Every-day Science: Volume 6. The Conquest of Nature

by Henry Smith Williams; Edward Huntington Williams

In the overhead method a shaft is sunk into the earth to a depth of several hundred feet, according to the depth of the ore, this shaft being lined with timbers for support. From this shaft horizontal tunnels are made in all directions in the ore deposits, and through these tunnels the ore is conveyed to the shaft and thence to the surface. As the ore is removed and the earth thus honeycombed in all directions, supports of various kinds must be made to prevent caving. For this purpose columns of the ore itself may be left, or supports of masonry or wood or steel may be introduced. Under certain circumstances, however, these supports are not employed, the earth being allowed gradually to cave in at the surface as the ore is removed, this being the method of mining known as "caving."

Where the ore deposit occurs in a favorable hillside the "milling" system is frequently employed. In working this system a large horizontal tunnel, twenty or more feet in diameter, is dug into the hillside. Perpendicular shafts are then sunk from the top of the hill, connected with openings leading directly into the top of the main horizontal shaft. By this arrangement the ore, when loosened in these perpendicular shafts, falls directly into the bins placed for its reception about the openings, or into the rows of cars in waiting to receive it. In this method dynamite and powder take the place of hand labor, the main mass of ore being dislodged and thrown into the shaft by blasting, instead of by hand labor.

But all these methods are overshadowed in magnitude by the great "open pit" systems, where the ore is taken from the surface and handled entirely by machinery, the only part played by the miner's pick being that of assisting in loosing certain fragments so that they may be more easily seized by the machines. Indeed, this system of mining partakes of the nature of quarrying rather than that of mining in the ordinary sense, the ore being scooped from the surface of the ground. One naturally thinks of a mine as being subterranean; but in the great open-pit mines in the Lake Superior region, which are the largest mines in the world, all the mining is done at the surface of the earth.

It should not be understood, however, that in such mines nature has left the red iron ore exposed at the surface in any great quantities. On the contrary, it is usually covered by a layer of earth ranging from a yard to ten or more yards in depth, and this, of course, must be removed before open-pit methods can be practised. Prospecting for such deposits is therefore just as necessary as in cases where the deposit is situated much deeper in the earth; and the business of prospecting by "test pit" men is as important an industry as ever.

When an available open-pit mine of sufficient extent has been located the gigantic task of "stripping" or removing the overlying layer of earth begins. Immense areas of land have been thus stripped in some of these undertakings, no difficulties being considered insurmountable. If a small river-bed lies in an unfavorable position, the course of the river is changed regardless of expense. Farms and farm houses are purchased and literally carted away, neither land nor houses representing values worth considering when compared with the stratum of ore beneath them. The single contract for stripping one area in the Lake Superior region was let for a sum amounting to half a million dollars.

As soon as a sufficiently large area has been stripped, railroads are constructed into the pit, steam shovels are run into place, and the actual work of mining begins. Five shovels full make a car-load, and under ordinary circumstances the five loads may be delivered in as many minutes.

The number of men required to manipulate one of these steam shovels is from ten to twelve. The ore itself is frequently so hard that the scoop of the shovel could not penetrate it until loosened and broken up, and it is the business of the gang of workmen to do this and slide the ore down within easy working distance of the shovel. This is mostly done by blasting with dynamite and powder, little of the actual labor being performed by hand. In blasting, a deep hole is first drilled into the ore near the top of the embankment, and into this hole a stick of dynamite is dropped and exploded. This enlarges the cavity sufficiently so that a quantity of blasting powder may be poured in and set off, tumbling the ore down within reach of the shovel.

This ore is frequently almost as hard as iron itself, many of the pieces thus dislodged being too large for convenient handling, either by the steam shovel or in the chutes at the wharves, and must be still further broken up. This is sometimes done by the men with picks; but in mining on a large scale, where the deposit is all of a very hard nature, crushing machines are used.

In this manner the steam shovel is kept constantly supplied with ore for the waiting train of cars. These trains are arranged on a track running parallel with the track from which the steam shovel operates, and at such a distance that the centre of the car will be directly under the opening in the bottom of the shovel when it is swung around on its crane. The engineer in charge of the locomotive drawing the train stops it in a position so that the first shovelful of ore will be dumped into the forward end of the first car. As each successive shovelful is deposited, representing about one-fifth of a car-load, the train is pulled or backed along the track about one-fifth of a car-length. In this manner it is only necessary for the steam shovel to be swung into the same position and dumped at the same point each time to insure the proper loading of the cars.