craunch
A portion of a stratum or vein left in excavating to support the roof.
craunch: a pillar left behind to hold up the mine roof
A craunch is a deliberate block of unmined ore or rock left in place within a working seam or vein to provide structural support for the overlying strata. It serves as a temporary or sometimes permanent pillar, preventing roof collapse in underground excavations where removal of all material would compromise stability. The term is most common in hard rock mining, particularly in metalliferous operations where veins are worked selectively rather than extracted completely.
The size and spacing of craunches depend on the strength of both the remaining ore and the hanging wall above. A weak roof or friable country rock demands larger pillars left at shorter intervals; strong geology allows smaller craunches spaced farther apart. Operators must calculate carefully: too little remaining material and the roof fails suddenly; too much and economic recovery suffers. In some operations, craunches are intentionally small and temporary, removed during later retreat mining once adjacent ground is sufficiently exposed to stand alone.
When things go wrong
Craunches can become a liability if they attract water infiltration or if surrounding ground weakens over time through weathering or oxidation. In coal mining, similar supporting pillars left in worked seams have been known to pillar burst, where the concentrated load on the remaining coal triggers violent failure. Abandoned craunches in old workings pose particular hazard: ground may subside decades later when pillars finally fail under stress or when water dissolves supporting minerals.
The word likely derives from an old mining term for rock broken or crushed in place, related to the verb "crunch." It appears more frequently in British and European mining literature than in North American practice, where the more common terms are pillar, rib, or stook. Understanding local terminology matters because a craunch in one mining district may be called something entirely different fifty miles away, yet the structural principle remains identical across all underground metal and coal extraction.