The Room and Pillar Mining is not so much a mining method as a strategy to prevent the collapse of the mine. It has been used since time immemorial in very large deposits and in extensive seams or strata when the aim is to prevent the surface layer from caving in. This is not always a problem, but especially when mining rocks such as rock salt, lime, sandstone or shale, where large quantities are to be extracted, caving in can cause serious damage at the surface. But also groundwater above the quarry, which is dammed by a water-impermeable separation layer, could penetrate the quarry through the resulting cracks during a collapse. In addition, chamber construction makes the use of supports and lining unnecessary in many cases, which is a very important benefit.
The basic principle is to excavate a part of the seam, but not exceeding the dimension at which the surface layer remains stable. In this way, a chamber is obtained whose ceiling is stable on its own and does not need support. Only the width of the chamber is relevant for this stability, the length is arbitrary. In principle, each tunnel is a chamber. The next chamber running parallel must keep a minimum distance, so that a wall remains that is stable enough to absorb the supporting pressure. Miners call those walls pillar and thats the reason why this is called Room and Pillar Mining.
The disadvantage of the process is obvious, part of the extracted raw material is needed as pillars and therefore cannot be mined. In the case of rocks, the proportion is between 10% and 20%, and up to 50% in the case of less stable layers above. The method is also used in coal mining, but the coal is relatively soft, so that up to 75% has to be left standing. In addition, besides the areas with chambers and panel pillars, larger areas are not mined in order to increase stability. They are called barrie pillars. Therefore, in some mines, the chambers are created in the width of the excavation machines with pillars of the same width. Then the chambers are filled with overburden and the pillars are mined too. Since the overburden settles, it is advisable to backfill the dismantled pillars as well.