PrintLesson 8.3: Mining by Unsupported Methods
You will recall from Lesson 4.3 of Module 4 that underground mining methods are traditionally placed 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.
We are going to focus on the class of unsupported methods in this lesson. If the rock is essentially self-supporting and only requires the addition of minimal artificial supports to achieve a stable opening, then one of the methods from the unsupported class will most likely be applicable.
The three important methods within this class are room and pillar, shrinkage stoping, and open stoping. After a brief summary here, we’ll look at each in more detail. We will not talk in detail about the unit and auxiliary operations associated with these methods, as these were covered earlier in the course. Suffice it to say that a conventional mining cycle is used for shrinkage stoping and open stoping, whereas both continuous and conventional cycles are employed with the room and pillar method, depending on the commodity being mined. Examples of commonly used equipment will be noted for the different 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 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 I believe globally as well, although I have not done that analysis. Examples of commodities mined by this method include coal, limestone, salt, trona, lead, and potash.
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.
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.