So far, we have studied mining methods that require exploitation workings to be held open, essentially intact, for the duration of mining. Specifically,
We will now study a class of methods in which the exploitation openings are designed to collapse; that is, caving of the ore or rock or both is intentional and is the very essence of the method.
We define caving methods as those associated with induced, controlled, massive caving of the ore body, the overlying rock, or both, concurrent with and essential to the conduct of mining.
There are three current methods that are considered to be caving methods:
Sublevel caving and block caving have application to inclined or vertical, massive deposits, almost exclusively metallic or nonmetallic.
Longwall mining is used in relatively flat-lying and tabular deposits such as coal, primarily, but the method is used to exploit some noncoal minerals, such as trona.
Longwall mining is a moderately priced method, and sublevel and block caving are among the cheapest of all the underground methods.
The caving class is truly unique because the exploitation openings are deliberately destroyed in the process of mining. Now, having said that, I suppose that before we go any further with this discussion of the caving methods, I should clarify one point. Do not think, even for one second, that because caving is the desired outcome, you are relieved of your engineering obligations for ground control! It is as important to apply rock mechanics to ensure that caving will occur as it is to prevent the occurrence of caving! In effect, the cross-sectional shape of the undercut area (i.e., the width-to-height ratio) must be sufficiently elongated to cause failure of the roof or back. Further, the development openings must be designed and located to withstand shifting and caving ground, as well as subsidence that usually extends to the surface. Thus, there is no shortage of significant ground control challenges with the caving methods. And it doesn’t stop with the engineering. In caving operations, the rate of production may be more important than in many other methods. Specifically, production must be maintained at a steady and continuous pace to avoid disruptions or hang-ups in the caving action.
Let’s take a look at each of the three methods in this class.
This method, which was developed in the U.S. after WW I, is well suited for mining in weak orebodies. Panels or blocks of ore are undercut. Once undercut, the weak orebody begins to cave under its own weight. The caved ore is drawn off through draw points. As the ore is drawn off, the orebody will continue to cave under its own weight. This process will continue until all of the orebody within the block or panel has been recovered.
Under proper conditions, this underground method can rival economically surface mining! It can do so because it is a bulk or high-volume mining method. But it is not a selective method. In the caving and drawing process, you take everything; and therefore this is not a viable method for following rich mineralized zones, for example. If you were in weak ground and you wanted to follow mineralized zones, which method would you first consider? Cut and fill? Yes! The method works well for low-grade ores where you have to mine large quantities cheaply to be profitable. Examples include low-grade copper and molybdenum deposits, diamond-bearing Kimberlite pipes, and certain iron ore deposits, among a few others.
Given the foregoing discussion, let’s summarize the conditions that we expect to see for a successful application of block caving.
Note the breakdown of the orebody into the categories of blocks, panels, and masses. These relate to the size of the cave area. If the ore is very, very weak you can have a wide stope. On the other hand, if you try to have a wide stop with a moderately weak ore, you are likely to get bridging within the stope. Caving stops at that point and it develops into a very dangerous as well as production-killing situation. Thus, the cave volume has to be reduced as the ore becomes marginally stronger. In some mines, they will talk about their panels, and in others, their blocks. Now, you know the reason for this difference in the words they use.
Let’s take a look at a video of block caving before going on. This video will give you a good understanding of the method. The are some other details of note as well:
Please watch the following video (3:44) entitled "Block Caving"
Here, below, is a view of a block caving operation, which depicts the information that you saw in the video, but in a slightly different form.
We can summarize what we learned from the video into the following steps for development and exploitation.
Development to prepare blocks for production caving is extensive and can take up to several years of advance work. On a per unit cost basis, the development for block caving is no greater than for sublevel caving.
This figure, below, shows these development activities.
Sublevel caving shares many similarities with block caving, with one notable exception, which is responsible for this method: the orebody is competent and will not cave under its own weight. The host rock, on the other hand, is weak and caves behind the ore as it is extracted. Consequently, the orebody needs to be drilled and shot to extract the ore. Once extracted, the hanging wall caves. Given the similarities, we don’t need to say much, and especially if you look at this diagram of a sublevel caving operation, below. There are a few points to be made, however.
