stowing

Stowing is the deliberate packing of waste material, usually coarse tailings or broken rock from development work, into the void left behind after ore extraction. Rather than leaving empty chambers underground, miners use stowing to stabilize the remaining pillars and walls, reduce subsidence at surface, and reclaim space for further mining operations in the same area.

The waste material used for stowing differs by site and depth. In hard rock mines, broken development waste or sorted coarse tailings work well; in coal and softer seams, even finer classified tailings can be pumped in as slurry. The material must have minimal value and must not contain sulfides or other compounds that will generate acid drainage or excessive heat as it sits underground. Particle size matters: oversized boulders create large voids and poor compaction, while fine material compacts too densely and may trap groundwater.

Placement and compaction

Stowing is performed behind workers as they retreat from a worked-out area, or into pillars during pillar extraction. In shallow workings, waste rock is thrown or shoveled into the void by hand. In deeper mines, broken rock falls into the worked space by gravity from slopes or chutes, or tailings are pumped in as a thickened paste or slurry through boreholes. Hydrostatic stowing, in which material settles from a slurry, requires no mechanical compaction but demands careful water management to prevent piping and loss of cohesion.

The key risk is incomplete filling. Soft or unsupported stowing can compress under the weight of overlying rock, reopening voids and triggering collapses. Badly stowed areas can also develop high pore pressures if water is trapped, causing sudden rupture and outbursts. Mine plans must account for predicted settlement and must verify that stow density and strength are adequate for the stress regime at depth.

Stowing appears most commonly in hardrock vein mining, particularly in narrow seam extraction where pillars are worked down, and in some deep coal mines. It is less practiced in bulk underground mines where waste is kept separate and surface disposal is planned from the outset. The term comes from stow, meaning to pack or cram closely, and reflects the practical goal of using waste productively rather than managing it as a disposal burden.