Underground mining

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 involves a larger capital investment and higher production cost per ton of ore than open pit mining. It 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. In some situations, the shallower portion of a large orebody will be mined by open pit methods, and the deeper portion will be mined by underground methods.  See also: Open-pit mining

Underground mine entries are by shaft, adit, incline, or spiral ramp (Fig. 1). 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.

Fig. 1  Underground mining entries and workings.

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

Exploration

Exploration and development constitute the preproduction stage of underground mining. Exploration refers to the delineation of a newly discovered mineral deposit or an extension of a known deposit and to its evaluation as a prospect. During exploration, the deposit is investigated in sufficient detail to estimate its tonnage and grade, its metallurgical recovery characteristics, and its suitability for mining by various methods.

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. Diamond core drilling provides intact samples of ore and rock for assaying and for detailed geologic and geotechnical study; percussion drilling provides chips of material for the recognition of ore and waste boundaries and for additional sampling. Underground exploration workings are used for bulk and detailed sampling, rock mechanics testing, and the siting of machinery for underground drilling.  See also: Drilling, geotechnical

The tonnage and the grade of the material available in a mineral deposit are interrelated. The cutoff grade is the weakest mineralization that can be mined at a profit. Ore reserves are calculated in respect to the amount of ore in place at potential cutoff grades, the tonnages and average grades in identified blocks of ore, and the ultimate tonnage and grade of ore that should be available under projected conditions of recovery and wall rock dilution in mining. The suitability of a deposit for mining is determined in testing and evaluation work related to the physical and chemical nature of the ore, hydrologic conditions, and the needs for ground control.  See also: Rock mechanics

Mine Development

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.

Adits

Access by adit generally provides for relatively low-cost underground mining. The broken ore from above the adit level can be brought to the portal in trains, conveyor belts, and rubber-tired trucks without the need for hoisting, and the workings can be drained without pumping. The driving of an adit is generally less expensive per unit distance of advance than the sinking of a shaft or the driving of an inclined access. In areas of low topographic relief and in the mining of deep orebodies, the sinking of a shaft will often be a more economical approach than the driving and maintaining of a considerably longer incline or adit from a remote part of the site.

Shafts

Production shafts are generally located in stable ground on the footwall side of a dipping deposit rather than in the deposit itself or in the hanging-wall side, where protective pillars would be needed to maintain stability as mining progresses. A shaft may be inclined to follow the dip of the deposit and avoid increasingly longer cross-cuts to the ore at greater depth, but vertical shafts are more common because of their lower construction and maintenance cost per unit of depth and their better efficiency for hoisting ore. Shafts are sunk as rectangular or circular openings 15–30 ft (5–9 m) in diameter; they are equipped with a headframe and hoisting system and are lined with timber, steel forms, or concrete for ground support. Smaller shafts 5– 15 ft (1.5–5 m) in diameter, generally for escapeways and ventilation, may be bored by mechanical drilling machines.

Inclines

Inclines equipped with hoists, declines for access by rubber-tired equipment, and gently inclined spiral ramps for diesel-powered truck haulage allow for direct access to relatively deep mine levels without having to transfer the ore and materials to hoisting systems.

Development workings

Development workings in the deposit consist of mine levels and sublevels, with drifts in the ore zone or in the more stable rock on the footwall side of the ore zone. Level workings serve as passageways for miners and low-profile equipment and as haulageways. In broken or unstable ground, passageways and haulageways are supported by timber sets and steel beams or arches; further stabilization is given by rock bolts, sometimes in combination with cable bolting and wire mesh, and the walls may be lined with concrete or spray-on shotcrete.

The raises that connect levels and sublevels provide for the removal of broken ore (chutes and ore passes), for access by miners, and for ventilation and supply routes.

