---
title: "Rat-holing"
description: "Rat-holing is a bulk-solids flow pattern in which material discharges through a narrow vertical channel directly above the hopper or silo outlet, while the surrounding material remains stagnant and progressively consolidates. The result is a funnel-flow condition gone to the extreme: most of the silo contents never move."
canonical_url: "https://sylio.co/glossary/rat-holing"
last_updated: "2026-06-28T02:29:33.252Z"
---

**Rat-holing** is a bulk-solids flow pattern in which material discharges through a narrow vertical channel directly above the [hopper](/glossary/hopper) or [silo](/glossary/silo) outlet, while the surrounding material remains stagnant and progressively consolidates. The result is a [funnel-flow](/glossary/mass-flow-vs-funnel-flow) condition gone to the extreme: most of the silo contents never move.

## Why rat-holing matters

- **Effective storage volume collapses** — only the narrow flowing column is usable
- **Stagnant material consolidates and ages** — eventually hardens beyond recovery without manual cleanout
- **First-in, last-out becomes never-out** — older material is trapped indefinitely
- **Catastrophic collapse risk** — when the rat-hole eventually breaks open under load it can release tonnes of compacted material suddenly into downstream equipment

## Causes

- Narrow outlet relative to silo diameter
- Steep but insufficiently steep cone angle
- Cohesive material below its [mass-flow](/glossary/mass-flow-vs-funnel-flow) threshold
- Failure of a discharge aid (vibrator, aeration) that previously prevented funnel flow

## Prevention

The structural remedy is to redesign the cone for mass flow — steeper angle, larger outlet, smoother wall finish. Where that is not feasible, [sonic horns](/glossary/sonic-horn) mounted on the cone wall continuously vibrate the stagnant material and break the rat-hole pattern, restoring closer-to-mass-flow behaviour.

## Related terms

- [Hopper](/glossary/hopper)
- [Silo](/glossary/silo)
- [Bridging](/glossary/bridging)
- [Mass flow vs funnel flow](/glossary/mass-flow-vs-funnel-flow)
- [Sonic horn](/glossary/sonic-horn)