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 accomplishing 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 crushing, and of fires are sufficient incentives to do away with wood as far as possible.
There are three princ.i.p.al systems of timber support to excavations,--by stulls, square-sets, and cribs.
Stulls are serviceable only where the deposit is so narrow that the opening can be bridged by single timbers between wall and wall (Figs. 28 and 43). This system can be applied to any dip and is most useful in narrow deposits where the walls are not too heavy. Stulls in inclined deposits are usually set at a slightly higher angle than that perpendicular to the walls, in order that the vertical pressure of the hanging wall will serve to tighten them in position. The "stull" system can, in inclined deposits, be further strengthened by building waste pillars against them, in which case the arrangement merges into the system of artificial pillars.
[Ill.u.s.tration: Fig. 28.--Longitudinal section of stull-supported stope.]
[Ill.u.s.tration: Fig. 29.--Longitudinal section showing square-set timbering.]
[Ill.u.s.tration: Fig. 30.--Square-set timbering on inclined ore-body.
Showing ultimate strain on timbers.]
Square-sets (Figs. 29 and 30), that is, trusses built in the opening as the ore is removed, are applicable to almost any dip or width of ore, but generally are applied only in deposits too wide, or to rock too heavy, for stulls. Such trusses are usually constructed on vertical and horizontal lines, and while during actual ore-breaking the strains are partially vertical, ultimately, however, when the weight of the walls begins to be felt, these strains, except in vertical deposits, come at an angle to lines of strength in the trusses, and therefore timber constructions of this type present little ultimate resistance (Fig. 30). Square-set timbers are sometimes set to present the maximum resistance to the direction of strain, but the difficulties of placing them in position and variations in the direction of strain on various parts of the stope do not often commend the method. As a general rule square-sets on horizontal lines answer well enough for the period of actual ore-breaking. The crushing or creeps is usually some time later; and if the crushing may damage the whole mine, their use is fraught with danger.
Reenforcement by building in waste is often resorted to. When done fully, it is difficult to see the utility of the enclosed timber, for entire waste-filling would in most cases be cheaper and equally efficient.
[Ill.u.s.tration: Fig. 31.--"Cribs."]
There is always, with wood constructions, as said before, the very pertinent danger of subsequent crushing and of subsidence in after years, and the great risk of fires. Both these disasters have cost Comstock and Broken Hill mines, directly or indirectly, millions of dollars, and the outlay on timber and repairs one way or another would have paid for the filling system ten times over. There are cases where, by virtue of the cheapness of timber, "square-setting"
is the most economical method. Again, there are instances where the ore lies in such a manner--particularly in limestone replacements--as to preclude other means of support. These cases are being yearly more and more evaded by the ingenuity of engineers in charge. The author believes it soon will be recognized that the situation is rare indeed where complete square-setting is necessarily without an economical alternative. An objection is sometimes raised to filling in favor of timber, in that if it become desirable to restope the walls for low-grade ore left behind, such stopes could only be entered by drawing the filling, with consequent danger of total collapse. Such a contingency can be provided for in large ore-bodies by installing an outer sh.e.l.l of sets of timber around the periphery of the stope and filling the inside with waste. If the crushing possibilities are too great for this method then, the subsequent recovery of ore is hopeless in any event. In narrow ore-bodies with crushing walls recovery of ore once left behind is not often possible.
The third sort of timber constructions are cribs, a "log-house" sort of structure usually filled with waste, and more fully discussed under artificial pillars (Fig. 31). The further comparative merits of timbering with other methods will be a.n.a.lyzed as the different systems are described.
FILLING WITH WASTE.--The system of filling stope-excavations completely with waste in alternating progress with ore-breaking is of wide and increasingly general application (Figs. 32, 33, 34, 35).
Although a certain amount of waste is ordinarily available in the stopes themselves, or from development work in the mine, such a supply must usually be supplemented from other directions. Treatment residues afford the easiest and cheapest handled material. Quarried rock ranks next, and in default of any other easy supply, materials from crosscuts driven into the stope-walls are sometimes resorted to.
In working the system to the best advantage, the winzes through the block of ore under attack are kept in alignment with similar openings above, in order that filling may be poured through the mine from the surface or any intermediate point. Winzes to be used for filling should be put on the hanging-wall side of the area to be filled, for the filling poured down will then reach the foot-wall side of the stopes with a minimum of handling. In some instances, one special winze is arranged for pa.s.sing all filling from the surface to a level above the princ.i.p.al stoping operations; and it is then distributed along the levels into the winzes, and thus to the operating stopes, by belt-conveyors.
