Underground mining is a method of extracting ore, minerals, and other geological materials from beneath the earth's surface. It is utilized when the mineral deposit cannot be recovered efficiently by surface mining, or when the deposit is located at a significant depth, thereby requiring a tunnel-based approach to reach it.
Methods used in underground mining are generally classified based on the type of deposit being mined and the type of ground conditions at the site. The three main types of underground mining are room-and-pillar, cut-and-fill, and block caving.
The process of underground mining is aimed at ensuring safety, economic viability, and minimal waste. Access to an underground mine can be via a horizontal or vertical tunnel, referred to as an adit, shaft, or decline.
In practical terms, underground mining is used in instances where the ore body is too deep for profitable open-pit mining when the orebody's quality or grades can offset the costs, and when the need for a smaller ground footprint is preferred, such as in comparison to open-pit mining. The technique of narrow vein stoping is an example of a method used in underground mining.
Underground mining methods used by companies like Anglo American plc (LSE: AAL) (OTC: NGLOY) include room and pillar, narrow vein stoping, and large-scale mechanized mining.
What is Room and Pillar in Underground Mining?
Room and pillar mining is a way to mine where tunnels are dug like a chessboard with big square pillars in between. As mining continues, these pillars are slowly removed. This technique is often used to mine coal.
Room-and-pillar mining is one of the oldest and most used methods in underground mining. It is primarily employed for relatively flat-lying, or gently dipping ore bodies where the surrounding rock is competent enough to support itself. The name derives from the method's characteristic use of "rooms" that are excavated and "pillars" that are left untouched to support the overburden.
In room-and-pillar mining, a network of entries, often in a grid-like pattern, like a chessboard, is driven into the mineral seam. These entries or "rooms" are then separated by rows of "pillars" made up of unexcavated material, providing structural support for the ceiling or "roof." Once the deposit is mined out, the pillars are sometimes left in place in a process known as "non-retreat" mining, or they are progressively removed in "retreat" or "pillar recovery" mining, causing controlled roof collapses to extract more material.
This method is generally considered safer than some other forms of underground mining. However, it does have a limitation in terms of recovery rate, as a significant amount of ore may be left behind in the form of pillars to prevent collapse.
What is Cut and Fill in Underground Mining?
Cut and fill mining is an underground mining method used to mine steeply dipping or irregular ore bodies. It is useful for mining at depths where surface support is unavailable.
Cut and fill is a very selective method, which allows for high recovery rates, and it is typically used where the quality of the ore varies significantly, or to minimize dilution of the ore. This method is often used to mine precious metals and base metals.
The process involves digging (cutting) into the ore, which creates an open space (known as a stope). Once a certain section of the ore has been excavated, the open space is then filled with waste material (backfill), providing a working platform for miners to stand on to access the next cut of ore above. This sequence of cutting and then filling is repeated until the entire ore body has been mined.
This method of mining can be relatively safe as the filling provides support to the walls and prevents collapse. However, it can be slow and expensive because of the additional process of backfilling. Furthermore, the process requires careful planning and management to ensure that the filling and cutting are coordinated properly.
The cut-and-fill process involves a cycle of steps that are repeated until the ore body is completely mined out.
What is Narrow Vein Stoping?
Narrow vein stoping is a mining method where minerals are dug out from long, thin areas of mineral-rich rock, known as 'veins'. This method also includes digging spaces for machinery and for access like tunnels and lifts. This approach is often used to mine platinum.
What is Large-Scale Mechanised Mining?
Large-scale mechanized mining refers to a modern and highly efficient form of mining that relies heavily on advanced machinery and automation. This type of mining is typically used when large quantities of minerals or ores need to be extracted over a wide area or from a large deposit.
Mechanized mining operations can include the use of longwall mining, block caving, and sub-level caving methods. These techniques can increase productivity, improve safety, and reduce the environmental impact of mining.
Because it costs a lot of money to buy the machines and set up the infrastructure, large-scale mechanized mining is usually used when the potential profit from the mineral deposit is big enough to cover these costs.
The techniques and machinery used can vary significantly depending on the specific mineral or ore being mined, the local geology, and other factors.
What is Longwall Mining?
Longwall mining is a very efficient way to mine coal underground. It uses a machine, called a shearer, that moves back and forth, cutting and taking out the coal.
The shearer is equipped with large, rotating cutting drums that slice into the coal seam. As the shearer moves along the face of the coal seam, hydraulic-powered supports (called shields) hold up the roof of the mine in the area where the coal has been removed. These shields, which are systematically moved as mining progresses, provide a safe working environment for miners and equipment.
Behind the shields, the mine roof is allowed to naturally collapse, which helps prevent surface subsidence (ground sinking). The coal that's been cut is transported by a conveyor belt to the surface.
Longwall mining is known for being able to get a lot of coal out of the ground — it can get about 75% or more from a coal seam, while room-and-pillar mining gets about 60%. It's usually used for flat coal seams. However, it can be expensive to set up and maintain, and it only works in certain types of ground.
What is Block Caving in Underground Mining?
Block caving is a cheap way to mine a lot of ore from underground, especially for less valuable minerals like copper and diamond. It works best when there is a lot of deep ore, evenly spread out.
The method involves undermining an ore body, causing it to collapse under its own weight into a series of production levels beneath it. The process is initiated by excavating a tunnel below the ore body and then, by drilling and blasting, creating a large cavern or 'undercut'. This leads to the formation of a network of cracks above the undercut.
Gravitationally induced stress on the cracks then causes the ore to break and cave into the undercut. The ore is subsequently drawn off through draw points and is transported to the surface for processing.
Block caving requires careful planning and monitoring due to the potential for surface subsidence as the orebody caves in, but its cost per tonne of extracted ore is generally low compared to other underground mining methods.
This technique is chosen when mineral deposits are deep and open-pit mining is not economically feasible. However, it does require a significant upfront investment in infrastructure and may take years before production begins.
This method often offers more safety to the surrounding environment compared to surface mining and is usually more cost-effective when mining high-grade ores.
What is Sub-Level Caving?
Sub-level caving is an underground mining method used to extract large volumes of ore from bodies that are steeply dipping or vertical. It is a productive and cost-effective mining method preferred for relatively low-cost, low-grade and large mineral deposits.
In sub-level caving, tunnels (sub-levels) are developed in the ore body, at different heights.
The ore is then blasted and falls under gravity to be collected at a haulage level beneath the blasting area. From there, it's transported to the surface for further processing.
Blasting occurs sequentially, starting from the highest level and moving down, allowing for the overlying rock mass and ore to cave and fill the void left by the removed ore. This continual caving process means that the mining operation can keep running continuously.
While sub-level caving can be a relatively cheap and efficient way to mine large volumes of ore, it can also cause substantial surface subsidence as the ground caves in, making it unsuitable for some locations. Additionally, it may result in a higher amount of dilution, which is the unwanted mixing of ore with waste material.
This concludes our overview of underground mining. We also provide an in-depth metals and mining guide and an overview of the history and evolution of mining, offering further insights into investing in the metals and mining sector. You may enjoy an article on the profitability of different miners or you could evaluate the investment potential of metals like gold and copper.