Shaft: Site Selection, Shape, Depth & Pillar | Deep Mining | Mining Technology

In this article we will discuss about:- 1. Selecting Site of an Incline or Pit 2. Shape, Size and Number of Shafts 3. Depth 4. Shaft Pillar.

Site Selection of an Incline or Pit:

A number of factors have to be considered before selecting the site of a shaft or incline. In mountainous regions, inclines and shafts should be always from possible path of land slides. The site should be above the highest flood level of a river or lake in the area, and the slope of the ground away from the shaft to facilitate drainage and movement of mineral-loaded tubs to the crusher and tippler.

Usually the crusher and tippler are located at a higher level for flow of mineral by gravity. A shaft or incline should be situated close to efficient transport facilities, like railway, road or canal to avoid the need for construction of long railway line as also curves and heavy cutting or filling for the line.

At the same time surface transport of mineral from incline or pit to processing plant should be minimum. If the railway has to cross a big river like the Damodar, the cost involved in bridge construction may rule out the possibility of constructing a siding.

Installation of an aerial ropeway for transport of mineral provides an alternative e.g., at Sasti Colliery. Broken ground like golf area and stopped area and a site near geological disturbances like faults, dykes and sills should be avoided so that un-productive work of long stone drift through dyke of fault should not be necessary soon after touching the mineral.

A plentiful supply of water, not far from the shaft or incline, is essential for billers, pithead baths, washeries, etc. Costly land should be avoided as a large number of buildings and other installations require an extensive area.

The space near the shaft should be adequate for dumping the debris during sinking and for the service buildings like office, lamp cabin, pithead bath, winding engine rooms, crèche, canteen, screening plant, repair shops, etc. In the case of metal mining if ore preparation mill or plant is to be located near the shaft, mill tailings occupy a large surface. The site should be level or nearly so over a wide stretch.

In some mines large dumping space near the shaft is required to dump underground stone or shale bands or to dump washery rejects if a washery is planned. Sufficient space should be earmarked for further expansion programme.

For a coal seam of moderate inclination the shaft may be in the middle (dip-rise direction), but where the seam has more inclination, it is convenient to so place the shaft as to have nearly one-third area on the dip side and two-third on the rise. This facilitates underground transport and drainage of rise areas towards the shaft. Shafts for sand stowing pipes are however on the rise side of the property. Ventilation shafts used mainly for upcast air where exhaust fans are used are also the rise side.

If the surface of the mine leasehold is not hilly and the land is nearly flat or uniformly sloping, it is convenient to have production shafts in the middle of the mine area as transport of the mineral on the surface can be arranged by locomotives, belt conveyors or by rope haulage to coal handling plant or mineral bunkers. Maintenance and supervision of surface transport arrangements is more efficient than underground.

An incline has to be situated near the outcrop and its site does not offer as much choice as a pit. The section of shaft site is, however, a major decision. Shaft sinking being a costly process, nature of strata through which shaft has to be sunk should be ascertained by a proving bore hole near the proposed shaft site. Trial pits 3 m to 5 deep, are sunk to test bearing pressure of the soil for structures associated with winding and other heavy installations.

The incline in a coal mine should have practically equal area upto a maximum of 600 m on the strike, on either side, for development by rope haulage. This limit may be considered at 1000 m for belt conveyors. Larger area on the strike requires more inclines. The shaft should have nearly equal width on either side along the strike for development.

Shape, Size and Number of Shafts:

It shafts are circular in shape; rectangular shafts are rare in this country, the exceptional cases being some of the shafts in metal mines. The finished diameter of a shaft varies from 4.2 m to 6.7 m.

Where a shaft accommodates a pair of single cages (a cage capable of an accommodating a single tube only), the diameter may be nearly 4.2 m; where a pair of tandem changes (a cage capable of accommodating two tubs) is used, the finished diameter may be 5 m to 6 m.

At Chinakuri colliery employing two single deck cages, each cage accommodating one 31/2 te mine car, the coal raising shaft is 6.0 m finished diameter for a planned production of 50,000 te per month. At Sudamdih colliery, the main coal winding shaft has 7.2 m finished diameter.

The main shaft for mine entry at Jaduguda mine is circular, 5 m finished diameter, fully lined. It is equipped with 2 multirope friction winders, one winder for cages, and the other for a skip having a counter balancing weight. Each cage has 2 decks and is for a payload of 3 1/2 te or 50 men. The skin has a capacity of 5 te payload. The cages run on rigid guides whereas the skip and its balancing counterweights run on rope guides.

Under the mining Regulations each seam should have a least two outlets to surface separated by a minimum of 13.5 m at any point. The general arrangement is to sink two shafts close to each other, separated by 30 to 60 m, for facility of a quicker intercommunication.

Unless the two shafts are connected by underground tunnel no development work inside the mine is permitted under the law. One of the shafts serves for mineral winding, and the other for ventilation, man riding and material transport. The upcast shaft, having air locks, may be used for small quantity of raising, which may be from a separate seam, or form a different level. This helps in grade control.

The nearness of the two shafts permits common boiler plant, pit-tip arrangement, screening plant, crusher plant and siding. The pit-bottom arrangements also may be so laid out that loads of one shaft may be directed to the other, if desired.

An extensive property, say 2.5 km² or more may have two or more pairs of shafts, situated far apart far mineral winding only if deposit is at a shallow depth. Geological disturbances such as faults or dykes may require divisions of the area into convenient zones and shafts may be sunk to serve each zone.

