Part 10 (1/2)
FILLING WITH BROKEN ORE SUBSEQUENTLY WITHDRAWN.--This order of support is called by various names, the favorite being ”shrinkage-stoping.”
The method is to break the ore on to the roof of the level, and by thus filling the stope with broken ore, provide temporary support to the walls and furnish standing floor upon which to work in making the next cut (Figs. 37, 38, and 39.) As broken material occupies 30 to 40% more s.p.a.ce than rock _in situ_, in order to provide working s.p.a.ce at the face, the broken ore must be drawn from along the level after each cut. When the area attacked is completely broken through from level to level, the stope will be full of loose broken ore, which is then entirely drawn off.
A block to be attacked by this method requires preliminary winzes only at the extremities of the stope,--for entry and for ventilation.
Where it is desired to maintain the winzes after stoping, they must either be strongly timbered and lagged on the stope side, be driven in the walls, or be protected by a pillar of ore (Fig.
37). The settling ore and the crus.h.i.+ng after the stope is empty make it difficult to maintain timbered winzes.
[Ill.u.s.tration: Fig. 37.--Longitudinal section of stope filled with broken ore.]
Where it can be done without danger to the mine, the empty stopes are allowed to cave. If such crus.h.i.+ng would be dangerous, either the walls must be held up by pillars of unbroken ore, as in the Alaska Treadwell, where large ”rib” pillars are left, or the open s.p.a.ces must be filled with waste. Filling the empty stope is usually done by opening frequent pa.s.ses along the base of the filled stope above, and allowing the material of the upper stope to flood the lower one. This program continued upwards through the mine allows the whole filling of the mine to descend gradually and thus requires replenishment only into the top. The old stopes in the less critical and usually exhausted territory nearer the surface are sometimes left without replenis.h.i.+ng their filling.
The weight of broken ore standing at such a high angle as to settle rapidly is very considerable upon the level; moreover, at the moment when the stope is entirely drawn off, the pressure of the walls as well is likely to be very great. The roadways in this system therefore require more than usual protection. Three methods are used: (_a_) timbering; (_b_) driving a sublevel in the ore above the main roadway as a stoping-base, thus leaving a pillar of ore over the roadway (Fig. 39); (_c_) by dry-walling the levels, as in the Baltic mine, Michigan (Figs. 34 and 35). By the use of sublevels the main roadways are sometimes driven in the walls (Fig. 38) and in many cases all timbering is saved. To recover pillars left below sublevels is a rather difficult task, especially if the old stope above is caved or filled. The use of pillars in subst.i.tution for timber, if the pillars are to be lost, is simply a matter of economics as to whether the lost ore would repay the cost of other devices.
[Ill.u.s.tration: Fig. 38.--Cross-section of ”shrinkage” stope.]
Frequent ore-chutes through the level timbers, or from the sublevels, are necessary to prevent lodgment of broken ore between such pa.s.ses, because it is usually too dangerous for men to enter the emptying stope to shovel out the lodged remnants. Where the ore-body is wide, and in order that there may be no lodgment of ore, the timbers over the level are set so as to form a trough along the level; or where pillars are left, they are made ”A”-shaped between the chutes, as indicated in Figure 37.
[Ill.u.s.tration: Fig. 39.--Cross-section of ”shrinkage” stope.]
The method of breaking the ore in conjunction with this means of support in comparatively narrow deposits can be on the rill, in order to have the advantage of down holes. Usually, however, flat-back or horizontal cuts are desirable, as in such an arrangement it is less troublesome to regulate the drawing of the ore so as to provide proper head room. Where stopes are wide, ore is sometimes cut arch-shaped from wall to wall to a.s.sure its standing. Where this method of support is not of avail, short, sharply tapering stulls are put in from the broken ore to the face (Fig. 39). When the cut above these stulls is taken out, they are pulled up and are used again.
This method of stoping is only applicable when:--
1. The deposit dips over 60, and thus broken material will freely settle downward to be drawn off from the bottom.
2. The ore is consistently payable in character. No selection can be done in breaking, as all material broken must be drawn off together.
3. The hanging wall is strong, and will not crush or spall off waste into the ore.
4. The ore-body is regular in size, else loose ore will lodge on the foot wall. Stopes opened in this manner when partially empty are too dangerous for men to enter for shoveling out remnants.
The advantages of this system over others, where it is applicable, are:--
(_a_) A greater distance between levels can be operated and few winzes and rises are necessary, thus a great saving of development work can be effected. A stope 800 to 1000 feet long can be operated with a winze at either end and with levels 200 or 220 feet apart.
(_b_) There is no shoveling in the stopes at all.
(_c_) No timber is required. As compared with timbering by stulling, it will apply to stopes too wide and walls too heavy for this method.
Moreover, little staging is required for working the face, since ore can be drawn from below in such a manner as to allow just the right head room.
(_d_) Compared to the system of filling with waste, coincidentally with breaking (second method), it saves altogether in some cases the cost of filling. In any event, it saves the cost of ore-pa.s.ses, of shoveling into them, and of the detailed distribution of the filling.
Compared with other methods, the system has the following disadvantages, that:
_A_. The ore requires to be broken in the stopes to a degree of fineness which will prevent blocking of the chutes at the level.
When pieces too large reach the chutes, nothing will open them but blasting,--to the damage of timbers and chutes. Some large rocks are always liable to be buried in the course of ore-breaking.
_B_. Practically no such perfection of walls exists, but some spalling of waste into the ore will take place. A crus.h.i.+ng of the walls would soon mean the loss of large amounts of ore.
_C_. There is no possibility of regulating the mixture of grade of ore by varying the working points. It is months after the ore is broken before it can reach the levels.
_D_. The breaking of 60% more ore than immediate treatment demands results in the investment of a considerable sum of money. An equilibrium is ultimately established in a mine worked on this system when a certain number of stopes full of completely broken ore are available for entire withdrawal, and there is no further acc.u.mulation. But, in any event, a considerable amount of broken ore must be held in reserve. In one mine worked on this plan, with which the writer has had experience, the annual production is about 250,000 tons and the broken ore represents an investment which, at 5%, means an annual loss of interest amounting to 7 cents per ton of ore treated.