[Ill.u.s.tration: Fig. 32.--Longitudinal section. Rill stope filled with waste.]
[Ill.u.s.tration: Fig. 33.--Longitudinal section. Horizontal stope filled with waste.]
[Ill.u.s.tration: Fig. 34.--Longitudinal section. Waste-filled stope with dry-walling of levels and pa.s.ses.]
In this system of stope support the ore is broken at intervals alternating with filling. If there is danger of much loss from mixing broken ore and filling, "sollars" of boards or poles are laid on the waste. If the ore is very rich, old canvas or cowhides are sometimes put under the boards. Before the filling interval, the ore pa.s.ses are built close to the face above previous filling and their tops covered temporarily to prevent their being filled with running waste. If the walls are bad, the filling is kept close to the face. If the unbroken ore requires support, short stulls set on the waste (as in Fig. 39) are usually sufficient until the next cut is taken off, when the timber can be recovered. If stulls are insufficient, cribs or bulkheads (Fig. 31) are also used and often buried in the filling.
[Ill.u.s.tration: Fig. 35.--Cross-section of Fig. 34 on line _A-B_.]
Both flat-backed and rill-stope methods of breaking are employed in conjunction with filled stopes. The advantages of the rill-stopes are so patent as to make it difficult to understand why they are not universally adopted when the dip permits their use at all. In rill-stopes (Figs. 32 and 34) the waste flows to its destination with a minimum of handling. Winzes and ore-pa.s.ses are not required with the same frequency as in horizontal breaking, and the broken ore always lies on the slope towards the pa.s.ses and is therefore also easier to shovel. In flat-backed stopes (Fig. 33) winzes must be put in every 50 feet or so, while in rill-stopes they can be double this distance apart. The system is applicable by modification to almost any width of ore. It finds its most economical field where the dip of the stope floor is over 45, when waste and ore, with the help of the "rill," will flow to their destination. For dips from under about 45 to about 30 or 35, where the waste and ore will not "flow" easily, shoveling can be helped by the use of the "rill" system and often evaded altogether, if flow be a.s.sisted by a sheet-iron trough described in the discussion of stope transport. Further saving in shoveling can be gained in this method, by giving a steeper pitch to the filling winzes and to the ore-pa.s.ses, by starting them from crosscuts in the wall, and by carrying them at greater angles than the pitch of the ore (Fig.
36). These artifices combined have worked out most economically on several mines within the writer"s experience, with the dip as flat as 30. For very flat dips, where filling is to be employed, rill-stoping has no advantage over flat-backed cuts, and in such cases it is often advisable to a.s.sist stope transport by temporary tracks and cars which obviously could not be worked on the tortuous contour of a rill-stope, so that for dips under 30 advantage lies with "flat-backed" ore-breaking.
[Ill.u.s.tration: Fig. 36.--Cross-section showing method of steepening winzes and ore pa.s.ses.]
On very wide ore-bodies where the support of the standing ore itself becomes a great problem, the filling system can be applied by combining it with square-setting. In this case the stopes are carried in panels laid out transversally to the strike as wide as the standing strength of the ore permits. On both sides of each panel a fence of lagged square-sets is carried up and the area between is filled with waste. The panels are stoped out alternately. The application of this method at Broken Hill will be described later. (See pages 120 and Figs. 41 and 42.) The same type of wide ore-body can be managed also on the filling system by the use of frequent "bulkheads"
to support the ore (Fig. 31).
Compared with timbering methods, filling has the great advantage of more effective support to the mine, less danger of creeps, and absolute freedom from the peril of fire. The relative expense of the two systems is determined by the cost of materials and labor.
Two extreme cases ill.u.s.trate the result of these economic factors with sufficient clearness. It is stated that the cost of timbering stopes on the Le Roi Mine by square-sets is about 21 cents per ton of ore excavated. In the Ivanhoe mine of West Australia the cost of filling stopes with tailings is about 22 cents per ton of ore excavated. At the former mine the average cost of timber is under $10 per M board-measure, while at the latter its price would be $50 per M board-measure; although labor is about of the same efficiency and wage, the cost in the Ivanhoe by square-setting would be about 65 cents per ton of ore broken. In the Le Roi, on the other hand, no residues are available for filling. To quarry rock or drive crosscuts into the walls might make this system cost 65 cents per ton of ore broken if applied to that mine. The comparative value of the filling method with other systems will be discussed later.