the roof in the working-places, and later to control the fall
of the roof while the pillars are mined. More effective
support and control of the roof may be secured by the use of
rock-filling alone or with timber. By the use of rock-filling
it is even possible to dispense with pillars of mineral; or, if
pillars are left, the use of rock-filling greatly facilitates subsequent
robbing operations. Rock-filling will be used whenever
a large proportion of barren material must be mined with the
ore. If rock-filling must be brought from the surface its use
will generally be confined to mines in which it is difficult to
support the roof in any other way. Rock-filling yields and
becomes consolidated under heavy pressure, and therefore
does not furnish a rigid support of the overlying strata, but
rather a cushion to control and equalize the subsidence.
With soft material, pillars must be large, even at moderate depths below the surface, and it involves less labour to leave long rectangular pillars than to form numerous square ones. This leads to the adoption of the room and pillar system so common in coal-mining. Fig. 5 is a mine in a bed of soft iron ore worked by a series Room- and Pillar-Mining. of inclined shafts, from which long horizontal rooms branch off right and left.
Fig. 5.
The usual method of working metal-mines is by overhand and underhand stoping, using rock-filling or pillars of mineral to support the roof. Fig. 6 represents a portion of one of the Lake Superior copper-mines worked by overhand stoping. A stope is that portion of the working assigned to a party of miners, and the block of ground is usually Stoping. divided into three or four stopes at varying heights above the main level, the lowest being known as the cutting-out stope, the others as the first and second back stopes in ascending order.
Fig. 6.
In steep pitching beds sufficient excavated material is allowed to remain in the stope for the support of the machines and men, the excess being drawn out from time to time and loaded into cars. The rest of the mineral is allowed to remain until the stope has so far advanced that its support is no longer needed. This method of mining requires but little timbering, only a single line of timber and lagging over the level, called the stull. When the roof is weak, or when it is undesirable to leave so much ore in the stopes, false stulls are sometimes erected in the upper part of the stope. The ore below the false stulls can then be drawn out without waiting for the completion of the top stope. When the mineral does not stand well in the pillar it will be necessary to erect a line of timbers with lagging so as to sheathe the under-side of the pillar and prevent its falling.
Fig. 7.
It is not desirable to leave large areas standing upon pillars in the mine, and as soon as the work on any level is completed the pillar below should be mined out as far as is safe, and the abandoned portion of the mine allowed to cave in and lessen the weight on the pillars elsewhere. The block or ground between levels is sometimes mined by underhand stoping (fig. 7.). In this case the advanced drift is run underneath the pillar, and the ground below is mined in descending steps. This plan has the advantage of requiring little or no timbering when the mineral is strong enough to stand well in the pillars and when the hanging wall is good. The main haulage tracks are laid at the bottom of the stope, which thus forms the level. In this method of mining the different stopes must be kept close together; otherwise there is much added labour in shovelling the broken ore down to the main level. This method has the advantage of permitting the ore to be sent to the surface as fast as it is mined instead of being left for some months in the stopes for the men to stand upon. It has the disadvantage that the distance from one level to the next cannot usually be more than fifty feet without increasing greatly the chances of injury to the men from falling rock. The method is then practicable and safe only with exceptionally strong mineral and roof. In metal-mines producing abundant rock-filling the overhand method of stoping, illustrated in fig. 8, is used.
Fig. 8.
In this the stoping contracts run vertically, and each party of contractors has one or more mills or timbered chutes through which the rich ore is conveyed to the level below and loaded in cars. The ore as mined is hand-picked and the barren material allowed to remain in the stope where it
