| METHODS OF ORE-BREAKING; UNDERHAND STOPES; OVERHAND STOPES; COMBINED STOPE. VALUING ORE IN COURSE OF BREAKING. |
There is a great deal of confusion in the application of the word "stoping." It is used not only specifically to mean the actual ore-breaking, but also in a general sense to indicate all the operations of ore-breaking, support of excavations, and transportation between levels. It is used further as a noun to designate the hole left when the ore is taken out. Worse still, it is impossible to adhere to miners' terms without employing it in every sense, trusting to luck and the context to make the meaning clear.
The conditions which govern the method of stoping are in the main:—
| a. | The dip. |
| b. | The width of the deposit. |
| c. | The character of the walls. |
| d. | The cost of materials. |
| e. | The character of the ore. |
Every mine, and sometimes every stope in a mine, is a problem special to itself. Any general consideration must therefore be simply an inquiry into the broad principles which govern the adaptability of special methods. A logical arrangement of discussion is difficult, if not wholly impossible, because the factors are partially interdependent and of varying importance.
For discussion the subject may be divided into:
| 1. | Methods of ore-breaking. |
| 2. | Methods of supporting excavation. |
| 3. | Methods of transport in stopes. |
METHODS OF ORE-BREAKING.
The manner of actual ore-breaking is to drill and blast off slices from the block of ground under attack. As rock obviously breaks easiest when two sides are free, that is, when corners can be broken off, the detail of management for blasts is therefore to set the holes so as to preserve a corner for the next cut; and as a consequence the face of the stope shapes into a series of benches (Fig. 22),—inverted benches in the case of overhand stopes (Figs. 20, 21). The size of these benches will in a large measure depend on the depth of the holes. In wide stopes with machine-drills they vary from 7 to 10 feet; in narrow stopes with hand-holes, from two to three feet.
| Fig. 20. |