Underground mining (hard rock)

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A three-dimensional model of a modern underground mine with shaft access Underground 3D model.jpg
A three-dimensional model of a modern underground mine with shaft access

Underground hard rock mining refers to various underground mining techniques used to excavate hard minerals, usually those containing metals [1] such as ore containing gold, silver, iron, copper, zinc, nickel, tin and lead, but also involves using the same techniques for excavating ores of gems such as diamonds or rubies. Soft rock mining refers to excavation of softer minerals such as salt, coal, or oil sands.

Mining The extraction of valuable minerals or other geological materials from the earth

Mining is the extraction of valuable minerals or other geological materials from the earth, usually from an ore body, lode, vein, seam, reef or placer deposit. These deposits form a mineralized package that is of economic interest to the miner.

Mineral Element or chemical compound that is normally crystalline and that has been formed as a result of geological processes

A mineral is, broadly speaking, a solid chemical compound that occurs naturally in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are usually excluded, but some minerals are often biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings often synthesize inorganic minerals that also occur in rocks.

Metal element, compound, or alloy that is a good conductor of both electricity and heat

A metal is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typically malleable or ductile. A metal may be a chemical element such as iron, or an alloy such as stainless steel.


Mine access

Underground access

Accessing underground ore can be achieved via a decline (ramp), inclined vertical shaft or adit.

Ore rock with valuable metals, minerals and elements

An ore is an occurrence of rock or sediment that contains sufficient minerals with economically important elements, typically metals, that can be economically extracted from the deposit. The ores are extracted from the earth through mining; they are then refined to extract the valuable element, or elements.

Shaft mining construction which connect underground deposits together or with the surface

Shaft mining or shaft sinking is excavating a vertical or near-vertical tunnel from the top down, where there is initially no access to the bottom.

Adit horizontal entrance shaft to an underground mine

An adit is an entrance to an underground mine which is horizontal or nearly horizontal, by which the mine can be entered, drained of water, ventilated, and minerals extracted at the lowest convenient level. Adits are also used to explore for mineral veins.

Decline portal Wiluna underground.jpg
Decline portal

A box cut is a small open cut built to supply a secure and safe entrance as access to a slope to an underground mine. Generally the box cut is sunk until sufficiently unweathered rock is found to permit the development of the decline. The portal is generally made safe with rock bolts, wire mesh, and shotcrete, which prevents accidental rock falls from closing access to the decline.

Overburden rock and subsoil that lies above a mineral deposit such as a coal seam

In mining, overburden is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from tailings, the material that remains after economically valuable components have been extracted from the generally finely milled ore. Overburden is removed during surface mining, but is typically not contaminated with toxic components. Overburden may also be used to restore an exhausted mining site to a semblance of its appearance before mining began.

Bedrock Lithified rock under the regolith

In geology, bedrock is the lithified rock that lies under a loose softer material called regolith at the surface of the Earth or other terrestrial planets. The broken and weathered regolith includes soil and subsoil. The surface of the bedrock beneath the soil cover is known as rockhead in engineering geology, and its identification by digging, drilling or geophysical methods is an important task in most civil engineering projects. Superficial deposits can be extremely thick, such that the bedrock lies hundreds of meters below the surface.

Declines are often started from the side of the high wall of an open cut mine when the ore body is of a payable grade sufficient to support an underground mining operation, but the strip ratio has become too great to support open cast extraction methods. They are also often built and maintained as an emergency safety access from the underground workings and a means of moving large equipment to the workings.

Ore access

Levels are excavated horizontally off the decline or shaft to access the ore body. Stopes are then excavated perpendicular (or near perpendicular) to the level into the ore.

Development mining vs. production mining

There are two principal phases of underground mining: development mining and production mining.

Development mining is composed of excavation almost entirely in (non-valuable) waste rock in order to gain access to the orebody. There are six steps in development mining: remove previously blasted material (muck out round), scaling (removing any unstable slabs of rock hanging from the roof and sidewalls to protect workers and equipment from damage), installing support or/and reinforcement using shotcrete etceteras, drill face rock, load explosives, and blast explosives. To start the mining, the first step is to make the path to go down. The path is defined as 'Decline' as describe above. Before the start of Decline all preplanning of Power facility, drilling arrangement, dewatering, ventilation and, muck withdrawal facilities are required. [2]

Shotcrete building material

Shotcrete, gunite or sprayed concrete is concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface, as a construction technique. It is typically reinforced by conventional steel rods, steel mesh, or fibers.

