stall

In underground mining, a stall is the excavated void created when ore or coal is removed from around a support pillar. The pillar itself remains standing to carry roof load, while the stall is the working space where extraction occurred. Stalls may be a few meters or many tens of meters across, depending on the geometry of the ore body, the thickness of the seam, and the pillar design chosen by the mining engineer.

The term originates from its literal sense: a compartment or bay in a structure. In pillar and stall mining (also called room and pillar mining), the layout consists of a regular grid where pillars are left unworked at set spacing, and stalls are the mined-out bays between them. This method is common in coal seams and bedded ore deposits where the ore is relatively flat-lying and the stall dimensions can be standardized. The ratio of pillar width to stall width determines the overall recovery rate and the safety margin: narrower stalls mean higher recovery but higher stress on pillars.

Stall geometry and stability

Stall walls are called ribs or rib sides. Where a stall is worked down to the full height of the seam, the exposed ribs can be unstable if the host rock is weak or if the overburden weight is high. Rib fall (slabbing or spalling of the rib wall) is a common hazard; it can be mitigated by leaving a thicker rib, slowing advance, or installing rock bolts or standing timber. The floor and roof of a stall must also be assessed: coal or ore left in the floor adds bearing capacity, while a weak roof requires roof support or caving.

Ventilation in stalls must be carefully controlled. Dead stalls (worked areas now abandoned) can accumulate dangerous gases such as methane or carbon dioxide if not properly sealed or degassed. Active stalls require adequate air flow to remove dust and gases from the working face. The network of stalls and their connecting roadways forms the mine's ventilation circuit.

Recovery from a stall is not always 100 percent. Ore or coal left as a protective rib, floor thickness, or in inaccessible corners remains in the ground. The economics of stall mining therefore involve a balance between maximizing extraction and ensuring that the remaining pillars can support the roof load over the mine's intended life. If a mine is eventually abandoned, pillars can sometimes be recovered by systematic retreat mining, working backward and allowing controlled caving of the stall roof.