In conventional mining and in the most common development procedures, headings are advanced in a cyclic sequence of drilling, blasting, mucking (removal of broken rock), and installing ground support. In continuous mining, the cycle is replaced by rapid excavation, a single operation in which headings are advanced by powerful tunnel boring and road header machines with teeth that break rock from the face. In situations where the uniformity and texture of the rock and ore permit development by continuous mining, the walls of the resulting passageways are smoother and more stable than would be provided by conventional cyclic operations involving blasting.  See also: Tunnel

The continuous mining procedure of raise boring is well established. Shaft boring is used in the sinking of small-diameter ventilation shafts and escapeways. The driving of mine level development headings by cutting and boring machinery is more common in coal, potash, and salt deposits and in relatively soft sandstones and shales than in hard ore and rock.

Hydraulic breakers provide successively smaller rock sizes at development headings, and the broken rock is collected at the face by mechanical loading machinery and transferred to the mine haulage system by mobile conveyors or rubber-tired load-haul-dump machines. Haulage beyond the transfer point has been done by electric-powered locomotives with trains of cars but now is increasingly done by rubber-tired electric- or diesel-powered shuttle cars or trucks and by conveyor belt systems. In shaft mines, the broken rock is collected in underground storage pockets and loaded into skips for hoisting to the surface.

The entire sequence in mine development—the advance of headings, breaking of rock, loading, haulage, and hoisting—is increasingly automated. Teleoperated and autonomous machines have become central to every stage in mining, and new mines are developed with the use of geographic information systems (GIS) technology to accommodate the extensive communication systems and mining methods that relate to operations by remote control.  See also: Geographic information systems

Mining Methods

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 wide-spread 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. Mining to an increasing depth with higher stress conditions and mining from a thicker portion of an orebody into thinner or less uniform portions will especially call for changes in method.

Naturally supported openings

The stopes remain open, essentially by their own strength, during ore extraction. Stability may be maintained to some extent by timbers, rock bolts, and accumulations of broken ore. The workings may collapse with time or may eventually need to be filled with waste material to protect workings in adjacent areas. Backfilling involves the placement of a paste of cemented waste rock or mill tailings. The methods range from gophering, an unsystematic small-scale practice, to carefully planned and executed systems using limits determined by rock mechanics investigations.

Open stoping

This is used in steeply dipping and thin orebodies with relatively strong ore and wall rock. In overhand methods the ore is stoped upward from a sill pillar by miners working on a staging composed of stulls (round timbers) and lagging (planks). With the drilling and blasting of successive small blocks of ore from the back (roof), the broken ore falls onto lower stagings and to the bottom of the stope; it is collected on the haulage level through draw points or chutes. In underhand stoping the ore is mined downward in a series of benches, and the broken ore is scraped or hauled into a raise or ore pass for collection on a lower mine level. The width of an open stope is limited by the strength of the ore and its capability to stand unsupported. Occasional pillars, generally of waste or low-grade zones in a vein, are left for support; timber stulls may be wedged between the stope walls for stability as well as for access, and rock bolts may also be used to maintain wall stability.

Sublevel stoping

Also referred to as longhole or blasthole stoping, sublevel stoping is practiced in steeply dipping and somewhat wider orebodies with strong ore and strong walls (Fig. 2). Sublevel drifts and raises or slots are driven at the ends of a large block of ore so that a series of thinner horizontal slices can be provided. Miners in the sublevels drill patterns of radial holes (ring or fan drilling) or quarrylike parallel holes (slashing). Beginning at the open face of the initial slot, the ore is blasted in successive increments, and the broken ore falls directly to the bottom of the stope. A crown pillar is generally left unmined at the top of the stope to support the next major level.

Fig. 2  Sublevel stoping, with ring drilling.

Vertical crater retreat

This is a method of sublevel stoping in which large-diameter blastholes are drilled in a parallel pattern between major levels, and the ore is broken from the bottom of the stope in a sequence of localized blasts. All of the drilling, loading, and blasting are done by miners and teleoperated machinery in the upper level, so there is no need for access to the ore from below as the stope progresses upward.