Production mining is further broken down into two methods, long hole and short hole. Short hole mining is similar to development mining, except that it occurs in ore. There are several different methods of long hole mining. Typically, long hole mining requires two excavations within the ore at different elevations below surface, (15 m – 30 m apart). Holes are drilled between the two excavations and loaded with explosives. The holes are blasted and the ore is removed from the bottom excavation.


Door for directing ventilation in an old lead mine. The ore hopper at the front is not part of the ventilation. Smallcleugh door.jpg
Door for directing ventilation in an old lead mine. The ore hopper at the front is not part of the ventilation.

One of the most important aspects of underground hard rock mining is ventilation. Ventilation is the primary method of clearing hazardous gases and/or dust which are created from drilling and blasting activity (e.g., silica dust, NOx), diesel equipment (e.g., diesel particulate, carbon monoxide), or to protect against gases that are naturally emanating from the rock (e.g., radon gas). Ventilation is also used to manage underground temperatures for the workers. In deep, hot mines ventilation is used to cool the workplace; however, in very cold locations the air is heated to just above freezing before it enters the mine. Ventilation raises are typically used to transfer ventilation from surface to the workplaces, and can be modified for use as emergency escape routes. The primary sources of heat in underground hard rock mines are virgin rock temperature, machinery, auto compression, and fissure water. Other small contributing factors are human body heat and blasting.

Ground support

Some means of support is required in order to maintain the stability of the openings that are excavated. This support comes in two forms; local support and area support.

Area ground support

Area ground support is used to prevent major ground failure. Holes are drilled into the back (ceiling) and walls and a long steel rod (or rock bolt) is installed to hold the ground together. There are three categories of rock bolt, differentiated by how they engage the host rock. [3] They are:

Mechanical bolts

  • Point anchor bolts (or expansion shell bolts) are a common style of area ground support. A point anchor bolt is a metal bar between 20 mm – 25 mm in diameter, and between 1 m – 4 m long (the size is determined by the mine's engineering department). There is an expansion shell at the end of the bolt which is inserted into the hole. As the bolt is tightened by the installation drill the expansion shell expands and the bolt tightens holding the rock together. Mechanical bolts are considered temporary support as their lifespan is reduced by corrosion as they are not grouted. [3]

Grouted bolts

  • Resin grouted rebar is used in areas which require more support than a point anchor bolt can give. The rebar used is of similar size as a point anchor bolt but does not have an expansion shell. Once the hole for the rebar is drilled, cartridges of polyester resin are installed in the hole. The rebar bolt is installed after the resin and spun by the installation drill. This opens the resin cartridge and mixes it. Once the resin hardens, the drill spinning tightens the rebar bolt holding the rock together. Resin grouted rebar is considered a permanent ground support with a lifespan of 20–30 years. [3]
  • Cable bolts are used to bind large masses of rock in the hanging wall and around large excavations. Cable bolts are much larger than standard rock bolts and rebar, usually between 10–25 metres long. Cable bolts are grouted with a cement grout. [3]

Friction bolts

  • Friction stabilizer (frequently called by the genericized trademark Split Set) are much easier to install than mechanical bolts or grouted bolts. The bolt is hammered into the drill hole, which has a smaller diameter than the bolt. Pressure from the bolt on the wall holds the rock together. Friction stabilizers are particularly susceptible to corrosion and rust from water unless they are grouted. Once grouted the friction increases by a factor of 3-4. [3]
  • Swellex is similar to Friction stabilizers, except the bolt diameter is smaller than the hole diameter. High pressure water is injected into the bolt to expand the bolt diameter to hold the rock together. Like the friction stabilizer, swellex is poorly protected from corrosion and rust. [3]

Local ground support

Local ground support is used to prevent smaller rocks from falling from the back and ribs. Not all excavations require local ground support.

Stope and retreat vs. stope and fill

Stope and retreat

Sub-Level Caving Subsidence reaches surface at the Ridgeway underground mine. Elura.png
Sub-Level Caving Subsidence reaches surface at the Ridgeway underground mine.

Using this method, mining is planned to extract rock from the stopes without filling the voids; this allows the wall rocks to cave in to the extracted stope after all the ore has been removed. The stope is then sealed to prevent access.

