mineral, and by the possibility of supporting the roof long
enough to permit the excavation of the mineral without
unnecessary risk or expense. In metal mining, when the
workable portions of the deposit are small and separated by
unworkable areas, the levels serve also the purpose of exploration,
and in such cases must not be so far apart as to risk
missing valuable mineral. In coal-mines main entries are often
100 yds. apart, while in metal-mines the distance between
levels rarely exceeds 50 yds. and sometimes is but 50 or 60 ft.
In irregular and uncertain deposits this work of development
should be kept at all times so far in advance of mining operations
as to ensure a regular and uniform output. In some cases,
where the barren areas are large, it may be necessary to have
two or three years’ supply of ore thus blocked out in advance.
A mine, however, may be over-developed, which results in loss
of interest on the capital unnecessarily locked up for years
by excessive development, and involves additional cost for the
maintenance of such openings until they are needed for active
mining operations.
Working.—When the development of a mine has advanced sufficiently the operation of working or extracting the mineral begins. The method to be adopted will vary with the thickness and character of the deposit, with its inclination, and to some extent with the character of the enclosing rocks, the depth below the surface, and other conditions. The safety of the men must be one of the first considerations of the mine operator. In most civilized countries the safety of mine workers is guarded by stringent laws and enforced by the careful supervision of mine inspectors on behalf of the government. The method of mining adopted must secure the extraction of the mineral at a minimum cost. The principal item in mining cost is that of labour, which is expended chiefly in breaking down the mineral, either by the use of hand tools or with the aid of powder. Labour is also expended in handling the mineral in the working places and in bringing it to the mine-cars in which it is brought to the surface. Narrow and contracted working-places are to be avoided, as in such places the cost of breaking ground is always large. Economy in handling makes it desirable to bring the mine-cars as near as may be to the point where the mineral is broken. This can be done in inclined deposits, it can often be done by the aid of mechanical appliances, though sometimes at an expense not warranted in the saving in the labour of loading. In steeply inclined beds the working-place can be so arranged that the mineral will fall or slide from the place where it is broken down to the main haulage road. The greatest difficulty is found where the inclination of the deposit is too great to permit the mine-cars to be brought into the working-place and yet not great enough to allow the mineral to fall or slide to a point where it can be loaded.
While it is always desirable to provide large working-places, the size of the working-place is limited by the thickness and strength of the overlying beds forming the roof or hanging wall of the mine. With thick and strong rocks the working-places may sometimes exceed 100 or even 200 ft. in width. Indeed in metal-mines 100 ft. Size of Working-Places. is the usual distance from one level to the next. With weak and thin beds forming the roof the working-places are often not wider than 20 or 30 ft. as in most coal-mines. While the width of the working-place is thus limited by the strength of the roof, its length is determined by other considerations—namely, the rapidity with which the mining work can be conducted and the length of time it is practicable to keep the working-place open, and also by the increased difficulty of handling the minerals sometimes experienced when the workings reach undue length. In long-wall and in the work of mining pillars the roof will be supported on one side only, the overhanging beds acting as cantilevers. The working-place in such case is considerably narrower than in rooms or stopes, and there is also greater difficulty in supporting the roof because the projecting beds tend to break close to the point of support where the strain is greatest. This tendency is overcome by the use of timber supports so disposed as to ensure the breaking of the overhanging roof at a safe distance from the working-face and prevent the interruption of the work that might otherwise result.
While it is always desirable to work the deposit so as to,
extract the mineral completely, it frequently happens that
this can only be done at greatly increased cost. In
the case of cheap and abundant minerals and low-grade
ore deposits it is sometimes necessary to
sacrifice a considerable proportion of the mineral, which is
Complete Extraction
of Mineral.
left for the support of the overlying strata. A similar sacrifice
in the shape of pillars is often necessary to support the surface,
either to avoid injury to valuable structures or to prevent
a flooding of the mine. As already noted large pillars must
always be left to protect shafts, adits and the more important
mine-passages necessary for drainage, ventilation and the
haulage of mineral. In the early history of mining there was
but little attempt at systematic development and working,
and the mines were often irregular and tortuous. Fig. 3 is
an old Mexican silver-mine of this type.
Fig. 3.
In such mines the mineral was carried out on the backs of men, and the water was laboriously raised by a long line of suction-pumps, operated by hand, each lifting the water a few feet only. With but slight modifications permitting the use of pumps and hoisting-machinery equally simple methods of mining may be seen to-day when the deposit is of small extent. Fig. 4 is a portion of a mine which consists of a series of irregular chambers with the roof supported on small pillars left at intervals for the purpose.
| Fig. 4. |
In the systematic mining of larger deposits, the simplest plan consists in mining large areas by means of numerous working-places under the protection of pillars of mineral left for the purpose, and later mining these pillars systematically, allowing the overlying rock beds to fall and fill the abandoned workings. In shallow mines the pillars are small and the saving of the mineral of minor importance. In deep mines the pillars may furnish the bulk of the product, and the control of the fall of the roof, so as to permit the successful extraction of the mineral, demands a well-schemed plan of operation. In the robbing of pillars, timber is necessary for the support of
