Print8.4.2 Caving Methods
So far, we have studied mining methods that require exploitation workings to be held open, essentially intact, for the duration of mining. Specifically,
- if the ore and rock are sufficiently competent, unsupported methods are adequate;
- if ore and rock are incompetent to moderately competent, then supported methods must be used.
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:
- Block caving
- Sublevel caving
- Longwall mining
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.