cave
In room-and-pillar mining, to extract a deposit of rock by breaking down a pillar which had been holding it in place.
cave: deliberately collapse a pillar to drop the ore above
In room-and-pillar mining, to cave means to extract ore or mineral by deliberately breaking down a support pillar that was left in place during earlier mining phases. The pillar fails under load, and the overlying rock mass drops into the void below, fragmenting as it falls. This is a planned, controlled extraction method rather than an unintended structural failure.
Room-and-pillar mines are laid out in a grid pattern: miners extract ore in rectangular rooms, leaving pillars between them to support the roof and maintain access. Early in the mine's life, these pillars are oversized for safety. As the operation matures and approaches closure, those pillars become valuable targets. Caving recovers ore that would otherwise remain locked in the support structure.
Caving triggers and sequence
The operator systematically weaken or remove portions of targeted pillars using drill-and-blast or mechanical excavation. The timing is critical: undermine too slowly and the roof may collapse unpredictably; remove too much too fast and the mine becomes unsafe. Many operations monitor pillar stress with instruments or observe crack patterns before triggering final collapse. Once a pillar fails, the overlying strata can settle anywhere from a few metres to tens of metres, depending on rock strength and pillar geometry.
Caving works best in competent (strong, stable) rock where the broken fragments remain largely confined to the extraction area. In weak or heavily fractured rock, subsidence can spread wider, damaging infrastructure above or opening unintended fractures in surrounding pillars. Operators must therefore survey and predict subsidence zones before caving begins.
The extracted rock is then mucked out (loaded and hauled away) through existing mine passages. This operation is labour-intensive and carried out under careful roof support conditions, as the mine geometry has now changed fundamentally. Once caving is complete in a section, that area is typically abandoned and not re-entered unless conditions allow further pillar recovery.