Part 9 (1/2)
This system can be advantageously applied only in the rare cases in which the walls require little or no support, and where very little or no waste requiring separation is broken with the ore in the stopes. To support the walls in bad ground in underhand stopes would be far more costly than with overhand stopes, for square-set timbering would be most difficult to introduce, and to support the walls with waste and stulls would be even more troublesome. Any waste broken must needs be thrown up to the level above or be stored upon specially built stages--again a costly proceeding.
A further drawback lies in the fact that the broken ore follows down the face of the stope, and must be shoveled off each bench.
It thus all arrives at a single point,--the winze,--and must be drawn from a single ore-pa.s.s into the level. This usually results not only in more shoveling but in a congestion at the pa.s.ses not present in overhand stoping, for with that method several chutes are available for discharging ore into the levels. Where the walls require no support and no selection is desired in the stopes, the advantage of the men standing on the solid ore to work, and of having all down holes and therefore drilled wet, gives this method a distinct place. In using this system, in order to protect the men, a pillar is often left under the level by driving a sublevel, the pillar being easily recoverable later. The method of sublevels is of advantage largely in avoiding the timbering of levels.
[Ill.u.s.tration: Fig. 23.--Longitudinal section of an underhand stope.]
OVERHAND STOPES.--By far the greatest bulk of ore is broken overhand, that is broken upward from one level to the next above. There are two general forms which such stopes are given,--”horizontal” and ”rill.”
[Ill.u.s.tration: Fig. 24.--Horizontal-cut overhand stope--longitudinal section.]
The horizontal ”flat-back” or ”long-wall” stope, as it is variously called, shown in Figure 24, is operated by breaking the ore in slices parallel with the levels. In rill-stoping the ore is cut back from the winzes in such a way that a pyramid-shaped room is created, with its apex in the winze and its base at the level (Figs. 25 and 26). Horizontal or flat-backed stopes can be applied to almost any dip, while ”rill-stoping” finds its most advantageous application where the dip is such that the ore will ”run,” or where it can be made to ”run” with a little help. The particular application of the two systems is dependent not only on the dip but on the method of supporting the excavation and the ore. With rill-stoping, it is possible to cut the breaking benches back horizontally from the winzes (Fig. 25), or to stagger the cuts in such a manner as to take the slices in a descending angle (Figs. 21 and 26).
[Ill.u.s.tration: Fig. 25.--Rill-cut overhand stope--longitudinal section.]
In the ”rill” method of incline cuts, all the drill-holes are ”down”
holes (Fig. 21), and can be drilled wet, while in horizontal cuts or flat-backed stopes, at least part of the holes must be ”uppers”
(Fig. 20). Aside from the easier and cheaper drilling and setting up of machines with this kind of ”cut,” there is no drill dust,--a great desideratum in these days of miners' phthisis. A further advantage in the ”rill” cut arises in cases where horizontal jointing planes run through the ore of a sort from which unduly large ma.s.ses break away in ”flat-back” stopes. By the descending cut of the ”rill” method these calamities can be in a measure avoided. In cases of dips over 40 the greatest advantage in ”rill” stoping arises from the possibility of pouring filling or timber into the stope from above with less handling, because the ore and material will run down the sides of the pyramid (Figs. 32 and 34). Thus not only is there less shoveling required, but fewer ore-pa.s.ses and a less number of preliminary winzes are necessary, and a wider level interval is possible. This matter will be gone into more fully later.
[Ill.u.s.tration: Fig. 26.--Rill-cut overhand stope-longitudinal section.]
COMBINED STOPES.--A combined stope is made by the coincident working of the underhand and ”rill” method (Fig. 27). This order of stope has the same limitations in general as the underhand kind. For flat veins with strong walls, it has a great superiority in that the stope is carried back more or less parallel with the winzes, and thus broken ore after blasting lies in a line on the gradient of the stope. It is, therefore, conveniently placed for mechanical stope haulage. A further advantage is gained in that winzes may be placed long distances apart, and that men are not required, either when at work or pa.s.sing to and from it, to be ever far from the face, and they are thus in the safest ground, so that timber and filling protection which may be otherwise necessary is not required. This method is largely used in South Africa.
[Ill.u.s.tration: Fig. 27.--Longitudinal section of a combined stope.]
MINIMUM WIDTH OF STOPES.--The minimum stoping width which can be consistently broken with hand-holes is about 30 inches, and this only where there is considerable dip to the ore. This s.p.a.ce is so narrow that it is of doubtful advantage in any case, and 40 inches is more common in narrow mines, especially where worked with white men. Where machine-drills are used about 4 feet is the minimum width feasible.
RESUING.--In very narrow veins where a certain amount of wall-rock must be broken to give working s.p.a.ce, it pays under some circ.u.mstances to advance the stope into the wall-rock ahead of the ore, thus stripping the ore and enabling it to be broken separately. This permits of cleaner selection of the ore; but it is a problem to be worked out in each case, as to whether rough sorting of some waste in the stopes, or further sorting at surface with inevitable treatment of some waste rock, is more economical than separate stoping cuts and inevitably wider stopes.
VALUING ORE IN COURSE OF BREAKING.--There are many ores whose payability can be determined by inspection, but there are many of which it cannot.
Continuous a.s.saying is in the latter cases absolutely necessary to avoid the treatment of valueless material. In such instances, sampling after each stoping-cut is essential, the unprofitable ore being broken down and used as waste. Where values fade into the walls, as in impregnation deposits, the width of stopes depends upon the limit of payability. In these cases, drill-holes are put into the walls and the drillings a.s.sayed. If the ore is found profitable, the holes are blasted out. The gauge of what is profitable in such situations is not dependent simply upon the average total working costs of the mine, for ore in that position can be said to cost nothing for development work and administration; moreover, it is usually more cheaply broken than the average breaking cost, men and machines being already on the spot.
CHAPTER XI.
Methods of Supporting Excavation.
TIMBERING; FILLING WITH WASTE; FILLING WITH BROKEN ORE; PILLARS OF ORE; ARTIFICIAL PILLARS; CAVING SYSTEM.
Most stopes require support to be given to the walls and often to the ore itself. Where they do require support there are five princ.i.p.al methods of accomplis.h.i.+ng it. The application of any particular method depends upon the dip, width of ore-body, character of the ore and walls, and cost of materials. The various systems are by:--
1. Timbering.
2. Filling with waste.
3. Filling with broken ore subsequently withdrawn.
4. Pillars of ore.
5. Artificial pillars built of timbers and waste.
6. Caving.
TIMBERING.--At one time timbering was the almost universal means of support in such excavations, but gradually various methods for the economical application of waste and ore itself have come forward, until timbering is fast becoming a secondary device. Aside from economy in working without it, the dangers of creeps, or crus.h.i.+ng, and of fires are sufficient incentives to do away with wood as far as possible.