You will notice there is no need to develop and undercut, nor bells and drawpoints. Instead, a series of sublevels is developed, and next, the ore above the sublevel is fan or ring drilled. The holes are charged and fired. The broken ore is then loaded out of the sublevel using an LHD (or rarely, a slusher). The LHD’s travel to ore passes where the ore is dumped and then collected at a lower haulage level.
The world’s largest sublevel caving mine is located above the Arctic Circle in Sweden. The Kiruna Mine is mining the Kirunavaara iron orebody. (See figure below.) This mine is famous not only for its size and longevity, but it has been a hotbed of innovation for mining technologies over the years. I really like this figure because it tracks the mine’s development over the decades. You can see that the orebody was mined by open pit for nearly 50 years, and then they went underground. You can trace their progress over the ensuing 50 years up to the present.
Looking at this next figure, you see the planned workings (sublevels) through 2019.
Block caving and sublevel caving require steeply dipping to nearly vertical deposits to enable the gravity flow of the rock. Longwall mining by contrast does not require this gravity flow of the caved material, and as such, it is well suited for tabular and nearly horizontal deposits.
Longwall mining applies to thin, bedded deposits, with uniform thickness and large horizontal extension. Typical deposits are coal seams, potash layers or conglomerates, trona, and gold reefs. Longwalls are found in nearly horizontal deposits of coal and trona, whereas in metal/nonmetal deposits, a steeper dip is tolerated. The difference is in the equipment that is used. Coal and trona are using massive suites of mechanized and semi-automated equipment that is unsuitable for use in greater than 5-10% grades. There are exceptions, but we’re not going to complicate this discussion with those.
Longwall mining takes its name from the characteristic long face or wall, which may be several hundred feet or more in length. The figure below illustrates this nicely. The ore is extracted in a slice along this long wall. The region adjacent to the face is kept open, i.e., free of obstructions, to allow space for miners and equipment. This region might extend 10’-20’ out from the wall. If we are looking at mining a gold reef, for example, a line of posts will be installed to support the roof or back and protect the active mining activity. At some distance back from the face, caving will be allowed to occur, and in most instances, this is necessary to relieve superimposed loads on the working face. If we are looking at a coal application, the process will be somewhat different. A significant percentage of the coal that is mined underground comes from longwall mines; and not just in the U.S. but around the world. As such, we’ll look more closely at longwall mining in coal.
Longwall mining of coal is a high production and high productivity method, employing sophisticated electrical, mechanical, and hydraulic systems, as well as computer-based monitoring and control systems. Most modern (coal) longwall faces are semi-automated. It is noteworthy that longwall operations in trona mines utilize essentially the same equipment and processes that are used in coal mine longwalls. As the long wall or face is mined and the roof supports advance forward with mining, the roof in the mined-out area will cave.
When the panel is initially mined, caving will be delayed. This is a worrisome period because all of the weight of the unsupported roof is transferred to the face and also the gateroad pillars. Sometimes caving may not start for 10 - 20 or more passes of the shearer. If the superimposed load becomes too great, the face and pillars can begin to crush. Thus, for longwall to work safely and productively, the cave must occur in a timely fashion. Once it has started to cave, it will generally continue with each pass of the shearer. Ground control experts will conduct a cavability analysis of the overburden before a decision is made to employ this mining method. This requirement for caving is the reason this method is classified in the caving class of methods.
Please be aware that the longwall panels within the coal deposit are created by room and pillar mining. Thus, many room and pillar (coal and trona) mines are also longwall mines, and in most of them, the room and pillar work is simply to develop the panels and the infrastructure to facilitate operation of the longwall. By that, I mean you need to have a well-developed ventilation, materials handling, and power systems to support a high production longwall. The room and pillar mining creates the mains, submains, and panel entries for these systems. Let’s look at a few figures. These won’t answer all of your questions right now, but these in combination with some videos that will come afterwards, should give you a good understanding of the method.
Let’s start with a plan view of a mine. This figure, below, shows a portion of the mains or submains and the longwall panels. Notice the three-entry gateroads that define the panels. One set of these will be known as the headgate entries and the other as the tailgate entries. The one of the longwall face will be known as the headgate and the other end of the face will be known as the tailgate. The entries adjacent to the mined out panel are the tailgate entries, and that defines the tailgate side of the panel. The longwall face is mined on retreat. That is to say, the gateroads are mined on advance, and then the longwall face retreats back to the submains or mains.