Room-and-pillar mining

This is also referred to as stope-and-pillar mining when done in a less regular pattern. Room-and-pillar mining is done in coal seams and in flat-lying or gently dipping ore and industrial mineral deposits (Fig. 3). It is a low-cost method of underground mining because fast-moving rubber-tired equipment can operate freely, especially in large rooms and haulageways. Thin-bedded deposits are generally mined in a single stage (pass) by conventional or continuous mining; thicker deposits are mined in a two-stage benching operation. In deposits of considerable thickness, an underground quarrying operation follows the first-stage opening of a development level for sufficient access by open-pit-type blasthole drills. Room-and-pillar mining is generally limited to depths on the order of 3000 ft (914 m) in hard-rock mines and to lesser depths in coal mines because of rock bursts and similar manifestations of high-stress concentration on the pillars. Extraction in mining generally amounts to about two-thirds of the ore in a bedded deposit, with the remaining ore being left in pillars; in places where pillars can be “robbed” and the roof allowed to settle, extraction can be increased to 90% or more.  See also: Rock burst

Fig. 3  Room-and-pillar mining; two-stage benching operation.

Shrinkage stoping

This is an overhand method in which broken ore accumulates in the stope, affording temporary support for the walls and a working platform for miners (Fig. 4). Shrinkage stoping is most applicable to steeply dipping veins with strong ore that will stand across a span and with relatively strong wall rock that would slough into the stope in places if left completely unsupported. When ore is broken, it has an expansion or swell factor; this necessitates a periodic drawing (shrinking) of some of the broken ore from the draw points and chutes to allow for continued access to the top of the stope. When all of the ore has been broken except for that left in pillars to protect the adjacent raises and mine levels, the entire content (the magazine) of the stope is drawn. The empty stope may be left open or filled with waste rock, and the pillars may eventually be mined.

Fig. 4  Shrinkage stoping, longitudinal section.

Artificially supported openings

In these methods, workings are kept open during mining by using waste material, timber, and hydraulic props. After the ore is extracted, the workings are filled to maintain stability or are allowed to cave.

Cut-and-fill stoping

This method, also referred to as drift-and-fill, is used in steeply dipping orebodies in which the ore has sufficient strength to be self-supporting but the walls are too weak to stand entirely without support (Fig. 5). Most cut-and-fill stoping is done overhand, with the drilling and blasting phase similar to that in shrinkage stoping; the broken ore, however, is removed from each new cut or slice along the back, and the floor of the stope is built up of waste material such as sand or mill tailings brought in by pipeline as a water slurry. The smooth and compacted or cemented fill material provides an especially suitable floor for rubber-tired machinery. Variations in cut-and-fill mining include the ramp-in-stope system, in which load-haul-dump equipment can move rapidly in and out of the stope on an inclined surface of fill material, and the less-mechanized system of resuing in narrow veins. In resuing, ore and waste material are broken separately and the waste material is left to accumulate as fill. One additional system, undercut-and-fill, is applied to bodies of weaker ore. It provides a solid artificial back of reinforced and cemented fill for the mining of successively underlying slices of ore.

Fig. 5  Cut-and-fill stoping with sand slurry and ramp.

Square set stoping

This is a labor-intensive and high-cost method that has been classically used in situations where the ore is too weak to stand across a wide or long back and the walls are not strong enough to support themselves. A square set, a skeletal box of keyed timbers, is filled and wedged into the available space as each small block of ore is removed by drilling and blasting. Mining continues by overhand or underhand stoping, and the stope becomes a network of interlocked square sets. The sets in the mined portion of the stope are filled with mill tailings or waste rock and pillars are left between mined-out stopes for additional wall support while the remainder of the deposit is being mined. Because of its high cost, square setting is no longer in use; it has been superseded in many mines by cut-and-fill, top slicing, and sublevel caving methods.

Longwall mining

This method is applicable to uniform and extensive but relatively thin deposits. Primarily a highly mechanized and increasingly automated coal mining method at depths where rock pressures are too high for safe room-and-pillar mining, it has also been used in potash deposits and to some extent in bedded iron, copper, and uranium orebodies. In the South African deep gold mines, a form of longwall mining is used in the thin-bedded ore zones.

In longwall mining, practically all of the coal or ore is recovered except for that left in safety pillars to protect surface structures.

