model aligning qualitatively with recent experiments.

Contribution

It introduces a theoretical analysis of capillary effects on sandpile stability, highlighting size-dependent critical angles and bulk failure mechanisms.

Findings

Critical angle unchanged in infinite systems

Critical angle increases for finite-sized systems

Failure occurs in the bulk, not surface

Abstract

Capillary forces significantly affect the stability of sandpiles. We analyze the stability of sandpiles with such forces, and find that the critical angle is unchanged in the limit of an infinitely large system; however, this angle is increased for finite-sized systems. The failure occurs in the bulk of the sandpile rather than at the surface. This is related to a standard result in soil mechanics. The increase in the critical angle is determined by the surface roughness of the particles, and exhibits three regimes as a function of the added-fluid volume. Our theory is in qualitative agreement with the recent experimental results of Hornbaker et al., although not with the interpretation they make of these results.