The equipment required for a longwall face is unique to this mining method. Let’s look at it in this figure below, and then when you see it in the videos, it will make more sense.
The armored face conveyor is a massive steel structure containing a chain conveyor. The shearer (or plow) rides on the AFC and cuts the coal. The cut coal falls into the AFC, and is transported to the headgate. At the headgate, the coal is crushed to a size suitable for transport on a conveyor belt and then fed at a controlled rate onto the belt. This panel belt feeds the outby belt system. It takes an enormous amount of power (1000s of hp) to operate the AFC, and there are drives at both the tail and headgates to power the chain conveyor. All of this equipment and the miners working at the face are protected by what has been called an umbrella of safety, i.e., the series of shields. As the coal is cut, the AFC snakes into place immediately adjacent to the face. On the right side of this figure, they are depicting this advance of the face, and you can see the shields that have moved into place.
This next figure illustrates more completely the relationship of the longwall panel to the overburden, the gateroads, and the longwall face itself.
Next, let’s take a look at some videos, each of which is less than five minutes in length. I think these videos are helpful for the details that they show. There are four of them, and they are addressing the same basic topic. However, in each one you can see certain important details more clearly than in the other videos. I suggest that you watch all of them twice, and don’t hesitate to pause them and look more closely at the image. In so doing, you’ll develop a more complete understanding of the equipment and the process.
I want to make one correction to the videos. Caterpillar or CAT as it is known, is a global manufacturer of excellent mining equipment. One important detail of their second video is not quite correct, however. All longwalls in the U.S. use shearers, not plows, including the longwalls operating in thin seams. The reasons for this are not important here, but please remember that shearers, not plows are applied through the U.S coal fields. In MNG 410, we take a more detailed look at longwall mines, including the conditions that favor the use of the plow.
I imagine that you have a reasonable understanding of modern longwall mining after studying the figures and the videos. Allow me to summarize the process and add a few additional details of interest.
Development of the panels is done with the room and pillar method. It generally will take at least two continuous mining sections to develop the longwall panels. From a mine planning perspective, the goal is to ensure that you have panels developed and ready to go. Often, the mine will have spare longwall equipment, and they will partially set up a longwall face on the next panel, so that minimal time will be lost in moving to the next panel and commencing with longwall production. This requires that panel development activities stay far enough ahead of the longwalls to ensure that the next panel can be set up before mining of the previous panel is completed. However, it is important that panel development does not get too far ahead, which would result in developed panels sitting idle for several months. This would represent a poor use of resources, but more importantly, the roof rock often begins to deteriorate when exposed to the moist mine atmosphere. This could result in roof problems before you have begun to mine in those panels.
Services to the longwall face will be placed in the headgate entries. This includes the panel belt, the staged loader and crusher, the hydraulic pumps for the shields, the electrical power centers, the computer control boxes, and the refuge chamber.
The tailgate is under additional roof stresses, and requires additional roof support, such as concrete pillars, timbered crib structures, and so on.
The number of gate roads is typically three, which is a legal requirement in the U.S. Under severe ground pressures, fewer gate entries are advantageous. In the western U.S., a few mines have received special permission to have only two gate roads. Outside of the U.S., you will find single entry gate roads. A reduction in the number of gateroads does create safety hazards that must be managed.
The basic production cycle is straightforward, as you no doubt saw in the videos. The cutter, whether a plow or shearer, mines along the width of the face. As the cutter moves, the shields advance forward. As they advance forward, the AFC is pushed to the face; and as such, it is ready for the next cut. A few details that are not apparent would include:
Modern longwall mining systems represent the highest level of technology and engineering achievement in mining method design. The design of the individual components has pushed the envelope on electrical, mechanical, and hydraulic component design. The productivity of these systems is unrivaled, and the raw tonnages achievable per shift are staggering compared to what was state-of-the-art 20 years ago. As cutting technology advances to allow continuous cutting of harder materials, you will see these systems applied in other commodities… but only if what is true?
This brings us to the end of our discussion of the supported and caving classes of underground mining methods.