The basic practice is to maintain a temporary opening in a uniform line along a working face and then to allow the roof to cave onto the floor or waste fill (gob) behind the active area. In a typical mechanized longwall coal operation, the roof support units are canopies with hydraulic-powered adjustable legs or chocks that are moved ahead as the coal is shaved into slices by shearing and plowing machinery with integrated conveyor systems. In the mining of South African gold reef deposits, longwall-type mining is done by drilling and blasting; the active area is kept open by hydraulic props and timber-concrete packs, and the mined-out areas are filled to some extent by waste rock or cemented mill tailings.

Longwall mining systems allow for a high abutment pressure to build up in solid ore or coal in advance of the face, a low-pressure zone to exist in the working area just behind the face, and a normal lithostatic pressure to build up again in the mined-out and caved or gob-filled area as the face is moved ahead.

Top slice mining

Seldom used today, this method has been applied to wide and steeply dipping deposits with weak ore and weak walls. It has been of use in recovering pillars that have been left between filled stopes. It is a relatively expensive and labor-intensive method with a requirement for abundant timber, but it permits nearly total extraction of the ore. Top slicing is ultimately a caving method of mining, but the ore must first be drilled and blasted, and temporary support is needed between the taking of each successive downward slice or horizontal cut of ore. Working begins in drifts and cross-cuts on a mining floor at the top of a raise; after the driving of a series of adjacent cross-cuts so that a slice of sufficient width has been taken, a mat of timber and scrap lumber is laid down on the floor and the supporting timbers are blasted to cave the overlying rock. A new slice is mined laterally from drifts and cross-cuts under the mat, with the mat supported by timber props (stulls). A mat is again laid down, supports are blasted, and subsequent slices are mined beneath the subsiding accumulation of timber mats and waste rock.

Caving methods

These methods are used in large orebodies with relatively weak ore and with weak wallrock that will collapse as the ore is removed. Geologic conditions must permit subsidence, and the ore must be sufficiently jointed or fractured to form fragments small enough to be handled in drawpoints and raises. Ore recovery in mining is generally quite high, but a certain amount of dilution from waste rock must be accepted.

Sublevel caving

This type is most suited to large and steeply dipping orebodies with weak walls and with ore that has enough stability to maintain sublevels (Fig. 6). It is similar to sublevel open stoping, but in this method the walls and the back are allowed to collapse. The ore is mined in downward increments that are drilled, blasted, and drawn from levels below the ore. Access drifts are driven on the footwall side of the orebody, sublevel cross-cuts are driven in ore, and fans of blastholes are drilled at intervals in the cross-cuts. A steplike succession of slices is mined in retreat from the hanging wall, with the wall rock collapsing and following the extraction of the ore. As each fan of holes is blasted, the broken ore caves into the sublevel, where it is loaded and transported to the ore pass. Broken waste rock fills the void as the ore is drawn. When an excess of waste rock begins to dilute the broken ore, the drawing is stopped and the next fan of holes is blasted.

Fig. 6  Sublevel caving, with stages of development and mining.

Block caving

This is applied to large and relatively uniform bodies in which both ore and waste will cave readily (Fig. 7). Production on the order of 50,000–75,000 tons (45,000–68,000 metric tons) per day can be achieved at a very low mining cost, but the capital cost of a block-caving mine is high. A mine is prepared for block-caving operations by establishing a principal haulage level, driving raises to production levels (slusher or grizzly levels), and driving a larger number of raises to workings on an undercut level beneath the orebody or block to be mined. Caving is initiated by drilling and blasting a slice of ore above the undercut level and, if necessary, by excavating narrow stopes at the boundaries of the block. With the drawing of the initially broken ore, the block begins to cave under its own weight. With further drawing, the entire column of ore and overburden rock continues to subside and break upward for as much as 4000 ft (1220 m) to the surface, where a depression forms. The ore, broken and crushed in caving, flows through cone-shaped draw holes and finger raises. The finger raises are carefully monitored at draw points on the grizzly level so that the caving action is kept uniform and salient channels of subsiding waste rock are not allowed to form prematurely. Broken ore collected from finger raises reaches the haulage level through transfer raises.  See also: Explosive; Mining; Prospecting

William C. Peters

Fig. 7  Block caving, with principal haulage level, driving raises to production (grizzly) levels, and raises to workings.