Part 9 (2/2)
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 crus.h.i.+ng or creeps is usually some time later; and if the crus.h.i.+ng 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 crus.h.i.+ng 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 crus.h.i.+ng possibilities are too great for this method then, the subsequent recovery of ore is hopeless in any event. In narrow ore-bodies with crus.h.i.+ng 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.
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