The extraction of ore from beneath the surface of the ground. Underground mining is also applied to deposits of industrial (nonmetallic) minerals and rocks, and underground or deep methods are used in coal mining. Some ores and industrial minerals can be recovered from beneath the ground surface by solution mining or in-place leaching using boreholes. See also Coal mining; Solution mining.
Underground mining entries and workings.
Underground mining is done where mineral deposits are situated beyond the economic depth of open pit mining; it is generally applied to steeply dipping or thin deposits and to disseminated or massive deposits for which the cost of removing the overburden and the maintaining of a slope angle in adjacent waste rock would be prohibitive. See also Open pit mining.
Underground mine entries are by shaft, adit, incline, or spiral ramp (see illustration). Development workings, passageways for gaining access to the orebody from stations on individual mine levels, are called drifts if they follow the trend of the mineralization, and cross-cuts if they are driven across the mineralization. Workings on successive mine levels are connected by raises, passageways that are driven upward. Winzes are passageways that are sunk downward, generally from a lowermost mine level.
In a fully developed mine with a network of levels, sublevels, and raises for access, haulage, pumping, and ventilation, the ore is mined from excavations referred to as stopes. Pillars of unmined material are left between stopes and other workings for temporary or permanent natural support. In large-scale mining methods and in methods where an orebody and its overlying waste rock are allowed to break and cave under their own weight, the ore is extracted in large collective units called blocks, panels, or slices. See also Mining.
Information on the size, shape, and attitude of a deposit and information for estimating the tonnage and grade of the ore is taken from drill holes and underground exploration workings. Underground exploration workings are used for bulk and detailed sampling, rock mechanics testing, and the siting of machinery for underground drilling.
Where high topographic relief allows for an acceptable tonnage of ore above a horizontal entry site, an adit or blind tunnel is driven as a cross-cut to the deposit or as a drift following the deposit from a portal at a favorable location for the surface plant, drainage facilities, and waste disposal. In situations where the deposit lies below or at a great distance from any portal site for an adit, entry must be made from a shaft collar or from an incline or decline portal. A large mine will commonly have a main multipurpose entry and several more shafts or adits to accommodate personnel, supplies, ventilation, communication, and additional production.
A fundamental condition in the choice of mining method is the strength of the ore and wall rock. Strong ore and rock permit relatively low-cost methods with naturally supported openings or with a minimum of artificial support. Weaker ore and wall rock necessitate more costly methods requiring widespread temporary or permanent artificial support such as rock bolting. Large deposits with weak ore and weak walls that collapse readily and provide suitably broken material for extraction may be mined by low-cost caving methods. Few mineral deposits are so uniform that a single method can be used without modification in all parts of the mine.
