Underground hard-rock mining
Underground hard-rock mining refers to various underground mining techniques used to excavate "hard" minerals, usually those containing metals, such as ore containing gold, silver, iron, copper, zinc, nickel, tin, and lead. It also involves the same techniques used to excavate ores of gems, such as diamonds and rubies. Soft-rock mining refers to the excavation of softer minerals, such as salt, coal, and oil sands.
Mine access
Underground access
Accessing underground ore can be achieved via a decline, inclined vertical shaft or adit.- Declines can be a spiral tunnel which circles either the flank of the deposit or circles around the deposit. The decline begins with a box cut, which is the portal to the surface. Depending on the amount of overburden and quality of bedrock, a galvanized steel culvert may be required for safety purposes. They may also be started into the wall of an open cut mine.
- Shafts are vertical excavations sunk adjacent to an ore body. Shafts are sunk for ore bodies where haulage to surface via truck is not economical. Shaft haulage is more economical than truck haulage at depth, and a mine may have both a decline and a ramp.
- Adits are horizontal excavations into the side of a hill or mountain. Adits are used for horizontal or near-horizontal ore bodies where there is no need for a ramp or shaft.
Ore access
Levels are excavated horizontally off the decline or shaft to access the ore body. Stopes are then excavated 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 waste rock in order to gain access to the orebody. There are six steps in development mining: remove previously blasted material, scaling, installing support or/and reinforcement using shotcrete or other supports, 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 a decline, all pre-planning of the power facility, drilling arrangement, de-watering, ventilation and, muck withdrawal facilities are required.
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 methods of long hole mining. Typically, long hole mining requires two excavations within the ore at different elevations below surface. Holes are drilled between the two excavations and loaded with explosives. The holes are blasted, and the ore is removed from the bottom excavation.
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, diesel equipment, or to protect against gases that are naturally emanating from the rock. 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 and walls and a long steel rod is installed to hold the ground together. There are three categories of rock bolt, differentiated by how they engage the host rock. They are:Mechanical bolts
- Point anchor 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. 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.
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.
- 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 6 and 25 metres long. Cable bolts are grouted with a cement grout.
Friction bolts
- Friction stabilizer 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.
- 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.
Local ground support
- Welded Wire Mesh is a metal screen with 10 cm x 10 cm openings. The mesh is held to the backs and walls using friction stabiliser bolts, point anchor bolts, or resin grouted rebar.
- Shotcrete is fibre-reinforced spray-on concrete which coats the back and ribs preventing smaller rocks from falling. Shotcrete thickness can be between 50 mm – 100 mm.
- Latex Membranes can be sprayed on the backs and ribs similar to shotcrete, but in smaller amounts.
Stope and retreat vs. stope and fill
Stope and retreat
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.Methods
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
- Cut and fill mining is a method of short-hole mining used in steeply dipping or irregular ore zones, in particular where the hanging wall limits the use of long-hole methods. The ore is mined in horizontal or slightly inclined slices, and then filled with waste rock, sand or tailings. Either fill option may be cemented with binders to add cohesion to the matrix or left uncemented. Cut and fill mining is an expensive but selective method, with the advantages of low ore loss and dilution.
- Drift and fill is similar to cut and fill, except that it is used in ore zones, which are wider than the method of drifting will allow to be mined. In this case, the first drift is developed in the ore, and is backfilled using consolidated fill. The second drift is driven adjacent to the first drift. This carries on until the ore zone is mined out to its full width, at which time the second cut is started atop of the first cut.
- Shrinkage stoping is a short-hole mining method which is suitable for steeply dipping orebodies. This method is similar to cut and fill mining with the exception that after being blasted, broken ore is left in the stope where it is used to support the surrounding rock and as a platform from which to work. Only enough ore is removed from the stope to allow for drilling and blasting the next slice. The stope is emptied when all of the ore has been blasted. Although it is very selective and allows for low dilution, since most of the ore stays in the stope until mining is completed, there is a delayed return on capital investments.
- VRM/ VCR: Vertical retreat mining also known as Vertical crater retreat is a method where mine is divided in vertical zones with depth of about 50 meters using open stoping, bottom-up mining. Long-hole large-diameter holes are drilled vertically into the ore body from the top using in-the-hole drills, and then blasting horizontal slices of the ore body into an undercut. Ore blasted in retrieval taken in phase. This retrieval is done from bottom of the section developed. Last cleaning of ore is done through remote controlled LHD machines. A system of primary and secondary stopes is often used in VCR mining, where primary stopes are mined in the first stage and then backfilled with cemented fill to provide wall support for the blasting of successive stopes. Side chambers will be mined in pre-planned sequence after the fill has solidified.