gunnies

When a lode of economically valuable mineral is extracted from underground, it leaves behind a cavity in the rock mass. This void space is called a gunny or gunnie, and it represents the three-dimensional shape of the ore body that was removed. Unlike a deliberately created chamber or stope that might be backfilled or used for further operations, a gunny is simply the negative space left by extraction, often unstable and prone to collapse.

Gunnies vary widely in size and shape depending on the ore deposit. A small pocket of galena or sphalerite might leave a gunny only a few cubic meters in volume, while extraction of a major copper or iron lode can create a cavity spanning hundreds of meters of strike length. The walls and roof of a gunny are typically jagged and irregular, following the geometry of the original mineralization rather than the neat rectangular shapes of a planned mining chamber.

Subsidence and Ground Stability

The primary concern with gunnies is ground stability. If a gunny lies near the surface or directly beneath inhabited or developed ground, the overlying rock and soil can subside, collapse, or create sinkholes. This hazard is particularly acute in older mining districts where extensive shallow workings created numerous gunnies with minimal documentation of their locations or dimensions. Modern mining operations typically track gunnies as part of their closure and rehabilitation plans, sometimes electing to backfill them with tailings, waste rock, or stabilized material to prevent future surface failures.

The term gunny appears in mining vocabulary primarily in historical and descriptive contexts, particularly in reports on legacy mining sites, subsidence investigations, and mine closure assessments. It is less common in operational settings, where extraction voids are more often referred to by the specific mining method used to create them, such as stope, pit, or shaft.