place. This method of mining and filling can be used when
the work is done in horizontal floors or in transverse chambers.
In the United States the Nevada square set system of timbering
is used in Connexion with rock filling (fig. 11). The use of the
heavy timbers and continuous framing which characterize
this system facilitates greatly the work of mining and maintaining
the haulage roads on the different floors, and gives more
rigid support to the unmined portions of the block of ground
above. These advantages compensate for the greater first
cost. Where each floor is timbered by itself with light timbers,
as is the practice on the continent of Europe, the consolidation
of the rock-filling under pressure gives rise to considerable
subsidence of the unmined ore, which has frequently settled
20 ft. or more before the upper part of the block is reached.
This occasions much added expense in the maintenance and
retimbering of the haulage roads on the upper floors. The
shrinkage of the rock-filling and the settlement of the workings
can be greatly lessened by the use of hard rock with a minimum
of fine stuff; but even so the advantage lies with the American
system of timbering.
Fig. 11.
The cost of filling has been greatly reduced by the system of flushing culm, sand, gravel and similar material, through pipes leading from the surface into mine workings. Material as coarse as 1 in. in diameter may be carried long distances underground with the use of little more than an equal volume of water. This method Sand Flushing. originated in the Pennsylvania anthracite mines in 1887, but has been employed in recent years on a large scale in Silesia, Westphalia and other European coalfields. In some cases it has been found advantageous to quarry and crush rock for the purpose of using it in this way. Examples of other mining methods will be found under Coal.
Where mineral deposits lie near the surface underground mining may be replaced by open excavations, and the reduced cost of mining makes it possible to remove the overlying soil and rock to considerable depths. The depth to which open working can be pushed depends upon the size and value of the mineral deposit and Open Workings. upon the expense of removing the over-burden. Open excavations several hundred feet in depth are not uncommon. Where practicable steam shovels are employed, even when it is necessary to break up the material beforehand by blasting. Steam shovels are not well adapted to deep excavation unless provision is. made for the rapid handling of the cars when filled. For deep workings the milling method is usually employed, in which the ore is excavated in funnel-shaped pits, each of which connects with underground haulage roads by a shaft. The ore is mined in the ordinary way, by pick and shovel if soft, or by the aid of powder if necessary, and the funnel-shaped bottom of the pit is maintained at such an angle that little or no shovelling is required to bring the excavated material to the shaft. Before. the bottom of these pits reaches the level of the haulage roads below, a new set of roads will have been driven at a lower level and connected with the excavations above by the shafts. The cost of mining by the milling method does not greatly exceed the cost of steam-shovel work. For the special methods by which placer deposits are mined see Gold.
Underground Haulage.—The excavated material is brought to the hoisting shaft, or sometimes directly to the surface, in small mine cars, moved by men or by animals, or by locomotives or wire-rope haulage. The size, shape and design of the cars depend on the size of the mine passage and of the hoisting compartments of the shafts; on whether the cars are to be trammed by hand or hauled in trains; whether they are loaded by shovel or by gravity from a chute; and whether they are to be hoisted to the surface or used only for underground transport. The cost of underground haulage is lessened. by the use of cars of large capacity. In the United States cars in the coal and iron mines hold from 2 to 4 tons. In Europe the capacity ranges from 1000 to 1500 ℔, though the tendency is to increase the size of the cars used. In mines of copper, lead and the precious metals, in which the cars are moved by hand, the usual load is from 1200 to 3000 ℔. These small cars are constructed so that the load may be dumped by pivoting the car bodies on the trucks. Larger cars are usually dumped by means of rotating or swinging cradles, the car bodies being rigidly attached to the axles or trucks. When loaded by shovel the car is made low to economize labour. Wooden rails, protected by iron straps, are sometimes used on underground roads for temporary traffic; but steel rails, similar to, though lighter than, those employed for railways are the rule. For hand tramming, animal and rope haulage, the rails weigh from 8 to 24 ℔ per yard, for locomotive haulage 30 to 40 ℔. Grades are made, whenever possible, in favour of the load, and of such degree that the power required to haul out the loaded cars shall be approximately equal to that for hauling back the empties, viz. about 12 of 1%. Sharp curves should be avoided, especially for mechanical haulage. Switches for turnouts and branches, &c., are similar to but simpler than those for railways.
In metal mines, where, as a rule, mechanical haulage is inapplicable, the cars are moved by men (trammers). This is expensive, but is made necessary by the small amount of material to be handled at any given point. The average speed is about 200 ft. per minute, and the distances preferably but a few hundred feet. Man and Animal Haulage. Animal haulage is employed chiefly in Collieries and large metal mines; sometimes for main haulage lines, but oftener for distributing empty cars and making up trains for mechanical haulage. In mines operated through shafts the animals are stabled underground, and when well fed and cared for, thrive notwithstanding their rather abnormal conditions of life. Mine cars are sometimes run long distances, singly or in trains, over roads which are given sufficient grade to impart considerable speed by gravity, say from 1 to 212%. The grades must not be too great for brake control nor for the hauling back of the empty cars. Cars may thus be run through long adits or through branch gangways to some central point for making up into trains. Near the top and bottom of hoisting shafts the tracks are usually graded to permit the cars to be run to and from the shaft by gravity.
Locomotive haulage is applicable to large mines, where trains of cars are hauled long distances on flat or undulating roads of moderate gradients. Steam locomotives have been largely superseded by compressed air or electric locomotives. Compressed air locomotives are provided with cylindrical
