Page:The New International Encyclopædia 1st ed. v. 13.djvu/615

MINING. opened up ahead of actual extraction of ore, so as to average the rich and lean portions and make the enterprise a permanent one. Veins often fork and send off stringers into the walls; they pinch and swell along their length and depth. They usually run out at their ends into small ramifications and finally cease. They may be cut off sharply by other cross-fractures and disturbances. They extend to considerable depths, having been followed in some cases as deep as 3000 feet or more.

As a typical case by which to illustrate the usual methods of procedure, we may assume that a vein has been located on the surface, that it extends a considerable distance, say a half-mile or mile, and dips at 60 degrees into the earth. Test pits and shallow shafts have indicated its value. The engineer, in opening a new deposit like this, would select as suitable a place as possible for his surface works, such as engine house, ore bins, and dump for waste rock, all in connection with a spot where the vein showed good ore. He would then sink a shaft or slope on the vein, and if it held good, would start drifts or levels at each 60 to 100 feet of descent. As soon as a level had advanced some distance from the shaft, say 100 feet or more, another party would be started near the shaft, working on the vein in the roof of the level. At first propped up on timbers, they would excavate a space, and clear away a working face, so that while the level was being driven ahead they could follow a short distance back, taking off a slice. Now in order that the loose rock and ore that are blasted down should not block the passageway, timbers would be set across the top of the level as at first run. The timbers called stulls would fit into sockets in the walls and on them would be laid rough plank or lagging, with taps or little hatchways at intervals for tapping out into cars the ore that would be blasted down upon them. This method is called ‘overhand stoping,’ and is the one usually adopted. When the first party of stopers had advanced far enough to warrant it, a second, and later a third, would be set at work following them up on other and higher slices. As soon as the levels had gone some distance, another shaft would be sunk to connect with them, not alone for hoisting, but to afford ventilation after blasting and for a safe line of escape for the men in case of accident.

Another method somewhat different from overhand stoping is sometimes adopted that is called underhand stoping. Suppose levels one and two had advanced some distance from the shaft, a small connecting shaft is then cut between them called a winze. It may be opened by sinking from the upper level or by an upraise from the lower. After it is cut, a party may begin on the upper level, and drilling in its floor may blast away the vein into the winze and allow it to fall to the level below to be removed. They may take off a vertical slice of the vein in this way, and gradually work each way from the winze. The upper level must then be kept passable with a floor of timber.

As these inclined shafts deepen and the vein is found to be rich and permanent, it is often advantageous no longer to use the inclined shaft, but rather to go out from the vein into the hanging wall on the surface, and sink a vertical shaft that will intersect the vein at some desirable depth. Above this point connections are made

with the levels by crosscuts through the hanging wall, and below it by cross-cuts through the footwall. Vertical shafts are always to be preferred, on account of the greater ease and speed of hoisting, but in a new enterprise the safer rule is to follow the ore until its quantity is proved. Variations on the above simple methods are introduced by the character of the wall-rock and the size of the ore body. If the wall-rock is bad, and tends to scale off and impede the workings, it must be propped up with heavy timbering. If the vein is thick, the timbers are built up either rough or squared, and so mortised at the ends that they fit together like the edges of a cube, six feet on the side. Others fit in with them, each stick entering into the four adjacent cubes, and in the end a framework of timber of great strength is built up. As soon as possible this is filled in with waste rock, which finally settles down and is practically as solid as the original vein. Unless precautions are observed in connection with keeping the walls firm and immovable, they may settle and do great damage both to surface buildings and underground workings.

In the Lake Superior iron mines producing soft ore, that lies under a too heavy burden of gravel to warrant stripping, a system has been adopted called the ‘caving system.’ The ores of this character on Lake Superior lie in great troughs or elongated basins. A shaft is sunk in the rock beyond the limits of the ore and drifts at various levels are run out into it. From the uppermost level upraises are made to the top of the ore and minor drifts extended to its outer limits. Light timbering and lagging protect the miner, who then at these outer limits begins to mine out the ore on each side of the end of his drift, letting the burden gradually cave in to the place whence the ore is taken. By multiplying these drifts in every direction all the ore is removed, and the burden, closing in all the time, keeps the mine shut and the miners protected from the weather. In the end a great pit results, sunk in the natural surface.

In small mines no particular system of timbering or taking out the ore is necessary, especially if the wall-rock is firm. Beyond the general plan of shafts and levels the workings follow the ore, and, without much systematic exploration, blast it and remove it to the surface. The objection to this method is that when the known rich spots are exhausted, further operations until more ore is located are all dead work, yielding no return and often causing the enterprise to shut down. In large mines where the wall-rock is firm, great excavations may be made with no timbering whatever.

If the vein or series of veins outcrop on a hillside, either parallel with its surface or crossing the neighboring valley, the ore may be won by adits or tunnels run in on a slight up grade. Such a tunnel will automatically drain all the portions of the vein above it and will make it an easy matter to take out the ore, which is merely loosened and sent down to the tunnel in winzes and shoots. But the portions below the tunnel will of necessity be reached by shafts from it and will require pumping. For this reason, unless the advantages of a tunnel are very great, most engineers prefer a vertical shaft at as early a stage in the mine as possible, because it is so easy and convenient to handle ore quickly and cheaply