In 2005, only 17% of metal mines were underground operations (Raw Materials Data, Stockholm 2010) – but there are many instances where underground extraction of minerals is more cost-effective than surface mining, typically when the given ore body is high grade or very deep.
Due to the smaller nature of underground operations, they typically have much lower production rates than surface mines but are also able to be more selective and reduce the amount of waste rock that is mined.
Depending on the mineral being mined and the nature of the deposit, there are many different underground mining methods that can be implemented. New technology has enabled greater productivity and safer mining in underground operations, which has also helped decrease the cost of such production methods.
Cut and Fill Mining
In this selective mining method, the void created by mining is backfilled with a mixture of waste rock or tailings (with cement sometimes added to strengthen the mixture), which helps support the walls of the empty space. This type of mining is typically done upwards from lower levels, so the fill is also used to provide a new working level for further mining.
Cut and fill mining can be expensive, but this method allows for highly selective mining with less waste rock or low-grade ores mined-out (which keeps production costs low). It’s typically used in situations where the orebody dips steeply or is very irregular, which makes mechanized long-hole methods difficult to implement.
The shrinkage stoping method is similar to cut and fill, but instead of removing the ore after blasting and backfilling, the initial broken ore is left in the void to create a working platform for the next level (and to support the wall stability of the stope).
After all the planned levels have been blasted, then all of the ore is removed for processing.
This method is very selective and keeps dilution low, but requires many active stopes because ore is not removed from each mining area until completion – meaning longer lead times for ore to get to the process plant compared to cut and fill operations.
This bulk underground mining method involves mining large amounts of material from a single stope – similar to cut and fill, this method starts at the bottom of a level and moves upward. Ore is removed from the bottom, and then more ore is blasted from a higher level that falls to the same level to be removed, with the process repeating up the orebody.
The supporting walls need to be very strong in order to support the large underground openings that will be created by this process.
Room & Pillar
Room-and-pillar mining is typically chosen for flat-lying (or at slightly dipping) ore bodies. Commonly used for base metal or uranium metal deposits, or bedded seams of coal/potash/salt, mining is done by creating openings (rooms) on a single level, leaving pillars of rock at regular intervals to support the weight of the material above (the roof).
In hard-rock deposits (i.e. copper, lead-zinc), drilling and blasting is required in order to break up the ore before being able to remove it.
After mining out levels, the pillars may be removed (to recover the remaining ore or material) and the roof is allowed to safely collapse and fill in the mined out area.
Block caving is essentially the underground version of open-pit mining. It’s the only underground mining method that can reach similar production rates to surface mining operations, up to over 100,000 tonnes per day.
The method involves undermining an ore body, then allowing it to collapse under its own weight. The orebody is drilled and blasted and the collapsed ore is removed through a haulage access, and as more material is removed the orebody caves in.
This mining method is useful because it allows for huge volumes of material to be mined at relatively low costs, which makes lower grade deposits economical to turn into mines or new pits. Many large-scale open-pit operations have plans to progress into block caving operations over time, including Oyu Tolgoi and Grasberg.
This mining method is typically used in situations where the orebody is both large and steeply dipping, and because of the depth below surface is not suitable for surface mining methods.