Stope and fill

Where large bulk ore bodies are to be mined at great depth, or where leaving pillars of ore is uneconomical, the open stope is filled with backfill, which can be a cement and rock mixture, a cement and sand mixture or a cement and tailings mixture. This method is popular as the refilled stopes provide support for the adjacent stopes, allowing total extraction of economic resources.

Mining methods

Schematic diagram of cut and fill mining Cut and fill schematic.png
Schematic diagram of cut and fill mining

The mining method selected is determined by the size, shape, orientation and type of orebody to be mined. The orebody can be narrow vein such as a gold mine in the Witwatersrand, the orebody can be massive similar to the Olympic Dam mine, South Australia, or Cadia-Ridgeway Mine, New South Wales. The width or size of the orebody is determined by the grade as well as the distribution of the ore. The dip of the orebody also has an influence on the mining method for example a narrow horizontal vein orebody will be mined by room and pillar or a longwall method whereas a vertical narrow vein orebody will be mined by an open stoping or cut and fill method. Further consideration is needed for the strength of the ore as well as the surrounding rock. An orebody hosted in strong self-supporting rock may be mined by an open stoping method and an orebody hosted in poor rock may need to be mined by a cut and fill method where the void is continuously filled as the ore is removed.

Selective mining methods

Block mining methods

Orebodies that do not cave readily are sometimes preconditioned by hydraulic fracturing, blasting, or by a combination of both. Hydraulic fracturing has been applied to preconditioning strong roof rock over coal longwall panels, and to inducing caving in both coal and hard rock mines.

Ore removal

In mines which use rubber tired equipment for coarse ore removal, the ore (or "muck") is removed from the stope (referred to as "mucked out" or "bogged") using center articulated vehicles (referred to as boggers or LHD (Load, Haul, Dump machine)). These pieces of equipment may operate using diesel engines or electric motors, and resemble a low-profile front end loader. LHD operated through electricity utilize trailing cables which are flexible and can be extended or retracted on a reel. [9]

The ore is then dumped into a truck to be hauled to the surface (in shallower mines). In deeper mines, the ore is dumped down an ore pass (a vertical or near vertical excavation) where it falls to a collection level. On the collection level, it may receive primary crushing via jaw or cone crusher, or via a rockbreaker. The ore is then moved by conveyor belts, trucks or occasionally trains to the shaft to be hoisted to the surface in buckets or skips and emptied into bins beneath the surface headframe for transport to the mill.

In some cases the underground primary crusher feeds an inclined conveyor belt which delivers ore via an incline shaft direct to the surface. The ore is fed down ore passes, with mining equipment accessing the ore body via a decline from surface.

Deepest mines

See also

Related Research Articles

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Rock mechanics is a theoretical and applied science of the mechanical behavior of rock and rock masses; compared to geology, it is that branch of mechanics concerned with the response of rock and rock masses to the force fields of their physical environment.

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Stoping is the process of extracting the desired ore or other mineral from an underground mine, leaving behind an open space known as a stope. Stoping is used when the country rock is sufficiently strong not to collapse into the stope, although in most cases artificial support is also provided.

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  2. https://www.math.uwaterloo.ca/~nwormald/papers/DOT2.pdf.;Decline design in underground mines using constrained path optimisation
  3. 1 2 3 4 5 6 Puhakka, Tulla (1997). Underground Drilling and Loading Handbook. Finland: Tamrock Corporation. pp. 153–170.
  4. 1 2 3 Puhakka, Tulla (1997). Underground Drilling and Loading Handbook. Finland: Tamrock Corporation. pp. 98–130.
  5. http://www.sciencechannel.com/tv-shows/dirty-great-machines/dirty-great-machines-videos/the-in-the-hole-drill/
  6. https://mining.cat.com/cda/files/2785508/7/Creighton_Eng.pdf.;VIEWPOINT: perspectives on modern mining; page 2; Quote: 'Vertical retreat mining (VRM) was introduced in the mid-1980s to replace the cut-and-fill mining method. The slot-slash mining method, a modified VRM, was introduced in the late 1990s and replaced the VRM mining.'
  7. "Mining & Metallurgy 101". www.miningbasics.com.
  8. Fowler, J. C. W.; Hebblewhite, B. K. (2003). "Mining publication" (PDF). New South Wales.
  9. http://www.mineweb.com/archive/greGreener underground mining
  10. "TauTona, Anglo Gold, South Africa". 2009.
  11. "Mineral deposits: from their origin to their environmental impacts". Taylor & Francis.

Further reading