Such division into zones may sometimes be desirable if a river, railway or major trunk road (e.g. G.T. road in Bengal and Bihar coalfields) exists on the surface as each of these features requires solid block of mineral to the left-in-situ. In each zone the mineral winding shaft may be so situated as to have 50% to 60% area on the rise side. Moreover the extent served by a mineral winding shaft should be such that the manager of the pit has not more than 2,000 to 2,500 men including surface workers in his unit for ease of supervisor and control.

Depth of Shaft:

The shaft should reach nearly 5 m below the lowest bed to be worked to accommodate cheese weights of the guide ropes. If guide ropes are clamped to the girders at underground decking level, shaft may extend only 2 m below the girder level. Where skip winding is adopted the shaft should be sunk deeper by a minimum of 6 m below the underground decking level, depending mainly upon the height of the skip.

In an incline mine in coal upcast ventilation pit should be so such that the pit bottom is in coal which is comparatively stronger than near the outcrop. This reduces the cost on supporting sides and roof of the gallery leading to the ventilation pit-bottom.

The return air is warm and laden with moisture, gases, etc. and has a weathering effect. Because of this consideration the upcast shaft should touch the coal seam in 3rd or 4th level if the mine is laid out on bord and pillar system.

The deepest shaft in coal mine in this country (600 m depth) is at Chinakuri Colliery.

Shaft Pillar:

Since the inclines and shafts serve as means of access to underground mines throughout their life they should be in strata which are not likely to subside or collapse. In a mine a solid block of rock known as shaft pillar should be kept on all sides of the shaft, and only essential rods should be driven through it. Any attempt to extract mineral from the shaft pillar on its periphery will weaken it, and verticality of shaft may be affected, or it may collapse.

On the surface some buildings and other installations have to be situated very close to the shaft for technical reasons, e.g., winding engine rooms, boiler plant (in case of steam winders) and fan house. Some other buildings, for the sake of convenience and easy supervision, are also installed close to shaft e.g., workshop, electrical sub-station, lamp cabin, pit head bath, office, store, etc.

Screening plant, coal bunkers and coal handling plant, crushers are usually situated near the shaft to reduce cost on surface transport though benefit of centralisation for a number of mines may sometimes dictate otherwise. Such buildings and installations last for the whole life of the mine and the shaft pillar should be of such a size as to support them.

The Mines Regulations do not prescribe any specific size of shaft pillar. It may be said in broad terms that in coal mines one side of a square shaft pillar should be equal to depth of the seam and the essential surface buildings or installations likely to last the whole life of the colliery, should be located within the area of the shaft pillar.

In most of the mines in our country where mineral is raised through vertical shafts, the standard arrangement is to bring the mineral loaded tubs to the shaft bottom and push them into the cage for hoisting to the surface. The cage loading arrangements are in shaft level which is a level road in coal on either side of the shaft.

Such shaft level in coal and the other roadways in coal leading to the shaft level, as also essential excavations like pit bottom sumps, underground sub-station, etc. in coal itself demand generous dimensions of the shaft pillar. If however, the shaft level is in stronger rock like stone, as in Chinakuri mine.

The shaft pillar in stone need be of comparatively much smaller size. In the case of skip winding loading of the skip is by a chute at the bottom of a coal bunker which is in stone below the coal gallery housing the tippler.

In coal mines worked by horizon mining and in the case of metalliferous mines the pit bottom/plate are in stone. In all such cases where the pit bottom loading arrangements are in stone, the shaft pillar is in stone and of smaller size compared to what it would have been in coal.

Some thumb rules based on experience have been advanced for the side of a shaft pillar in coal.

Considerating D depth of shaft in m,

t = thickness of the seam in m,

R – radius of shaft pillar in m.

1. DRON’S Rule – Area of the shaft pillar = area to be supported + D/6 on all sides.

2. Foster’s rule – R = 3√Dt

3. Wadin’s Rule – For shafts upto 100 m depth, size should not be less than 36.5 x 36.5 m. Thereafter for every 36.5m depth, increase size by 9 m.

4. Mining Engineer’s Rule – For shallow shaft, Minimum radius for shaft pillar is 18 m. For deeper shaft –

R = 18.3 + D√t/32.8

5. O’Donahue’s Formula for Inclined Seams:

If D – depth of shaft; x = angle of dip of coal seam, then y will be D sin x cos x (See Fig. 6.6).

S = Margin of safety, usually equals to 5% to 1.0% of the depth;

Then width of pillar on the rise side = S + D/7 + 2y/3

Do dip side = S + D/7 – y/3

Do along strike = S + D/7

To calculate the size of a shaft pillar the scientific approach is based on the “angle of draw”. Precise observations and scientific studies have not been made in this country to ascertain the angle of draw for the various coalfields but the Central Mining Research Station has conducted experiments in a few cases.

In Fig. 6.6 is the surface area which needs to be supported. If the angle of a draw is known the shaft pillar may be set out as shown in the figure. It should be that angle draw to the dip of the workings and to the rise of the workings have different values.

In the case of an incline or adit there is no such thing as “shaft pillars”. In a coal mine developed on bord and pillar method two pillars of coal are left unextracted on either side of the incline for its support and essential buildings and installations can be located to the rise side of the incline mouth. In a metalliferios mine also a solid block of rock is left in-situ on either side of the incline and its dimensions depend upon the strength of the rock.

After the deposit has been entered through an incline or pit, arrangements have to be made at the top and bottom of the pit/incline to deal with the planned output of the mine in an efficient manner.