9.2.1: Deposit and Spatial Terms

If we are in a horizontal deposit and we mine an opening into the deposit, that opening will have a top and a bottom. We often use those terms. We might say, “this mine has good top,” and we mean that the rock structure at the top of the opening is competent. Or, we might say, “that seam is underlain by clay, and the bottom is terrible,” and we mean that it is difficult for equipment to move around because of the poor condition of the bottom. In a tabular deposit, we frequently refer to the top as the roof, and the bottom as the floor. Conceptually, this aligns with our everyday experiences – if someone refers to the roof or the floor, we know what they mean. Likewise, if we refer to the top or bottom of our mined opening, we intuitively know what is meant. As I indicated, these four terms, top, bottom, roof, and floor, work well for tabular deposits, and are used in coal, trona, potash, salt, and stone mines, for example.

When we move into metal mines, we will not hear those terms. There are a few reasons for this. The mining methods and culture evolved differently in metal versus coal and some of the nonmetals, and they each evolved their own terminology. It’s similar to the situation with the word we use to describe a soft drink. In some parts of the country we would ask for a bottle of “pop.” In another region, we would request a bottle of “soda.” If you go into an area and use the “wrong” term, people may snicker and smile, but they will know what you mean. The same is true for some of these mining terms, except that in addition to smiling, they will likely think that you are a rookie and perhaps are not really a mining person. If, in fact, you are a mining engineer, then that would not be a good thing!

The second reason for the difference in terms relates to the spatial complexity of many metal mines and metalliferous the deposit. In a steeply dipping deposit, for example, the concept of a roof and floor is less useful than hanging wall, footwall, and back. And, of course, there can be crossover in terms. A mining engineer working in a tabular limestone deposit will refer to the top of the opening as the roof, unless they have a background in the metal industry, and then they will refer to it as the back. Don’t despair! As you look at the figures, study the definitions, and read more about the methods, it will become perfectly clear. On with the definitions!

Initially, students are uncertain about which is the footwall and hanging wall. If you have doubts, remember this: these two terms only have meaning if the deposit is dipping at an angle; and pretend the opening in the deposit is a sliding board, and that you are going to slide down – and when you do, your butt will ride on the footwall. Now you’ve got a silly but effective way to keep it straight. Speaking of the footwall, you will notice that the shaft and the workings are located in the country rock and on the footwall side of the deposit, not the hanging wall. There’s a good reason for that. Why do you think that is the case?

All right, onward with the definitions! Here are two more of major importance.

We’ve talked on various occasions about deposits that dip, and so you already have a familiarity with the term. A synonym that is sometimes used is pitching. The terms steeply pitching or a steeply dipping are synonymous. This is true whether it is a seam, vein, deposit, or orebody.

If you’re a surveyor or a geologist, then you are very familiar with the concept of strike. The dip of a deposit can have a profound impact on the type of mine that we develop, but not so for the strike! The strike provides us a direction on the compass to orient our workings, but little more than that for the mining engineer. Now, for structural and economic geologists… they can get excited by the strike of the vein!

In the process of excavating an opening, i.e., mining, we will often leave behind sections of the orebody, for the sole purpose of providing structural support to the overlying layers of rock. These unmined blocks are called pillars, and they are necessary to prevent a collapse of the mined openings. Certain pillars are given specific names, as follows.

Crown and sill pillars are commonly found in mines for steeply pitching deposits, and barrier and yield pillars are commonly found in tabular deposits that are nearly horizontal.

Finally, within this first group, three more terms:

9.2.2: Directional Terms

The directional terms are quite useful and not too difficult to remember. Let me give you an example to illustrate these terms before I give you the formal definition. Imagine that we are all in a big classroom, and the classroom represents the mined out opening. This classroom has rows of chairs and then near the front of the room there is a table on which I lay my books and materials, and then we have a chalkboard on the front wall. I stand behind the table to lecture. There is a door into the classroom on the rear wall. Got the picture?

Ok, let’s imagine that we are mining in the direction of the chalkboard, i.e., we are advancing in the direction of the chalkboard. We’re going to drill holes into the chalkboard, load the holes with explosives, blast, and load out the broken material. We’ve just made the classroom bigger! The chalkboard where we drilled and blasted is known as the face or working face. It is also known as the breast. The act of mining in this horizontal direction is known as breast stoping.

Now, let’s suppose that I’d like to make the classroom higher rather than longer. So, I am going to drill holes into the ceiling, load the holes with powder, shoot them, and load out the broken material. When I advance in this upward direction, it is said to be overhand stoping. Similarly, if I wanted to enlarge our classroom to a lower level, I would drill down into the floor, blast, and load the broken material. When I advance in a downward direction, it is called underhand stoping.

