Room and pillar mining is an underground extraction method that involves removing a mineral deposit across a horizontal plane, creating a vast network of open spaces beneath the earth’s surface. This technique is one of the oldest and most widely used forms of underground mining globally, having been employed in Europe as early as the 13th century. The system systematically leaves behind columns of unmined material, which serve as permanent or temporary supports for the overburden (the rock layer above the deposit). By balancing material removal with ground support, room and pillar mining allows for the safe recovery of minerals while minimizing the risk of roof collapse. The method’s ability to be highly mechanized and its relative simplicity ensure its continued use in certain mining sectors.
Defining the Geometry of Room and Pillar
The name of the mining technique clearly describes its resulting geometry, which consists of two core components: rooms and pillars. Rooms are the voids or excavations created when the valuable ore or mineral is removed from the deposit. These rooms form the main working areas and haulage ways within the mine. Interspersed throughout these excavations are the pillars, which are columns of unmined rock left in place to support the mine’s roof. This structure creates a recognizable grid-like pattern that resembles a checkerboard.
The pillars bear the load of the overlying rock strata, effectively transferring the weight from the excavated areas. The size and spacing of these support columns are not arbitrary but represent a complex engineering decision based on rock mechanics. Engineers must calculate the pillar dimensions based on the strength of the mineral being mined, the load-bearing capacity of the rock above and below the deposit, and the overall stress load from the overburden. If the pillars are too small, they risk being crushed by the weight, an event known as a “squeeze,” which can trigger a chain reaction of collapses.
The Step-by-Step Mining Cycle
The operational sequence in a room and pillar mine follows a systematic cycle after the initial access shafts or slopes are established. The traditional approach involves a sequence of cutting, drilling, blasting, loading, and hauling the material. The process begins with drilling holes into the working face, which are then packed with explosives and detonated to break up the mineral. After the blast, the broken material (muck) is loaded onto haulage vehicles for transport out of the mine. Simultaneously, the roof must be secured using roof bolting equipment to prevent falls of rock from the newly exposed ceiling.
Modern operations often employ continuous mining machines, especially in coal mining, which significantly streamlines this cycle. These machines use a rotating drum equipped with tungsten carbide bits to mechanically cut the mineral from the face, eliminating the need for drilling and blasting. The material is then automatically collected and transferred to a conveyor system or shuttle cars. This methodical advance creates the parallel rooms and connecting cross-cuts in a predetermined sequence designed to optimize production and air circulation.
Minerals Extracted and Geological Suitability
The room and pillar method requires specific geological conditions. The primary constraint is that the deposit must be relatively flat-lying, or tabular, meaning it is spread out in a layer or seam. While a gentle dip is acceptable, a steep angle makes the method unsuitable. This technique is commonly used for deposits at shallow to moderate depths, ideally less than 600 meters for soft rock. Greater depths create high stress, demanding excessively large pillars that reduce recovery. The mineral body must also possess sufficient strength to form stable pillars and a competent roof.
A wide range of minerals is extracted using this technique, with coal being the most common application. The method is particularly advantageous for deposits of lower commercial value where causing surface subsidence is not desirable, or when the ore body is uniformly thick and continuous.
Common Minerals Extracted
- Coal
- Evaporite deposits such as rock salt, potash, and trona
- Industrial minerals like gypsum and limestone
- Metallic ores such as zinc and lead
Pillar Recovery and Mine Closure
The final and most intensive stage involves the controlled removal of the support columns, known as pillar recovery or retreat mining. This process maximizes mineral extraction, increasing the overall recovery rate from a typical 40–60% up to 90% or higher. It is called retreat mining because miners work backward toward the mine entrance, sequentially removing the pillars. Pillar recovery is the most hazardous phase due to the intentional weakening of the roof support, requiring specialized mobile roof supports to protect workers. As the last sections are removed, the overlying rock mass is allowed to collapse in a controlled manner into the void behind the work area, a process called caving.