The development operations in amine are undertaken by the owner of the mine lease hold if the feasibility report prepared by a firm of Mining Consultats promises mining of the mineral deposit a safe and practical proposition with a reasonable return on the investment. Mining Engineers Dairy - Metalliferous Mines India
An underground mine, in its early stages, requires a carefully planned network of shafts, drifts and raises which serves the following purposes:-
1. Enables one to know the quality of mineral and extent as well as shape, thickness and dip of orebody, nature f country rock, strength or ore and wall rocks, nature and extent of overburden, etc.
2. Permits prospecting to be carried out from underground workings, etc.
3. Provides access and transport facilities for men, material and excavated mineral.
4. Permits arrangement for drainage of water and ventilation of mines.
5. Enables mine planners to decide upon method of stoping.
The formation of such network of shafts, drifts and raises mentioned above is known as development of the mine. The site of sinking the shaft are then decided by the mine owner and the work of sinking of shafts, whether inclined or vertical, is usually entrusted to an established firm of shaft sinking contractors and rarely done departmentally by the mining company. The shaft sinking is a costly process and takes 2-3 years depending on the depth, nature of strata to pass through, and the difficulties one may encounter during sinking. The period of shaft sinking normally utilized by the Mine Executive to procure equipment for underground development, other constructions on the surface and for building up the infrastructure for the mining project.
A shaft is always sunk in the footwall in a metal mine. During the process of sinking and shaft walling, suitable openings are lert un-walled at those points (horizons) from where levels or drifts have to be driven later for access to the mineral deposit. The work of level driving at upper horizons and the shaft sinking may go on simultaneously as this ensures speedy development of upper horizons. In the case of a deep shaft the work of sinking may drag on when stoping of some blocks at shallow depth is in progress.
Level vertical distance between two levels is known as level interval. Level driving is a narrow work involving more cost per m³ than stoping; each level requires timbering, haulage track, piplline, loading machines and hauling machines. Some of these requirements have to be maintained during the life of the stopes above. The cost of the plats, crushers and ore chutes can be minimized by concentrating haulage on alternative or every 3rd level.
During lateral development in a metalliferous mine the interval between levels varies from 15 m to 100 m and depends upon the following considerations.
The drives and the corss-cuts , drifts, etc. branching off the levels give a correct idea the deposit and the wall rocks their dip, etc. It is therefore an advantage to have levels at short intervals if
Ø The deposit has not been proved adequately during exploration stage,
Ø The deposit is irregular in dip, quantity and extent, and
Ø Some of the orebody has been missed during initial prospecting stage.
High grade ores or ores containing workable grade in pockets, require levels to be close containing workable grade in pockets, require levels to be close together to avoid missing the ore. Considering the cost of development and equipment each level, constructing plants and loading points at each level, a short level interval is a disadvantage requiring large initial expenditure on unproductive work. When stoping, some ore has to be left in situ, for example, in the pillars and sill pillars for support of levels and in the rib pillars for support of winzes and raises. This loss of ore is naturally more with shorter level intervals than when the level interval is more.
Thicker the ore body, lesser the level interval and if the orebody is thin, the level interval is more. The reason for this is that in thick deposits there is a sufficient supply of ore between near levels. Also the pressures in stoping would be great in thick deposits so that the level has to be mined out quickly. In slightly inclined deposits the levels can be established rather close to each other vertically and yet have a sufficiently long wall and a sufficiently supply of mineral between them. In steeply inclined or vertical deposits levels are established further apart vertically as the length of the sections between the levels is shorter.Cost of maintenance of levels generally should be such that topes are completed and abandoned within the time that they can be kept open without undue maintenance cost. Speed of stoping and character of country rock are related factors.
A retreating system where the stopes are started from the boundary of the mine lease hold (or other artificial boundary line) may allow a larger level interval than an advancing system.
Levels sometimes area of unsupported hanging walls then limits level interval unless some form of filled stoping its practicable.
The question of selecting the level intervals is also governed by the anticipated least cost per tonne of ore mined by the contemplated method of mining. The economic level interval in any mine is decided by the management on the basis of experience, taking into consideration the factors stated above. At Jaduguda Mine, the level interval beyond a depth of 100 m was 65 m with an interval half way. But upto 100 m, a level interval of 30-50 m was kept to have better knowledge of the behaviour of the orebody and to get its pecise delineation. This also offered possibility stope blocks.
Whenever any work is being done in a mine the following points should be borne in mind as they are the cardinal points for any mining activity, safety dominating all other considerations.