8.4.1 The Supported Class of Mining Methods
Supported methods are those that require significant artificial support to maintain stability in the exploitation openings of the mine. We like to use pillars of the original rock mass as the ultimate form of ground control in an underground mine, because they are capable of providing near-rigid support. However, some orebodies are too weak to employ pillars for support. Often, the host rock is not very strong. Pillars are not an option, as a primary means of support, and consequently attempts to remove the ore will likely result in structural failure of the opening. It may not occur immediately, but over the time period that the opening would be needed, failure is likely to occur. Thus, the supported methods are used when production openings will not remain standing during their life. Support methods are also used when surface subsidence cannot be tolerated.
Whoa, but wait a minute. Didn’t we say that caving methods are applicable when the rock caves readily? Ok, then why would we use a supported method, rather than a caving method for these weak orebodies? Good question, for which there are two good answers! One is that caving to the surface will cause a subsidence zone, i.e., the surface will literally collapse and sink by an amount close to the thickness of material removed in the mine. If you are in the middle of “nowhere,” it doesn’t matter. If there are bodies of water, e.g., rivers and lakes, or towns, for example, you will not want to subside them! The second answer to the question is that supported methods can be extremely selective, whereas caving methods are just the opposite.
We could say that the supported class is employed when the other two classes of methods, i.e., unsupported and caving, are not applicable.
The supported class of methods is intended for application to rock ranging in competency from moderate to incompetent, and includes:
- Stull stoping
- Square-set stoping
Cut-and-fill and stull stoping is applicable to moderately competent rock, and square-setting is for the least competent rock. Mechanization requirements and labor costs have all but eliminated stull stoping and square-set stoping. Nonetheless, I’ll briefly explain them because it may give you some ideas if you ever want to recover a very small part of the deposit and no other approach is possible.