Print4.3.2: Underground Mining Methods
Underground mining methods become necessary when the stripping ratio becomes uneconomical, or occasionally when the surface use of the land would prohibit surface mining. Underground methods are traditionally broken into three classes: unsupported, supported, and caving methods. These classes reflect the competency of the orebody and host rock more than anything else. If you excavate an underground opening in the ore or the rock, is the opening stable -- i.e., will it remain open for an extended period, or will it begin to fall in? If it is unstable, i.e., the surrounding ore or rock breaks up and falls into the opening, how much support would be required to keep the opening from caving in? The answers to these questions lead us to choose mining methods from one of the three classes. Unsupported methods require the addition of minimal artificial supports to secure a stable opening, whereas the supported methods require the addition of major support to keep the openings from caving in. Finally the third class is, at first glance, counterintuitive: in general our goal is to create stable openings underground for obvious reasons, but the methods in this class will only work if the host rock or orebody will cave easily under its own weight -- the caving methods actually depend on this caving action to function safely and productively!
Unsupported Methods
Room and Pillar mining
This method of mining is used to recover bedded deposits that are horizontal or nearly horizontal when the orebody and the surrounding rock are reasonably competent. Parallel openings are mined in the ore, i.e., rooms, and blocks of ore, i.e., pillars, are left in place to support the overlying strata. Other than the pillars, little artificial support is required and often consists of bolts placed into the overlying strata to pin the layers together, making them behave like a strong laminated beam. A few examples of commodities mined by this method would include coal, lead, limestone, and salt. Historically, if the pillars were irregular in size and placement, which is more likely to occur in certain metal and nonmetal deposits, this method was known as stope and pillar, rather than room and pillar. You will still hear the word stope and pillar being used, but the distinction is now largely irrelevant. This method accounts for the vast majority of all underground mining in the U.S, and likely the world. Watch this video (2:58) created by Caterpillar showing the use of their equipment in room and pillar mining.
Shrinkage stoping
Shrinkage stoping is used to recover steeply dipping orebodies when the ore and host rock are reasonably competent. A stope, i.e., a large section of the mine where active production is occurring, is mined, but the broken ore is not removed, but rather is left in place to support the walls of the stope until the time when all of the broken ore will be removed. Since rock swells, i.e., increases in volume when it is broken, it is necessary to draw off some of the broken ore as the stope is progressively mined. The name of this method derives from this drawing off or shrinkage of the stope. A modern and important variant of this method is known as vertical crater retreat (VCR) mining. A few examples of commodities mined by this method include iron and palladium. Watch this video (3:01) created by Atlas Copco demonstrating sublevel stoping mining method.
Open stoping
This type of mining is used to recover steeply dipping orebodies in competent rock. The ore is removed from the stope as soon as it is mined. Sublevel stoping and big-hole stoping are the important variants in use today. A few examples of commodities mined by this method include iron and lead/zinc.
Supported Methods
Supported methods historically included cut and fill stoping, stull stoping, and square set stoping. However, the last two are no longer used due to their extreme cost. We’ll confine our discussion to cut and fill stoping.
Cut and fill
Cut and fill is used to recover ore from weaker strength materials, in which the openings will not remain stable after the ore is removed, and the overlying strata cannot be allowed to cave. A slice of the orebody is mined and immediately after the ore is removed, backfill is placed into the opening to support the ore above. The next slice is removed, the cut is then backfilled, and the process repeats. As you might imagine, this is a very expensive method to use, and consequently, it would be used only for the recovery of high value ores. An example of a commodity mined by this method is gold. Watch this video (2:58) created by Altas Copco on Cut and Fill mining method.
Caving methods
Caving methods include block caving, sublevel caving, and longwall mining. For emphasis, allow me to repeat what I said earlier: caving methods are used in settings where the ore or the host rock is so weak that it cannot support its own weight for any period of time; the methods only work if the rock or the ore will readily cave under its own weight.
Block caving
This method is used in weak and massive orebodies, in which the ore is undercut, and then as the broken ore is removed the remainder of the orebody collapses into this void, and as more ore is withdrawn, the caving continues. Typically the host rock is fairly strong, although ultimately it tends to cave into the void created from removing the ore. The fracturing and caving often break through to the surface. Watch this video (3:16) created by Atlas Copco on Block Caving Mining Method
Sublevel caving
This type of caving is used in strong and massive orebodies in which the host rock is very weak and quickly caves into the void created by removing the core. As in block caving, the cave will ultimately reach the surface. Watch this video (3:05) created by Atlas Copco on sublevel caving mining methods.
Longwall mining
Longwall mining is a type of caving, applied to a horizontal tabular deposit such as coal. While block and sublevel caving are essentially vertically advancing metal mining methods, longwall mining is applied to relatively thin and flat-lying deposits – most often coal, but occasionally an industrial mineral such as trona. The coal seam is extracted completely between the access roads, and then as mining retreats, the overlying strata caves into the void left by removing the coal. Watch this video (5:31) created by Clearcut Mining Solutions showing logwall mining method.
Our goal in attempting to classify mining methods is to make it easier to learn the methods, because methods in a given class tend to work best in similar circumstances. Similarly, there tend to be just a few factors that differentiate the methods. By examining the classification scheme, we make it easier to remember the methods and the characteristics under which they can or cannot be used. It’s also useful to note that there is nothing sacred about the choice of a method. If five years down the road the characteristics of the deposit are changing, then another method will be employed. There are examples of mines utilizing three different mining methods over a 15-year period, as they adapt the mining method to the evolving geological conditions. Sometimes, one method is employed as the primary mining method, but another is used on retreat to recover pillars, for example. We’ll look at some of those cases later as well.