Finally, I can squeeze one last example out of this classroom setting to help you understand the terms inby and outby. These two terms are very useful to state a relative position. Typically, the relative position is between the working face and the entrance to the mine or some portion of the mine. In our classroom that we are pretending is a mine opening, we have a working face (chalkboard wall) and an entrance (the door in the rear wall). Remember the table near the front of the room. If I am standing behind the table, close to the chalkboard, I am standing inby the table, and you are sitting outby the table. If we had a video camera set up in the fourth row back, the students in rows one through three would be sitting inby the camera, and the students sitting in row five to the back of the room would be sitting outby the camera.

This business of inby and outby may seem a little strange, but these words are extremely useful. There is for example, a regulation that prohibits miners from working inby the last row of roof bolts. Or another that allows certain electrical equipment to be used only if it is outby the last open crosscut. Hopefully, with this example, the following terms will be clearer.

9.2.3: Excavation Terms

These terms describe the types of openings that are created to facilitate mining of the deposit. The adjectives primary, secondary, and tertiary are used to characterize some of the terms. This is to indicate the relative importance of the opening in the same sense as saying interstate highways are primary roadways, two-lane streets in a city are secondary roadways, and the alleys between some streets are tertiary roadways. Some of these terms are specific to certain mining methods, and when we study those methods, they will become clearer. I want you to understand all of them, but for right now, focus on the ones that I have highlighted in red.

If you haven’t done so already, this would be a good time to look at the figures.

This figure from the text illustrates the basic infrastructure for a mine in a steeply pitching deposit. Generally, these mines are spatially complex.

Figure 9.2.1: Basic infrastructure for a mine in a steeply pitching deposit

Source: Figure 1.1 from Introductory Mining Engineering, 2nd Ed., Hartman, H.L, and J.M. Mutmansky

The footwall and hanging walls are not labeled, but I am sure you can identify them after our previous discussion. The answer to the question of why the permanent structures such as the shaft are located in the footwall side of the deposit is: as areas are mined out within the deposit, it is likely that some caving will occur, and the caving can be a consequence or cause of fractures propagating through the hanging wall, and potentially ending at the surface. If there were any structures such as a shaft in the hanging wall of the deposit, they would be destroyed by the normal mining operation.

You’ll note the main levels versus the sublevels, and don’t miss the decline. Also, take note of the exploration-drilling program that is ongoing throughout the life of the mine.

I like the next figure because it shows the footwall drift clearly as well as the sublevels and some of the unit operations within the sublevels. The ore passes down to the haulage level are easy to see in this figure, too.

Figure 9.2.2: Sublevel and unit operations

Source: Atlas Copco Underground Methods

Finally, this third figure allows me to illustrate and elaborate on more of the terms. In thick-bedded deposits, it is often necessary to mine the bed in multiple steps known as lifts or benches. In this example, they are taking the seam in two lifts, and you can see the work underway on the benches. Specifically, you should be able to identify the locations where they are breast stoping, underhand stoping, and overhand stoping. You can also see the pillars that are being left behind to support the overlying strata.

Figure 9.2.3: Taking the seam in two lifts

Source: Atlas Copco Underground Methods

Now that we have a common vocabulary to describe the elements of underground mines, as well as an understanding of these basic elements, we are ready to look at the underground mining methods. In the next two lessons, we will look at the methods. I think it is useful to put the methods into a context of their usage within the industry. That is to say, some methods are rarely applied, and others are frequently applied. Let’s finish up this lesson by taking a closer look at this.

9.2.4: Prevalence of Underground Mining Methods

The actual number of underground mines will fluctuate somewhat year-to-year, as a few close and a few open. In recent years, there has been a more significant shift in the number, as several underground coal mines closed due to market conditions. Even though the actual numbers will vary, the proportions are relatively stable. For the purposes of this illustration, we will use underground mines in the U.S., and I’ll round up the number slightly for ease of comparison. There are approximately 600 underground mines in the U.S. Of those, approximately 400 are coal and approximately 200 are noncoal. The noncoal includes metals, industrial minerals, and stone mines.

Every one of the 400 coal mines utilizes the room and pillar method, neglecting the 2 or 3 underground anthracite mines that employ a hybrid method. All of the other underground mining methods are distributed among the 200 noncoal mines. Let’s look at that group in more detail.

The disproportionate number of room and pillar mines is quite noteworthy. Roughly 75% of the underground noncoal mines employ this method. If you add the coal mines into the mix, roughly 90% of all underground mines are using the room and pillar method. Let me ask you a question. If you were to become an expert in the design of one mining method, which would you choose? Well, certainly your career options would be much better if you chose room and pillar! This is not to say that you don’t need to learn about the other methods! You do!!! However, in our curriculum, we do emphasize this method more than the others, and now you know why!

Let’s look at the commodities mined by the three most prevalent methods: room and pillar, cut and fill, and open stoping; as well as the two rarely used methods (in this country) of shrinkage stoping and block caving.

Please be aware that not all commodities are accounted for in this table. There are additional industrial minerals mined by an underground method, which are not accounted for in this data. Despite the very small discrepancies in the totals, the data illustrates accurately the prevalence of the mining methods by commodity.

Review your knowledge of Module 9 terms by engaging with the interactive activities.